Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Last month I wrote: It seems clear that our qsort.c is doing a pretty awful job of picking qsort pivots, while glibc is mostly managing not to make that mistake. I re-ran Gary's test script using the just-committed improvements to qsort.c, and got pretty nice numbers (attached --- compare to http://archives.postgresql.org/pgsql-performance/2006-02/msg00227.php). So it was wrong to blame his problems on the pivot selection --- the culprit was that ill-considered switch to insertion sort. regards, tom lane 100 runtimes for latest port/qsort.c, sorted ascending: Time: 335.481 ms Time: 335.606 ms Time: 335.932 ms Time: 336.039 ms Time: 336.182 ms Time: 336.231 ms Time: 336.711 ms Time: 336.721 ms Time: 336.971 ms Time: 336.982 ms Time: 337.036 ms Time: 337.190 ms Time: 337.223 ms Time: 337.312 ms Time: 337.350 ms Time: 337.423 ms Time: 337.523 ms Time: 337.528 ms Time: 337.565 ms Time: 337.566 ms Time: 337.732 ms Time: 337.741 ms Time: 337.744 ms Time: 337.786 ms Time: 337.790 ms Time: 337.898 ms Time: 337.905 ms Time: 337.952 ms Time: 337.976 ms Time: 338.017 ms Time: 338.123 ms Time: 338.206 ms Time: 338.306 ms Time: 338.514 ms Time: 338.594 ms Time: 338.597 ms Time: 338.683 ms Time: 338.705 ms Time: 338.729 ms Time: 338.748 ms Time: 338.816 ms Time: 338.958 ms Time: 338.963 ms Time: 338.997 ms Time: 339.074 ms Time: 339.106 ms Time: 339.134 ms Time: 339.159 ms Time: 339.226 ms Time: 339.260 ms Time: 339.289 ms Time: 339.341 ms Time: 339.500 ms Time: 339.585 ms Time: 339.595 ms Time: 339.774 ms Time: 339.897 ms Time: 339.927 ms Time: 340.064 ms Time: 340.133 ms Time: 340.172 ms Time: 340.219 ms Time: 340.261 ms Time: 340.323 ms Time: 340.708 ms Time: 340.761 ms Time: 340.785 ms Time: 340.900 ms Time: 340.986 ms Time: 341.339 ms Time: 341.564 ms Time: 341.707 ms Time: 342.155 ms Time: 342.213 ms Time: 342.452 ms Time: 342.515 ms Time: 342.540 ms Time: 342.928 ms Time: 343.548 ms Time: 343.663 ms Time: 344.192 ms Time: 344.952 ms Time: 345.152 ms Time: 345.174 ms Time: 345.444 ms Time: 346.848 ms Time: 348.144 ms Time: 348.842 ms Time: 354.550 ms Time: 356.877 ms Time: 357.475 ms Time: 358.487 ms Time: 364.178 ms Time: 370.730 ms Time: 493.098 ms Time: 648.009 ms Time: 849.345 ms Time: 860.616 ms Time: 936.800 ms Time: 1727.085 ms ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Added to TODO: * Improve port/qsort() to handle sorts with 50% unique and 50% duplicate value [qsort] This involves choosing better pivot points for the quicksort. --- Dann Corbit wrote: -Original Message- From: [EMAIL PROTECTED] [mailto:pgsql-hackers- [EMAIL PROTECTED] On Behalf Of Tom Lane Sent: Wednesday, February 15, 2006 5:22 PM To: Ron Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) Ron [EMAIL PROTECTED] writes: How are we choosing our pivots? See qsort.c: it looks like median of nine equally spaced inputs (ie, the 1/8th points of the initial input array, plus the end points), implemented as two rounds of median-of-three choices. With half of the data inputs zero, it's not too improbable for two out of the three samples to be zeroes in which case I think the med3 result will be zero --- so choosing a pivot of zero is much more probable than one would like, and doing so in many levels of recursion causes the problem. Adding some randomness to the selection of the pivot is a known technique to fix the oddball partitions problem. However, Bentley and Sedgewick proved that every quick sort algorithm has some input set that makes it go quadratic (hence the recent popularity of introspective sort, which switches to heapsort if quadratic behavior is detected. The C++ template I submitted was an example of introspective sort, but PostgreSQL does not use C++ so it was not helpful). I think. I'm not too sure if the code isn't just being sloppy about the case where many data values are equal to the pivot --- there's a special case there to switch to insertion sort, and maybe that's getting invoked too soon. Here are some cases known to make qsort go quadratic: 1. Data already sorted 2. Data reverse sorted 3. Data organ-pipe sorted or ramp 4. Almost all data of the same value There are probably other cases. Randomizing the pivot helps some, as does check for in-order or reverse order partitions. Imagine if 1/3 of the partitions fall into a category that causes quadratic behavior (have one of the above formats and have more than CUTOFF elements in them). It is doubtful that the switch to insertion sort is causing any sort of problems. It is only going to be invoked on tiny sets, for which it has a fixed cost that is probably less that qsort() function calls on sets of the same size. It'd be useful to get a line-level profile of the behavior of this code in the slow cases... I guess that my in-order or presorted tests [which often arise when there are very few distinct values] may solve the bad partition problems. Don't forget that the algorithm is called recursively. regards, tom lane ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster -- Bruce Momjian http://candle.pha.pa.us SRA OSS, Inc. http://www.sraoss.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
My introsort is almost complete and its the fastest variant of quicksort I can find, I'll submit it to -patches in the next couple days as-well.On 3/2/06, Bruce Momjian pgman@candle.pha.pa.us wrote: Added to TODO:* Improve port/qsort() to handle sorts with 50% unique and 50% duplicatevalue [qsort]This involves choosing better pivot points for the quicksort. ---Dann Corbit wrote: -Original Message- From: [EMAIL PROTECTED] [mailto:pgsql-hackers- [EMAIL PROTECTED]] On Behalf Of Tom Lane Sent: Wednesday, February 15, 2006 5:22 PM To: Ron Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) Ron [EMAIL PROTECTED] writes: How are we choosing our pivots? See qsort.c: it looks like median of nine equally spaced inputs (ie, the 1/8th points of the initial input array, plus the end points), implemented as two rounds of median-of-three choices.With half of the data inputs zero, it's not too improbable for two out of the three samples to be zeroes in which case I think the med3 result will be zero --- so choosing a pivot of zero is much more probable than one would like, and doing so in many levels of recursion causes the problem. Adding some randomness to the selection of the pivot is a known technique to fix the oddball partitions problem.However, Bentley and Sedgewick proved that every quick sort algorithm has some input set that makes it go quadratic (hence the recent popularity of introspective sort, which switches to heapsort if quadratic behavior is detected.The C++ template I submitted was an example of introspective sort, but PostgreSQL does not use C++ so it was not helpful). I think.I'm not too sure if the code isn't just being sloppy about the case where many data values are equal to the pivot --- there's a special case there to switch to insertion sort, and maybe that's getting invoked too soon. Here are some cases known to make qsort go quadratic: 1. Data already sorted 2. Data reverse sorted 3. Data organ-pipe sorted or ramp 4. Almost all data of the same value There are probably other cases.Randomizing the pivot helps some, as does check for in-order or reverse order partitions. Imagine if 1/3 of the partitions fall into a category that causes quadratic behavior (have one of the above formats and have more than CUTOFF elements in them). It is doubtful that the switch to insertion sort is causing any sort of problems.It is only going to be invoked on tiny sets, for which it has a fixed cost that is probably less that qsort() function calls on sets of the same size. It'd be useful to get a line-level profile of the behavior of this code in the slow cases... I guess that my in-order or presorted tests [which often arise when there are very few distinct values] may solve the bad partition problems.Don't forget that the algorithm is called recursively. regards, tom lane ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster --Bruce Momjian http://candle.pha.pa.usSRA OSS, Inc. http://www.sraoss.com+ If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)---TIP 6: explain analyze is your friend-- Jonah H. Harris, Database Internals Architect EnterpriseDB Corporation732.331.1324
