Re: [PERFORM] Releasing memory during External sorting?
On Fri, 2005-09-23 at 12:48 -0400, Ron Peacetree wrote: > > I have some indications from private tests that very high memory settings > >may actually hinder performance of the sorts, though I cannot explain that > >and wonder whether it is the performance tests themselves that have issues. > > > Hmmm. Are you talking about amounts so high that you are throwing the OS > into paging and swapping thrash behavior? If not, then the above is weird. Thanks for your thoughts. I'll retest, on the assumption that there is a benefit, but there's something wrong with my earlier tests. Best Regards, Simon Riggs ---(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: [PERFORM] Releasing memory during External sorting?
From: Tom Lane <[EMAIL PROTECTED]> Sent: Sep 23, 2005 2:15 PM Subject: Re: [PERFORM] Releasing memory during External sorting? >Mark Lewis <[EMAIL PROTECTED]> writes: >> operations != passes. If you were clever, you could probably write a >> modified bubble-sort algorithm that only made 2 passes. A pass is a >> disk scan, operations are then performed (hopefully in memory) on what >> you read from the disk. So there's no theoretical log N lower-bound on >> the number of disk passes. >Given infinite memory that might be true, but I don't think I believe it >for limited memory. If you have room for K tuples in memory then it's >impossible to perform more than K*N useful comparisons per pass (ie, as >each tuple comes off the disk you can compare it to all the ones >currently in memory; anything more is certainly redundant work). So if >K < logN it's clearly not gonna work. > Actually, it's far better than that. I recall a paper I saw in one of the algorithms journals 15+ years ago that proved that if you knew the range of the data, regardless of what that range was, and had n^2 space, you could sort n items in O(n) time. Turns out that with very modest constraints on the range of the data and substantially less extra space (about the same as you'd need for Replacement Selection + External Merge Sort), you can _still_ sort in O(n) time. >It's possible that you could design an algorithm that works in a fixed >number of passes if you are allowed to assume you can hold O(log N) >tuples in memory --- and in practice that would probably work fine, >if the constant factor implied by the O() isn't too big. But it's not >really solving the general external-sort problem. > If you know nothing about the data to be sorted and must guard against the worst possible edge cases, AKA the classic definition of "the general external sorting problem", then one can't do better than some variant of Replacement Selection + Unbalanced Multiway Merge. OTOH, ITRW things are _not_ like that. We know the range of the data in our DB fields or we can safely assume it to be relatively constrained. This allows us access to much better external sorting algorithms. For example Postman Sort (the 2005 winner of the PennySort benchmark) is basically an IO optimized version of an external Radix Sort. Ron ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Releasing memory during External sorting?
Yep. Also, bear in mind that the lg(n!)= ~ nlgn - n lower bound on the number of comparisions: a= says nothing about the amount of data movement used. b= only holds for generic comparison based sorting algorithms. As Knuth says (vol 3, p180), Distribution Counting sorts without ever comparing elements to each other at all, and so does Radix Sort. Similar comments can be found in many algorithms texts. Any time we know that the range of the data to be sorted is substantially restricted compared to the number of items to be sorted, we can sort in less than O(lg(n!)) time. DB fields tend to take on few values and are therefore "substantially restricted". Given the proper resources and algorithms, O(n) sorts are very plausible when sorting DB records. All of the fastest external sorts of the last decade or so take advantage of this. Check out that URL I posted. Ron -Original Message- From: Mark Lewis <[EMAIL PROTECTED]> Sent: Sep 23, 2005 1:43 PM To: Tom Lane <[EMAIL PROTECTED]> Subject: Re: [PERFORM] Releasing memory during External sorting? operations != passes. If you were clever, you could probably write a modified bubble-sort algorithm that only made 2 passes. A pass is a disk scan, operations are then performed (hopefully in memory) on what you read from the disk. So there's no theoretical log N lower-bound on the number of disk passes. Not that I have anything else useful to add to this discussion, just a tidbit I remembered from my CS classes back in college :) -- Mark On Fri, 2005-09-23 at 13:17 -0400, Tom Lane wrote: > Ron Peacetree <[EMAIL PROTECTED]> writes: > > 2= No optimal external sorting algorithm should use more than 2 passes. > > 3= Optimal external sorting algorithms should use 1 pass if at all possible. > > A comparison-based sort must use at least N log N operations, so it > would appear to me that if you haven't got approximately log N passes > then your algorithm doesn't work. > > regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PERFORM] Releasing memory during External sorting?
