Re: [HACKERS] gistchoose vs. bloat

[Heikki Linnakangas]

On 21.01.2013 15:19, Heikki Linnakangas wrote:

On 21.01.2013 15:06, Tom Lane wrote:

Jeff Davispg...@j-davis.com writes:

On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:

I looked at this patch. ISTM we should not have the option at all but
just do it always. I cannot believe that always-go-left is ever a
preferable strategy in the long run; the resulting imbalance in the
index will surely kill any possible benefit. Even if there are some
cases where it miraculously fails to lose, how many users are going to
realize that applies to their case and make use of the option?



Sounds good to me.



If I remember correctly, there was also an argument that it may be
useful for repeatable test results. That's a little questionable for
performance (except in those cases where few penalties are identical
anyway), but could plausibly be useful for a crash report or something.


Meh. There's already a random decision, in the equivalent place and for
a comparable reason, in btree (cf _bt_findinsertloc). Nobody's ever
complained about that being indeterminate, so I'm unconvinced that
there's a market for it with gist.


I wonder if it would work for gist to do something similar to
_bt_findinsertloc, and have a bias towards the left page, but sometimes
descend to one of the pages to the right. You would get the cache
locality of usually descending down the same subtree, but also fill the
pages to the right. Jeff / Alexander, want to give that a shot?


I did some experimenting with that. I used the same test case Alexander 
did, with geonames data, and compared unpatched version, the original 
patch, and the attached patch that biases the first best tuple found, 
but still sometimes chooses the other equally good ones.


testname| initsize | finalsize | idx_blks_read | idx_blks_hit
+--+---+---+--
 patched-10-4mb | 75497472 |  90202112 |   5853604 | 10178331
 unpatched-4mb  | 75145216 |  94863360 |   5880676 | 10185647
 unpatched-4mb  | 75587584 |  97165312 |   5903107 | 10183759
 patched-2-4mb  | 74768384 |  81403904 |   5768124 | 10193738
 origpatch-4mb  | 74883072 |  82182144 |   5783412 | 10185373

All these tests were performed with shared_buffers=4MB, so that the 
index won't fit completely in shared buffers, and I could use the 
idx_blk_read/hit ratio as a measure of cache-friendliness. The 
origpath test was with a simplified version of Alexander's patch, see 
attached. It's the same as the original, but with the 
randomization-option and gist-build related code removed. The patched-10 
and patched-2 tests are two variants with my patch, with 1/10 and 1/2 
chance of moving to the next equal tuple, respectively. The differences 
in cache hit ratio might be just a result of different index sizes. I 
included two unpatched runs above to show that there's a fair amount of 
noise in these tests. That's because of the random nature of the test 
case; it picks rows to delete and insert at random.


I think the conclusion is that all of these patches are effective. The 
1/10 variant is less effective, as expected, as it's closer in behavior 
to the unpatched behavior than the others. The 1/2 variant seems as good 
as the original patch.



I performed another test, by inserting 100 duplicate values on an 
empty table.


  testname  | finalsize | idx_blks_read | idx_blks_hit
+---+---+--
 unpatched  |  89030656 | 21350 |  2972033
 patched-10 |  88973312 | 21450 |  2971920
 patched-2  |  88481792 | 22947 |  2970260
 origpatch  |  61186048 |761817 |  2221500

The original patch produces a much smaller index in this test, but since 
the inserts are distributed randomly, the pages are accessed randomly 
which is bad for caching.


A table full of duplicates isn't very realistic, but overall, I'm 
leaning towards my version of this patch (gistchoose-2.patch). It has 
less potential for causing a regression in existing applications, but is 
just as effective in the original scenario of repeated delete+insert.