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
--On Donnerstag, Februar 16, 2006 10:39:45 -0800 Dann Corbit [EMAIL PROTECTED] wrote: He refers to counting sort and radix sort (which comes in most significant digit and least significant digit format). These are also called distribution (as opposed to comparison) sorts. These sorts are O(n) as a function of the data size, but really they are O(M*n) where M is the average key length and n is the data set size. (In the case of MSD radix sort M is the average length to completely differentiate strings) So if you have an 80 character key, then 80*log(n) will only be faster I suppose you meant 80 * n here? than n*log(n) when you have 2^80th elements -- in other words -- never. I think this is wrong. You can easily improve Radix sort by a constant if you don't take single bytes as the digits but rather k-byte values. At least 2 byte should be possible without problems. This would give you 40 * n time, not 80 * n. And you assume that the comparision of an 80-byte wide value only costs 1, which might (and in many cases will be imho) wrong. Actually it migh mean to compare 80 bytes as well, but I might be wrong. What I think as the biggest problem is the digit representation necessary for Radix-Sort in cases of locales which sort without looking at spaces. I assume that would be hard to implement. The same goes for the proposed mapping of string values onto 4/8-byte values. Mit freundlichem Gruß Jens Schicke -- Jens Schicke [EMAIL PROTECTED] asco GmbH http://www.asco.de Mittelweg 7 Tel 0531/3906-127 38106 BraunschweigFax 0531/3906-400 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Fri, Feb 17, 2006 at 09:18:39AM +0100, Jens-Wolfhard Schicke wrote: What I think as the biggest problem is the digit representation necessary for Radix-Sort in cases of locales which sort without looking at spaces. I assume that would be hard to implement. The same goes for the proposed mapping of string values onto 4/8-byte values. Actually, this is easy. The standard C library provides strxfrm() and other locale toolkits like ICU provide ucol_getSortKey(). Windows provides LCMapString(). Just pass each string through this and take the first four bytes of the result to form your integer key. Have a nice day, -- Martijn van Oosterhout kleptog@svana.org http://svana.org/kleptog/ Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a tool for doing 5% of the work and then sitting around waiting for someone else to do the other 95% so you can sue them. signature.asc Description: Digital signature
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Hi, David, David Lang schrieb: In SQL_ASCII, just take the first 4 characters (or 8, if using a 64-bit sortKey as elsewhere suggested). The sorting key doesn't need to be a one-to-one mapping. that would violate your second contraint ( f(a)==f(b) iff (a==b) ) no, it doesn't. When both strings are equal, then the first characters are equal, too. If they are not equal, the constraint condition does not match. The first characters of the strings may be equal as f(a) may be equal to f(b) as to the other constraint. Markus ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Feb 16, 2006, at 2:17 PM, Mark Lewis wrote:Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). ...and with some work, floats (I think just the exponent would work, if nothing else). bytea. Probably just about anything.Interesting. If you abandon the idea that collisions should be impossible (they're not indexes) or extremely rare (they're not hashes), it's pretty easy to come up with a decent hint to avoid a lot of dereferences. --Scott Lamb http://www.slamb.org/
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Fri, Feb 17, 2006 at 08:18:41AM -0800, Scott Lamb wrote: On Feb 16, 2006, at 2:17 PM, Mark Lewis wrote: Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). ...and with some work, floats (I think just the exponent would work, if nothing else). bytea. Probably just about anything. Interesting. If you abandon the idea that collisions should be impossible (they're not indexes) or extremely rare (they're not hashes), it's pretty easy to come up with a decent hint to avoid a lot of dereferences. Yep, pretty much for any datatype you create a mapping function to map it to a signed int32. All you have to guarentee is that f(a) f(b) implies that a b. Only if f(a) == f(b) do you need to compare a and b. You then change the sorting code to have an array of (Datum,int32) (ouch, double the storage) where the int32 is the f(Datum). And in the comparison routines you first check the int32. If they give an order you're done. On match you do the full comparison. For integer types (int2,int4,int8,oid) the conversion is straight forward. For float you'd use the exponent and the first few bits of the mantissa. For strings you'd have to bail, or use a strxfrm equivalent. NULL would be INT_MAX pr INT_MIN depending on where you want it. Thing is, even if you don't have such a function and always return zero, the results will still be right. Not a new idea, but it would be very nice to implement. If would produce nice speedups for type where comparisons are expensive. And more importantly, the bulk of the comparisons can be moved inline and make the whole cache-friendlyness discussed here much more meaningful. Have a nice day, -- Martijn van Oosterhout kleptog@svana.org http://svana.org/kleptog/ Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a tool for doing 5% of the work and then sitting around waiting for someone else to do the other 95% so you can sue them. signature.asc Description: Digital signature
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, 2006-02-16 at 21:33 -0800, David Lang wrote: In SQL_ASCII, just take the first 4 characters (or 8, if using a 64-bit sortKey as elsewhere suggested). The sorting key doesn't need to be a one-to-one mapping. that would violate your second contraint ( f(a)==f(b) iff (a==b) ) if you could drop that constraint (the cost of which would be extra 'real' compares within a bucket) then a helper function per datatype could work as you are talking. I think we're actually on the same page here; you're right that the constraint above ( f(a)==f(b) iff a==b ) can't be extended to data types with more than 32 bits of value space. But the constraint I listed was actually: if a==b then f(a)==f(b) Which doesn't imply 'if and only if'. It's a similar constraint to hashcodes; the same value will always have the same hash, but you're not guaranteed that the hashcodes for two distinct values will be unique. -- Mark ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Mark Lewis [EMAIL PROTECTED] writes: I think we're actually on the same page here; you're right that the constraint above ( f(a)==f(b) iff a==b ) can't be extended to data types with more than 32 bits of value space. But the constraint I listed was actually: if a==b then f(a)==f(b) I believe Martijn had it right: the important constraint is f(a) f(b) implies a b which implies by commutativity f(a) f(b) implies a b and these two together imply a == b implies f(a) == f(b) Now you can't do any sorting if you only have the equality rule, you need the inequality rule. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
At 06:35 AM 2/16/2006, Steinar H. Gunderson wrote:
On Wed, Feb 15, 2006 at 11:30:54PM -0500, Ron wrote:
Even better (and more easily scaled as the number of GPR's in the CPU
changes) is to use
the set {L; L+1; L+2; t1; R-2; R-1; R}
This means that instead of 7 random memory accesses, we have 3; two
of which result in a burst access for three elements each.