Mark Lewis <[EMAIL PROTECTED]> writes: > operations != passes. If you were clever, you could probably write a > modified bubble-sort algorithm that only made 2 passes. A pass is a > disk scan, operations are then performed (hopefully in memory) on what > you read from the disk. So there's no theoretical log N lower-bound on > the number of disk passes. Given infinite memory that might be true, but I don't think I believe it for limited memory. If you have room for K tuples in memory then it's impossible to perform more than K*N useful comparisons per pass (ie, as each tuple comes off the disk you can compare it to all the ones currently in memory; anything more is certainly redundant work). So if K < logN it's clearly not gonna work. It's possible that you could design an algorithm that works in a fixed number of passes if you are allowed to assume you can hold O(log N) tuples in memory --- and in practice that would probably work fine, if the constant factor implied by the O() isn't too big. But it's not really solving the general external-sort problem. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Releasing memory during External sorting?
operations != passes. If you were clever, you could probably write a modified bubble-sort algorithm that only made 2 passes. A pass is a disk scan, operations are then performed (hopefully in memory) on what you read from the disk. So there's no theoretical log N lower-bound on the number of disk passes. Not that I have anything else useful to add to this discussion, just a tidbit I remembered from my CS classes back in college :) -- Mark On Fri, 2005-09-23 at 13:17 -0400, Tom Lane wrote: > Ron Peacetree <[EMAIL PROTECTED]> writes: > > 2= No optimal external sorting algorithm should use more than 2 passes. > > 3= Optimal external sorting algorithms should use 1 pass if at all possible. > > A comparison-based sort must use at least N log N operations, so it > would appear to me that if you haven't got approximately log N passes > then your algorithm doesn't work. > > 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 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Releasing memory during External sorting?
Ron Peacetree <[EMAIL PROTECTED]> writes: > 2= No optimal external sorting algorithm should use more than 2 passes. > 3= Optimal external sorting algorithms should use 1 pass if at all possible. A comparison-based sort must use at least N log N operations, so it would appear to me that if you haven't got approximately log N passes then your algorithm doesn't work. 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: [PERFORM] Releasing memory during External sorting?
From: Simon Riggs <[EMAIL PROTECTED]> Sent: Sep 23, 2005 5:37 AM Subject: [PERFORM] Releasing memory during External sorting? >I have concerns about whether we are overallocating memory for use in >external sorts. (All code relating to this is in tuplesort.c) > A decent external sorting algorithm, say a Merge Sort + Radix (or Distribution Counting) hybrid with appropriate optimizations for small sub- files, should become more effective / efficient the more RAM you give it. >The external sort algorithm benefits from some memory but not much. > That's probably an artifact of the psql external sorting code and _not_ due to some fundamental external sorting issue. >Knuth says that the amount of memory required is very low, with a value >typically less than 1 kB. > "Required" means the external sort can operate on that little memory. How Much memory is required for optimal performance is another matter. >I/O overheads mean that there is benefit from having longer sequential >writes, so the optimum is much larger than that. I've not seen any data >that indicates that a setting higher than 16 MB adds any value at all to a >large external sort. > It should. A first pass upper bound would be the amount of RAM needed for Replacement Selection to create a run (ie sort) of the whole file. That should be ~ the amount of RAM to hold 1/2 the file in a Replacement Selection pass. At the simplest, for any file over 32MB the optimum should be more than 16MB. > I have some indications from private tests that very high memory settings >may actually hinder performance of the sorts, though I cannot explain that >and wonder whether it is the performance tests themselves that have issues. > Hmmm. Are you talking about amounts so high that you are throwing the OS into paging and swapping thrash behavior? If not, then the above is weird. >Does anyone have any clear data that shows the value of large settings >of work_mem when the data to be sorted is much larger than memory? (I am >well aware of the value of setting work_mem higher for smaller sorts, so >any performance data needs to reflect only very large sorts). > This is not PostgreSQL specific, but it does prove the point that the performance of external sorts benefits greatly from large amounts of RAM being available: http://research.microsoft.com/barc/SortBenchmark/ Looking at the particulars of the algorithms listed there should shed a lot of light on what a "good" external sorting algorithm looks like: 1= HD IO matters the most. 1a= Seeking behavior is the largest factor in poor performance. 2= No optimal external sorting algorithm should use more than 2 passes. 3= Optimal external sorting algorithms should use 1 pass if at all possible. 4= Use as much RAM as possible, and use it as efficiently as possible. 5= The amount of RAM needed to hide the latency of a HD subsytem goes up as the _square_ of the difference between the bandwidth of the HD subsystem and memory. 6= Be cache friendly. 7= For large numbers of records whose sorting key is substantially smaller than the record itself, use a pointer + compressed key representation and write the data to HD in sorted order (Replace HD seeks with RAM seeks. Minimize RAM seeks). 8= Since your performance will be constrained by HD IO first and RAM IO second, up to a point it is worth it to spend more CPU cycles to save on IO. Given the large and growing gap between CPU IO, RAM IO, and HD IO, these issues are becoming more important for _internal_ sorts as well. >Feedback, please. > >Best Regards, Simon Riggs > Hope this is useful, Ron ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