Isn't that improvement going to disappear competely if you choose a bad
pivot?
Only if you _consistently_ (read: the vast majority of the time:
quicksort is actually darn robust) choose a _pessimal_, not just
bad, pivot quicksort will degenerate to the O(N^2) behavior
everyone worries about. See Corman Rivest for a proof on this.
Even then, doing things as above has benefits:
1= The worst case is less bad since the guaranteed O(lgs!) pivot
choosing algorithm puts s elements into final position.
Worst case becomes better than O(N^2/(s-1)).
2= The overhead of pivot choosing can overshadow the benefits using
more traditional methods for even moderate values of s. See
discussions on the quicksort variant known as samplesort and
Sedgewick's PhD thesis for details. Using a pivot choosing algorithm
that actually does some of the partitioning (and does it more
efficiently than the usual partitioning algorithm does) plus using
partition-in-place (rather then Lomuto's method) reduces overhead
very effectively (at the cost of more complicated / delicate to get
right partitioning code). The above reduces the number of moves used
in a quicksort pass considerably regardless of the number of compares used.
3= Especially in modern systems where the gap between internal CPU
bandwidth and memory bandwidth is so great, the overhead of memory
accesses for comparisons and moves is the majority of the overhead
for both the pivot choosing and the partitioning algorithms within
quicksort. Particularly random memory accesses. The reason (#GPRs -
1) is a magic constant is that it's the most you can compare and move
using only register-to-register operations.
In addition, replacing as many of the memory accesses you must do
with sequential rather than random memory accesses is a big deal:
sequential memory access is measured in 10's of CPU cycles while
random memory access is measured in hundreds of CPU cycles. It's no
accident that the advances in Grey's sorting contest have involved
algorithms that are both register and cache friendly, minimizing
overall memory access and using sequential memory access as much as
possible when said access can not be avoided. As caches grow larger
and memory accesses more expensive, it's often worth it to use a
BucketSort+QuickSort hybrid rather than just QuickSort.
...and of course if you know enough about the data to be sorted so as
to constrain it appropriately, one should use a non comparison based
O(N) sorting algorithm rather than any of the general comparison
based O(NlgN) methods.
SIDE NOTE: IIRC glibc's qsort is actually merge sort. Merge sort
performance is insensitive to all inputs, and there are way to
optimize it as well.
glibc-2.3.5/stdlib/qsort.c:
/* Order size using quicksort. This implementation incorporates
four optimizations discussed in Sedgewick:
I can't see any references to merge sort in there at all.
Well, then I'm not the only person on the lists whose memory is faulty ;-)
The up side of MergeSort is that its performance is always O(NlgN).
The down sides are that it is far more memory hungry than QuickSort and slower.
Ron
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Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Hi, Ron, Ron wrote: ...and of course if you know enough about the data to be sorted so as to constrain it appropriately, one should use a non comparison based O(N) sorting algorithm rather than any of the general comparison based O(NlgN) methods. Sounds interesting, could you give us some pointers (names, URLs, papers) to such algorithms? Thanks a lot, Markus -- Markus Schaber | Logical TrackingTracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Last night I implemented a non-recursive introsort in C... let me test it a bit more and then I'll post it here for everyone else to try out.On 2/16/06, Markus Schaber [EMAIL PROTECTED] wrote:Hi, Ron, Ron wrote: ...and of course if you know enough about the data to be sorted so as to constrain it appropriately, one should use a non comparison based O(N) sorting algorithm rather than any of the general comparison based O(NlgN) methods.Sounds interesting, could you give us some pointers (names, URLs,papers) to such algorithms?Thanks a lot,Markus--Markus Schaber | Logical TrackingTracing International AG Dipl. Inf. | Software Development GISFight against software patents in EU! www.ffii.org www.nosoftwarepatents.org---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org-- Jonah H. Harris, Database Internals Architect EnterpriseDB Corporation732.331.1324
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, Feb 16, 2006 at 08:22:55AM -0500, Ron wrote: 3= Especially in modern systems where the gap between internal CPU bandwidth and memory bandwidth is so great, the overhead of memory accesses for comparisons and moves is the majority of the overhead for both the pivot choosing and the partitioning algorithms within quicksort. Particularly random memory accesses. The reason (#GPRs - 1) is a magic constant is that it's the most you can compare and move using only register-to-register operations. But how much of this applies to us? We're not sorting arrays of integers, we're sorting pointers to tuples. So while moves cost very little, a comparison costs hundreds, maybe thousands of cycles. A tuple can easily be two or three cachelines and you're probably going to access all of it, not to mention the Fmgr structures and the Datums themselves. None of this is cache friendly. The actual tuples themselves could be spread all over memory (I don't think any particular effort is expended trying to minimize fragmentation). Do these algorithms discuss the case where a comparison is more than 1000 times the cost of a move? Where this does become interesting is where we can convert a datum to an integer such that if f(A) f(B) then A B. Then we can sort on f(X) first with just integer comparisons and then do a full tuple comparison only if f(A) = f(B). This would be much more cache-coherent and make these algorithms much more applicable in my mind. Have a nice day, -- Martijn van Oosterhout kleptog@svana.org http://svana.org/kleptog/ Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a tool for doing 5% of the work and then sitting around waiting for someone else to do the other 95% so you can sue them. signature.asc Description: Digital signature
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Markus Schaber wrote: Ron wrote: ...and of course if you know enough about the data to be sorted so as to constrain it appropriately, one should use a non comparison based O(N) sorting algorithm rather than any of the general comparison based O(NlgN) methods. Sounds interesting, could you give us some pointers (names, URLs, papers) to such algorithms? Most of these techniques boil down to good ol' bucket sort. A simple example: suppose you have a column of integer percentages, range zero to 100. You know there are only 101 distinct values. So create 101 buckets (e.g. linked lists), make a single pass through your data and drop each row's ID into the right bucket, then make a second pass through the buckets, and write the row ID's out in bucket order. This is an O(N) sort technique. Any time you have a restricted data range, you can do this. Say you have 100 million rows of scientific results known to be good to only three digits -- it can have at most 1,000 distinct values (regardless of the magnitude of the values), so you can do this with 1,000 buckets and just two passes through the data. You can also use this trick when the optimizer is asked for fastest first result. Say you have a cursor on a column of numbers with good distribution. If you do a bucket sort on the first two or three digits only, you know the first page of results will be in the first bucket. So you only need to apply qsort to that first bucket (which is very fast, since it's small), and you can deliver the first page of data to the application. This can be particularly effective in interactive situations, where the user typically looks at a few pages of data and then abandons the search. I doubt this is very relevant to Postgres. A relational database has to be general purpose, and it's hard to give it hints that would tell it when to use this particular optimization. Craig ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Craig A. James [EMAIL PROTECTED] writes: You can also use this trick when the optimizer is asked for fastest first result. Say you have a cursor on a column of numbers with good distribution. If you do a bucket sort on the first two or three digits only, you know the first page of results will be in the first bucket. So you only need to apply qsort to that first bucket (which is very fast, since it's small), and you can deliver the first page of data to the application. This can be particularly effective in interactive situations, where the user typically looks at a few pages of data and then abandons the search. I doubt this is very relevant to Postgres. A relational database has to be general purpose, and it's hard to give it hints that would tell it when to use this particular optimization. Actually, LIMIT does nicely for that hint; the PG planner has definitely got a concept of preferring fast-start plans for limited queries. The real problem in applying bucket-sort ideas is the lack of any datatype-independent way of setting up the buckets. Once or twice we've kicked around the idea of having some datatype-specific sorting code paths alongside the general-purpose one, but I can't honestly see this as being workable from a code maintenance standpoint. regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
-Original Message- From: [EMAIL PROTECTED] [mailto:pgsql-hackers- [EMAIL PROTECTED] On Behalf Of Markus Schaber Sent: Thursday, February 16, 2006 5:45 AM To: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index Hi, Ron, Ron wrote: ...and of course if you know enough about the data to be sorted so as to constrain it appropriately, one should use a non comparison based O(N) sorting algorithm rather than any of the general comparison based O(NlgN) methods. Sounds interesting, could you give us some pointers (names, URLs, papers) to such algorithms? He refers to counting sort and radix sort (which comes in most significant digit and least significant digit format). These are also called distribution (as opposed to comparison) sorts. These sorts are O(n) as a function of the data size, but really they are O(M*n) where M is the average key length and n is the data set size. (In the case of MSD radix sort M is the average length to completely differentiate strings) So if you have an 80 character key, then 80*log(n) will only be faster than n*log(n) when you have 2^80th elements -- in other words -- never. If you have short keys, on the other hand, distribution sorts will be dramatically faster. On an unsigned integer, for instance, it requires 4 passes with 8 bit buckets and so 16 elements is the crossover to radix is faster than an O(n*log(n)) sort. Of course, there is a fixed constant of proportionality and so it will really be higher than that, but for large data sets distribution sorting is the best thing that there is for small keys. You could easily have an introspective MSD radix sort. The nice thing about MSD radix sort is that you can retain the ordering that has occurred so far and switch to another algorithm. An introspective MSD radix sort could call an introspective quick sort algorithm once it processed a crossover point of buckets of key data. In order to have distribution sorts that will work with a database system, for each and every data type you will need a function to return the bucket of bits of significance for the kth bucket of bits. For a character string, you could return key[bucket]. For an unsigned integer it is the byte of the integer to return will be a function of the endianness of the CPU. And for each other distinct data type a bucket function would be needed or a sort could not be generated for that type using the distribution method. ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, 2006-02-16 at 12:35 +0100, Steinar H. Gunderson wrote: glibc-2.3.5/stdlib/qsort.c: /* Order size using quicksort. This implementation incorporates four optimizations discussed in Sedgewick: I can't see any references to merge sort in there at all. stdlib/qsort.c defines _quicksort(), not qsort(), which is defined by msort.c. On looking closer, it seems glibc actually tries to determine the physical memory in the machine -- if it is sorting a single array that exceeds 1/4 of the machine's physical memory, it uses quick sort, otherwise it uses merge sort. -Neil ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, 2006-02-16 at 12:15 -0500, Tom Lane wrote: Once or twice we've kicked around the idea of having some datatype-specific sorting code paths alongside the general-purpose one, but I can't honestly see this as being workable from a code maintenance standpoint. regards, tom lane It seems that instead of maintaining a different sorting code path for each data type, you could get away with one generic path and one (hopefully faster) path if you allowed data types to optionally support a 'sortKey' interface by providing a function f which maps inputs to 32- bit int outputs, such that the following two properties hold: f(a)=f(b) iff a=b if a==b then f(a)==f(b) So if a data type supports the sortKey interface you could perform the sort on f(value) and only refer back to the actual element comparison functions when two sortKeys have the same value. Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). Depending on the overhead, you might not even need to maintain 2 independent search code paths, since you could always use f(x)=0 as the default sortKey function which would degenerate to the exact same sort behavior in use today. -- Mark Lewis ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Hi, Mark, Mark Lewis schrieb: It seems that instead of maintaining a different sorting code path for each data type, you could get away with one generic path and one (hopefully faster) path if you allowed data types to optionally support a 'sortKey' interface by providing a function f which maps inputs to 32- bit int outputs, such that the following two properties hold: f(a)=f(b) iff a=b if a==b then f(a)==f(b) Hmm, to remove redundancy, I'd change the = to a and define: if a==b then f(a)==f(b) if ab then f(a)=f(b) Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). With int2 or some restricted ranges of oid and int4, we could even implement a bucket sort. Markus ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, Feb 16, 2006 at 02:17:36PM -0800, Mark Lewis wrote: It seems that instead of maintaining a different sorting code path for each data type, you could get away with one generic path and one (hopefully faster) path if you allowed data types to optionally support a 'sortKey' interface by providing a function f which maps inputs to 32- bit int outputs, such that the following two properties hold: f(a)=f(b) iff a=b if a==b then f(a)==f(b) Note this is a property of the collation, not the type. For example strings can be sorted in many ways and the sortKey must reflect that. So in postgres terms it's a property of the btree operator class. It's something I'd like to do if I get A Round Tuit. :) Have a nice day, -- Martijn van Oosterhout kleptog@svana.org http://svana.org/kleptog/ Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a tool for doing 5% of the work and then sitting around waiting for someone else to do the other 95% so you can sue them. signature.asc Description: Digital signature
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
Markus Schaber [EMAIL PROTECTED] writes: Hmm, to remove redundancy, I'd change the = to a and define: if a==b then f(a)==f(b) if ab then f(a)=f(b) Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). How exactly do you imagine doing this for text? I could see doing it for char(n)/varchar(n) where n=4 in SQL_ASCII though. -- greg ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, 2006-02-16 at 17:51 -0500, Greg Stark wrote: Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). How exactly do you imagine doing this for text? I could see doing it for char(n)/varchar(n) where n=4 in SQL_ASCII though. In SQL_ASCII, just take the first 4 characters (or 8, if using a 64-bit sortKey as elsewhere suggested). The sorting key doesn't need to be a one-to-one mapping. -- Mark Lewis ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Thu, 16 Feb 2006, Mark Lewis wrote: On Thu, 2006-02-16 at 17:51 -0500, Greg Stark wrote: Data types which could probably provide a useful function for f would be int2, int4, oid, and possibly int8 and text (at least for SQL_ASCII). How exactly do you imagine doing this for text? I could see doing it for char(n)/varchar(n) where n=4 in SQL_ASCII though. In SQL_ASCII, just take the first 4 characters (or 8, if using a 64-bit sortKey as elsewhere suggested). The sorting key doesn't need to be a one-to-one mapping. that would violate your second contraint ( f(a)==f(b) iff (a==b) ) if you could drop that constraint (the cost of which would be extra 'real' compares within a bucket) then a helper function per datatype could work as you are talking. David Lang ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
[HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Gary Doades [EMAIL PROTECTED] writes: If I run the script again, it is not always the first case that is slow, it varies from run to run, which is why I repeated it quite a few times for the test. For some reason I hadn't immediately twigged to the fact that your test script is just N repetitions of the exact same structure with random data. So it's not so surprising that you get random variations in behavior with different test data sets. I did some experimentation comparing the qsort from Fedora Core 4 (glibc-2.3.5-10.3) with our src/port/qsort.c. For those who weren't following the pgsql-performance thread, the test case is just this repeated a lot of times: create table atest(i int4, r int4); insert into atest (i,r) select generate_series(1,10), 0; insert into atest (i,r) select generate_series(1,10), random()*10; \timing create index idx on atest(r); \timing drop table atest; I did this 100 times and sorted the reported runtimes. (Investigation with trace_sort = on confirms that the runtime is almost entirely spent in qsort() called from our performsort --- the Postgres overhead is about 100msec on this machine.) Results are below. It seems clear that our qsort.c is doing a pretty awful job of picking qsort pivots, while glibc is mostly managing not to make that mistake. I haven't looked at the glibc code yet to see what they are doing differently. I'd say this puts a considerable damper on my enthusiasm for using our qsort all the time, as was recently debated in this thread: http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php We need to fix our qsort.c before pushing ahead with that idea. regards, tom lane 100 runtimes for glibc qsort, sorted ascending: Time: 459.860 ms Time: 460.209 ms Time: 460.704 ms Time: 461.317 ms Time: 461.538 ms Time: 461.652 ms Time: 461.988 ms Time: 462.573 ms Time: 462.638 ms Time: 462.716 ms Time: 462.917 ms Time: 463.219 ms Time: 463.455 ms Time: 463.650 ms Time: 463.723 ms Time: 463.737 ms Time: 463.750 ms Time: 463.852 ms Time: 463.964 ms Time: 463.988 ms Time: 464.003 ms Time: 464.135 ms Time: 464.372 ms Time: 464.458 ms Time: 464.496 ms Time: 464.551 ms Time: 464.599 ms Time: 464.655 ms Time: 464.656 ms Time: 464.722 ms Time: 464.814 ms Time: 464.827 ms Time: 464.878 ms Time: 464.899 ms Time: 464.905 ms Time: 464.987 ms Time: 465.055 ms Time: 465.138 ms Time: 465.159 ms Time: 465.194 ms Time: 465.310 ms Time: 465.316 ms Time: 465.375 ms Time: 465.450 ms Time: 465.535 ms Time: 465.595 ms Time: 465.680 ms Time: 465.769 ms Time: 465.865 ms Time: 465.892 ms Time: 465.903 ms Time: 466.003 ms Time: 466.154 ms Time: 466.164 ms Time: 466.203 ms Time: 466.305 ms Time: 466.344 ms Time: 466.364 ms Time: 466.388 ms Time: 466.502 ms Time: 466.593 ms Time: 466.725 ms Time: 466.794 ms Time: 466.798 ms Time: 466.904 ms Time: 466.971 ms Time: 466.997 ms Time: 467.122 ms Time: 467.146 ms Time: 467.221 ms Time: 467.224 ms Time: 467.244 ms Time: 467.277 ms Time: 467.587 ms Time: 468.142 ms Time: 468.207 ms Time: 468.237 ms Time: 468.471 ms Time: 468.663 ms Time: 468.700 ms Time: 469.235 ms Time: 469.840 ms Time: 470.472 ms Time: 471.140 ms Time: 472.811 ms Time: 472.959 ms Time: 474.858 ms Time: 477.210 ms Time: 479.571 ms Time: 479.671 ms Time: 482.797 ms Time: 488.852 ms Time: 514.639 ms Time: 529.287 ms Time: 612.185 ms Time: 660.748 ms Time: 742.227 ms Time: 866.814 ms Time: 1234.848 ms Time: 1267.398 ms 100 runtimes for port/qsort.c, sorted ascending: Time: 418.905 ms Time: 420.611 ms Time: 420.764 ms Time: 420.904 ms Time: 421.706 ms Time: 422.466 ms Time: 422.627 ms Time: 423.189 ms Time: 423.302 ms Time: 425.096 ms Time: 425.731 ms Time: 425.851 ms Time: 427.253 ms Time: 430.113 ms Time: 432.756 ms Time: 432.963 ms Time: 440.502 ms Time: 440.640 ms Time: 450.452 ms Time: 458.143 ms Time: 459.212 ms Time: 467.706 ms Time: 468.006 ms Time: 468.574 ms Time: 470.003 ms Time: 472.313 ms Time: 483.622 ms Time: 492.395 ms Time: 509.564 ms Time: 531.037 ms Time: 533.366 ms Time: 535.610 ms Time: 575.523 ms Time: 582.688 ms Time: 593.545 ms Time: 647.364 ms Time: 660.612 ms Time: 677.312 ms Time: 680.288 ms Time: 697.626 ms Time: 833.066 ms Time: 834.511 ms Time: 851.819 ms Time: 920.443 ms Time: 926.731 ms Time: 954.289 ms Time: 1045.214 ms Time: 1059.200 ms Time: 1062.328 ms Time: 1136.018 ms Time: 1260.091 ms Time: 1276.883 ms Time: 1319.351 ms Time: 1438.854 ms Time: 1475.457 ms Time: 1538.211 ms Time: 1549.004 ms Time: 1744.642 ms Time: 1771.258 ms Time: 1959.530 ms Time: 2300.140 ms Time: 2589.641 ms Time: 2612.780 ms Time: 3100.024 ms Time: 3284.125 ms Time: 3379.792 ms Time: 3750.278 ms Time: 4302.278 ms Time: 4780.624 ms Time: 5000.056 ms Time: 5092.604 ms Time: 5168.722 ms Time: 5292.941 ms Time: 5895.964 ms Time: 7003.164 ms Time: 7099.449 ms Time: 7115.083 ms Time: 7384.940 ms Time: 8214.010 ms Time: 8700.771 ms Time: 9331.225 ms Time: 10503.360 ms Time: 12496.026 ms Time: 12982.474 ms Time: 15192.390 ms
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Tom Lane wrote: For some reason I hadn't immediately twigged to the fact that your test script is just N repetitions of the exact same structure with random data. So it's not so surprising that you get random variations in behavior with different test data sets. It seems clear that our qsort.c is doing a pretty awful job of picking qsort pivots, while glibc is mostly managing not to make that mistake. I haven't looked at the glibc code yet to see what they are doing differently. I'd say this puts a considerable damper on my enthusiasm for using our qsort all the time, as was recently debated in this thread: http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php We need to fix our qsort.c before pushing ahead with that idea. [snip] Time: 28314.182 ms Time: 29400.278 ms Time: 34142.534 ms Ouch! That confirms my problem. I generated the random test case because it was easier than including the dump of my tables, but you can appreciate that tables 20 times the size are basically crippled when it comes to creating an index on them. Examining the dump and the associated times during restore it looks like I have 7 tables with this approximate distribution, thus the ridiculously long restore time. Better not re-index soon! Is this likely to hit me in a random fashion during normal operation, joins, sorts, order by for example? So the options are: 1) Fix the included qsort.c code and use that 2) Get FreeBSD to fix their qsort code 3) Both I guess that 1 is the real solution in case anyone else's qsort is broken in the same way. Then at least you *could* use it all the time :) Regards, Gary. ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Gary Doades [EMAIL PROTECTED] writes: Is this likely to hit me in a random fashion during normal operation, joins, sorts, order by for example? Yup, anytime you're passing data with that kind of distribution through a sort. So the options are: 1) Fix the included qsort.c code and use that 2) Get FreeBSD to fix their qsort code 3) Both I guess that 1 is the real solution in case anyone else's qsort is broken in the same way. Then at least you *could* use it all the time :) It's reasonable to assume that most of the *BSDen have basically the same qsort code. Ours claims to have come from NetBSD sources, but I don't doubt that they all trace back to a common ancestor. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Gary Doades [EMAIL PROTECTED] writes: Ouch! That confirms my problem. I generated the random test case because it was easier than including the dump of my tables, but you can appreciate that tables 20 times the size are basically crippled when it comes to creating an index on them. Actually... we only use qsort when we have a sorting problem that fits within the allowed sort memory. The external-sort logic doesn't go through that code at all. So all the analysis we just did on your test case doesn't necessarily apply to sort problems that are too large for the sort_mem setting. The test case would be sorting 20 index entries, which'd probably occupy at least 24 bytes apiece of sort memory, so probably about 5 meg. A problem 20 times that size would definitely not fit in the default 16MB maintenance_work_mem. Were you using a large value of maintenance_work_mem for your restore? regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
This behavior is consistent with the pivot choosing algorithm assuming certain distribution(s) for the data. For instance, median-of-three partitioning is known to be pessimal when the data is geometrically or hyper-geometrically distributed. Also, care must be taken that sometimes is not when there are many equal values in the data. Even pseudo random number generator based pivot choosing algorithms are not immune if the PRNG is flawed in some way. How are we choosing our pivots? At 06:28 PM 2/15/2006, Tom Lane wrote: I did some experimentation comparing the qsort from Fedora Core 4 (glibc-2.3.5-10.3) with our src/port/qsort.c. For those who weren't following the pgsql-performance thread, the test case is just this repeated a lot of times: create table atest(i int4, r int4); insert into atest (i,r) select generate_series(1,10), 0; insert into atest (i,r) select generate_series(1,10), random()*10; \timing create index idx on atest(r); \timing drop table atest; I did this 100 times and sorted the reported runtimes. (Investigation with trace_sort = on confirms that the runtime is almost entirely spent in qsort() called from our performsort --- the Postgres overhead is about 100msec on this machine.) Results are below. It seems clear that our qsort.c is doing a pretty awful job of picking qsort pivots, while glibc is mostly managing not to make that mistake. I haven't looked at the glibc code yet to see what they are doing differently. I'd say this puts a considerable damper on my enthusiasm for using our qsort all the time, as was recently debated in this thread: http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php We need to fix our qsort.c before pushing ahead with that idea. regards, tom lane 100 runtimes for glibc qsort, sorted ascending: Time: 459.860 ms snip Time: 488.852 ms Time: 514.639 ms Time: 529.287 ms Time: 612.185 ms Time: 660.748 ms Time: 742.227 ms Time: 866.814 ms Time: 1234.848 ms Time: 1267.398 ms 100 runtimes for port/qsort.c, sorted ascending: Time: 418.905 ms snip Time: 20865.979 ms Time: 21000.907 ms Time: 21297.585 ms Time: 21714.518 ms Time: 25423.235 ms Time: 27543.052 ms Time: 28314.182 ms Time: 29400.278 ms Time: 34142.534 ms ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
I wrote: Gary Doades [EMAIL PROTECTED] writes: Ouch! That confirms my problem. I generated the random test case because it was easier than including the dump of my tables, but you can appreciate that tables 20 times the size are basically crippled when it comes to creating an index on them. Actually... we only use qsort when we have a sorting problem that fits within the allowed sort memory. The external-sort logic doesn't go through that code at all. So all the analysis we just did on your test case doesn't necessarily apply to sort problems that are too large for the sort_mem setting. I increased the size of the test case by 10x (basically s/10/100/) which is enough to push it into the external-sort regime. I get amazingly stable runtimes now --- I didn't have the patience to run 100 trials, but in 30 trials I have slowest 11538 msec and fastest 11144 msec. So this code path is definitely not very sensitive to this data distribution. While these numbers aren't glittering in comparison to the best-case qsort times (~450 msec to sort 10% as much data), they are sure a lot better than the worst-case times. So maybe a workaround for you is to decrease maintenance_work_mem, counterintuitive though that be. (Now, if you *weren't* using maintenance_work_mem of 100MB or more for your problem restore, then I'm not sure I know what's going on...) We still ought to try to fix qsort of course. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Ron [EMAIL PROTECTED] writes: How are we choosing our pivots? See qsort.c: it looks like median of nine equally spaced inputs (ie, the 1/8th points of the initial input array, plus the end points), implemented as two rounds of median-of-three choices. With half of the data inputs zero, it's not too improbable for two out of the three samples to be zeroes in which case I think the med3 result will be zero --- so choosing a pivot of zero is much more probable than one would like, and doing so in many levels of recursion causes the problem. I think. I'm not too sure if the code isn't just being sloppy about the case where many data values are equal to the pivot --- there's a special case there to switch to insertion sort, and maybe that's getting invoked too soon. It'd be useful to get a line-level profile of the behavior of this code in the slow cases... regards, tom lane ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
-Original Message- From: [EMAIL PROTECTED] [mailto:pgsql-hackers- [EMAIL PROTECTED] On Behalf Of Tom Lane Sent: Wednesday, February 15, 2006 5:22 PM To: Ron Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) Ron [EMAIL PROTECTED] writes: How are we choosing our pivots? See qsort.c: it looks like median of nine equally spaced inputs (ie, the 1/8th points of the initial input array, plus the end points), implemented as two rounds of median-of-three choices. With half of the data inputs zero, it's not too improbable for two out of the three samples to be zeroes in which case I think the med3 result will be zero --- so choosing a pivot of zero is much more probable than one would like, and doing so in many levels of recursion causes the problem. Adding some randomness to the selection of the pivot is a known technique to fix the oddball partitions problem. However, Bentley and Sedgewick proved that every quick sort algorithm has some input set that makes it go quadratic (hence the recent popularity of introspective sort, which switches to heapsort if quadratic behavior is detected. The C++ template I submitted was an example of introspective sort, but PostgreSQL does not use C++ so it was not helpful). I think. I'm not too sure if the code isn't just being sloppy about the case where many data values are equal to the pivot --- there's a special case there to switch to insertion sort, and maybe that's getting invoked too soon. Here are some cases known to make qsort go quadratic: 1. Data already sorted 2. Data reverse sorted 3. Data organ-pipe sorted or ramp 4. Almost all data of the same value There are probably other cases. Randomizing the pivot helps some, as does check for in-order or reverse order partitions. Imagine if 1/3 of the partitions fall into a category that causes quadratic behavior (have one of the above formats and have more than CUTOFF elements in them). It is doubtful that the switch to insertion sort is causing any sort of problems. It is only going to be invoked on tiny sets, for which it has a fixed cost that is probably less that qsort() function calls on sets of the same size. It'd be useful to get a line-level profile of the behavior of this code in the slow cases... I guess that my in-order or presorted tests [which often arise when there are very few distinct values] may solve the bad partition problems. Don't forget that the algorithm is called recursively. regards, tom lane ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Ouch! That confirms my problem. I generated the random test case because it was easier than including the dump of my tables, but you can appreciate that tables 20 times the size are basically crippled when it comes to creating an index on them. I have to say that I restored a few gigabyte dump on freebsd the other day, and most of the restore time was in index creation - I didn't think too much of it though at the time. FreeBSD 4.x. Chris ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Wed, 2006-02-15 at 19:59 -0500, Tom Lane wrote: I get amazingly stable runtimes now --- I didn't have the patience to run 100 trials, but in 30 trials I have slowest 11538 msec and fastest 11144 msec. So this code path is definitely not very sensitive to this data distribution. The worst-case behavior of replacement-selection is very close to its average behavior, while the worst-case behavior of QuickSort is terrible (N2) – a strong argument in favor of replacement-selection. Despite this risk, QuickSort is widely used because, in practice, it has superior performance. p.8, AlphaSort: A Cache-Sensitive Parallel External Sort, Nyberg et al, VLDB Journal 4(4): 603-627 (1995) I think your other comment about flipping to insertion sort too early (and not returning...) is a plausible cause for the poor pg qsort behaviour, but the overall spread of values seems as expected. Some test results I've seen seem consistent with the view that increasing memory also increases run-time for larger settings of work_mem/maintenance_work_mem. Certainly, as I observed a while back, having a large memory settings doesn't help you at all when you are doing final run merging on the external sort. Whatever we do, we should look at the value high memory settings bring to each phase of a sort separately from the other phases. There is work underway on improving external sorts, so I hear (not me). Plus my WIP on randomAccess requirements. Best Regards, Simon Riggs ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
On Wed, 2006-02-15 at 18:28 -0500, Tom Lane wrote: It seems clear that our qsort.c is doing a pretty awful job of picking qsort pivots, while glibc is mostly managing not to make that mistake. I haven't looked at the glibc code yet to see what they are doing differently. glibc qsort is actually merge sort, so I'm not surprised it avoids this problem. -Neil ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Tom Lane [EMAIL PROTECTED] wrote
I did this 100 times and sorted the reported runtimes.
I'd say this puts a considerable damper on my enthusiasm for using our
qsort all the time, as was recently debated in this thread:
http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php
100 runtimes for glibc qsort, sorted ascending:
Time: 866.814 ms
Time: 1234.848 ms
Time: 1267.398 ms
100 runtimes for port/qsort.c, sorted ascending:
Time: 28314.182 ms
Time: 29400.278 ms
Time: 34142.534 ms
By did this 100 times do you mean generate a sequence of at most
20*100 numbers, and for every 20 numbers, the first half are all
zeros and the other half are uniform random numbers? I tried to confirm it
by patching the program mentioned in the link, but seems BSDqsort is still a
little bit leading.
Regards,
Qingqing
---
Result
sort#./sort
[3] [glibc qsort]: nelem(2000), range(4294901760) distr(halfhalf)
ccost(2) : 18887.285000 ms
[3] [BSD qsort]: nelem(2000), range(4294901760) distr(halfhalf) ccost(2)
: 18801.018000 ms
[3] [qsortG]: nelem(2000), range(4294901760) distr(halfhalf) ccost(2) :
22997.004000 ms
---
Patch to sort.c
sort#diff -c sort.c sort1.c
*** sort.c Thu Dec 15 12:18:59 2005
--- sort1.c Wed Feb 15 22:21:15 2006
***
*** 35,43
{BSD qsort, qsortB},
{qsortG, qsortG}
};
! static const size_t d_nelem[] = {1000, 1, 10, 100, 500};
! static const size_t d_range[] = {2, 32, 1024, 0xL};
! static const char *d_distr[] = {uniform, gaussian, 95sorted,
95reversed};
static const size_t d_ccost[] = {2};
/* factor index */
--- 35,43
{BSD qsort, qsortB},
{qsortG, qsortG}
};
! static const size_t d_nelem[] = {500, 1000, 2000};
! static const size_t d_range[] = {0xL};
! static const char *d_distr[] = {halfhalf};
static const size_t d_ccost[] = {2};
/* factor index */
***
*** 180,185
--- 180,192
swap(karray[i], karray[nelem-i-1]);
}
}
+ else if (!strcmp(distr, halfhalf))
+ {
+ int j;
+ for (i = 0; i nelem/20; i++)
+ for (j = 0; j 10; j++)
+ karray[i*20 + j] = 0;
+ }
return array;
}
---(end of broadcast)---
TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Qingqing Zhou [EMAIL PROTECTED] writes: By did this 100 times do you mean generate a sequence of at most 20*100 numbers, and for every 20 numbers, the first half are all zeros and the other half are uniform random numbers? No, I mean I ran the bit of SQL script I gave 100 separate times. regards, tom lane ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Tom Lane [EMAIL PROTECTED] wrote Qingqing Zhou [EMAIL PROTECTED] writes: By did this 100 times do you mean generate a sequence of at most 20*100 numbers, and for every 20 numbers, the first half are all zeros and the other half are uniform random numbers? No, I mean I ran the bit of SQL script I gave 100 separate times. I must misunderstand something here -- I can't figure out that why the cost of the same procedure keep climbing? Regards, Qingqing ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Qingqing Zhou [EMAIL PROTECTED] wrote I must misunderstand something here -- I can't figure out that why the cost of the same procedure keep climbing? Ooops, I mis-intepret the sentence -- you sorted the results ... Regards, Qingqing ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Qingqing Zhou [EMAIL PROTECTED] writes: Tom Lane [EMAIL PROTECTED] wrote No, I mean I ran the bit of SQL script I gave 100 separate times. I must misunderstand something here -- I can't figure out that why the cost of the same procedure keep climbing? No, the run cost varies randomly depending on the random data supplied by the test script. The reason the numbers are increasing is that I sorted them for ease of inspection. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index
At 08:21 PM 2/15/2006, Tom Lane wrote:
Ron [EMAIL PROTECTED] writes:
How are we choosing our pivots?
See qsort.c: it looks like median of nine equally spaced inputs (ie,
the 1/8th points of the initial input array, plus the end points),
implemented as two rounds of median-of-three choices.
OK, this is a bad way to do median-of-n partitioning for a few
reasons. See Sedgewick's PhD thesis for details.
Basically, if one is using median-of-n partitioning to choose a
pivot, one should do it in =one= pass, and n for that pass should be
= the numbers of registers in the CPU. Since the x86 ISA has 8
GPR's, n should be = 8. 7 for instance.
Special purposing the median-of-n code so that the minimal number of
comparisons and moves is used to sort the sample and then
partitioning in place is the best way to do it. In addition, care
must be taken to deal with the possibility that many of the keys may be equal.
The (pseudo) code looks something like this:
qs(a[],L,R){
if((R-L) SAMPLE_SIZE){ // Not worth using qs for too few elements
SortSample(SAMPLE_SIZE,a[],L,R);
// Sorts SAMPLE_SIZE= n elements and does median-of-n
partitioning for small n
// using the minimal number of comparisons and moves.
// In the process it ends up partitioning the first n/2 and last
n/2 elements
// SAMPLE_SIZE is a constant chosen to work best for a given CPU.
// #GPRs - 1 is a good initial guess.
// For the x86 ISA, #GPRs - 1 = 7. For native x86-64, it's 15.
// For most RISC CPUs it's 31 or 63. For Itanium, it's 127 (!)
pivot= a[(L+R)1]; i= L+(SAMPLE_SIZE1); j= R-(SAMPLE_SIZE1);
for(;;){
while(a[++i] pivot);
while(a[--j] pivot);
if(i = j) break;
if(a[i] a[j]) swap(a[i],a[j]);
}
if((i-R) = (j-L)){qs(a,L,i-1);}
else{qs(a,i,R);}
else{OofN^2_Sort(a,L,R);}
// SelectSort may be better than InsertSort if KeySize in bits
RecordSize in bits
} // End of qs
Given that the most common CPU ISA in existence has 8 GPRs,
SAMPLE_SIZE= 7 is probably optimal:
t= (L+R);
the set would be {L; t/8; t/4; t/2; 3*t/4; 7*t/8; R;}
== {L; t3; t2; t1; (3*t)2; (7*t)3; R} as the locations.
Even better (and more easily scaled as the number of GPR's in the CPU
changes) is to use
the set {L; L+1; L+2; t1; R-2; R-1; R}
This means that instead of 7 random memory accesses, we have 3; two
of which result in a
burst access for three elements each.
That's much faster; _and_ using a sample of 9, 15, 31, 63, etc (to
max of ~GPRs -1) elements is more easily done.
It also means that the work we do sorting the sample can be taken
advantage of when starting
inner loop of quicksort: items L..L+2, t, and R-2..R are already
partitioned by SortSample().
Insuring that the minimum number of comparisons and moves is done in
SortSample can be down by using a code generator to create a
comparison tree that identifies which permutation(s) of n we are
dealing with and then moving them into place with the minimal number of moves.
SIDE NOTE: IIRC glibc's qsort is actually merge sort. Merge sort
performance is insensitive to all inputs, and there are way to
optimize it as well.
I'll leave the actual coding to someone who knows the pg source
better than I do.
Ron
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