- Heikki


duplicatetest.sh
Description: Bourne shell script


gistbloat.sh
Description: Bourne shell script
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 8b60774..75778f6 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -379,6 +379,7 @@ gistchoose(Relation r, Page p, IndexTuple it,	/* it has compressed entry */
 	GISTENTRY	entry,
 identry[INDEX_MAX_KEYS];
 	bool		isnull[INDEX_MAX_KEYS];
+	int			look_further_on_equal = -1;
 
 	Assert(!GistPageIsLeaf(p));
 
@@ -446,6 +447,8 @@ gistchoose(Relation r, Page p, IndexTuple it,	/* it has compressed entry */
 
 if (j  r-rd_att-natts - 1)
 	best_penalty[j + 1] = -1;
+
+look_further_on_equal = -1;
 			}
 			else if (best_penalty[j] == usize)
 			{
@@ -468,12 +471,46 @@ 

Re: [HACKERS] gistchoose vs. bloat

[Heikki Linnakangas]

On 21.01.2013 15:19, Heikki Linnakangas wrote:

On 21.01.2013 15:06, Tom Lane wrote:

Jeff Davispg...@j-davis.com writes:

On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:

I looked at this patch. ISTM we should not have the option at all but
just do it always. I cannot believe that always-go-left is ever a
preferable strategy in the long run; the resulting imbalance in the
index will surely kill any possible benefit. Even if there are some
cases where it miraculously fails to lose, how many users are going to
realize that applies to their case and make use of the option?



Sounds good to me.



If I remember correctly, there was also an argument that it may be
useful for repeatable test results. That's a little questionable for
performance (except in those cases where few penalties are identical
anyway), but could plausibly be useful for a crash report or something.


Meh. There's already a random decision, in the equivalent place and for
a comparable reason, in btree (cf _bt_findinsertloc). Nobody's ever
complained about that being indeterminate, so I'm unconvinced that
there's a market for it with gist.


I wonder if it would work for gist to do something similar to
_bt_findinsertloc, and have a bias towards the left page, but sometimes
descend to one of the pages to the right. You would get the cache
locality of usually descending down the same subtree, but also fill the
pages to the right. Jeff / Alexander, want to give that a shot?


I did some experimenting with that. I used the same test case Alexander 
did, with geonames data, and compared unpatched version, the original 
patch, and the attached patch that biases the first best tuple found, 
but still sometimes chooses the other equally good ones.


testname| initsize | finalsize | idx_blks_read | idx_blks_hit
+--+---+---+--
 patched-10-4mb | 75497472 |  90202112 |   5853604 | 10178331
 unpatched-4mb  | 75145216 |  94863360 |   5880676 | 10185647
 unpatched-4mb  | 75587584 |  97165312 |   5903107 | 10183759
 patched-2-4mb  | 74768384 |  81403904 |   5768124 | 10193738
 origpatch-4mb  | 74883072 |  82182144 |   5783412 | 10185373

All these tests were performed with shared_buffers=4MB, so that the 
index won't fit completely in shared buffers, and I could use the 
idx_blk_read/hit ratio as a measure of cache-friendliness. The 
origpath test was with a simplified version of Alexander's patch, see 
attached. It's the same as the original, but with the 
randomization-option and gist-build related code removed. The patched-10 
and patched-2 tests are two variants with my patch, with 1/10 and 1/2 
chance of moving to the next equal tuple, respectively. The differences 
in cache hit ratio might be just a result of different index sizes. I 
included two unpatched runs above to show that there's a fair amount of 
noise in these tests. That's because of the random nature of the test 
case; it picks rows to delete and insert at random.


I think the conclusion is that all of these patches are effective. The 
1/10 variant is less effective, as expected, as it's closer in behavior 
to the unpatched behavior than the others. The 1/2 variant seems as good 
as the original patch.



I performed another test, by inserting 100 duplicate values on an 
empty table.


  testname  | finalsize | idx_blks_read | idx_blks_hit
+---+---+--
 unpatched  |  89030656 | 21350 |  2972033
 patched-10 |  88973312 | 21450 |  2971920
 patched-2  |  88481792 | 22947 |  2970260
 origpatch  |  61186048 |761817 |  2221500

The original patch produces a much smaller index in this test, but since 
the inserts are distributed randomly, the pages are accessed randomly 
which is bad for caching.


A table full of duplicates isn't very realistic, but overall, I'm 
leaning towards my version of this patch (gistchoose-2.patch). It has 
less potential for causing a regression in existing applications, but is 
just as effective in the original scenario of repeated delete+insert.


- Heikki


duplicatetest.sh
Description: Bourne shell script


gistbloat.sh
Description: Bourne shell script
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 8b60774..75778f6 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -379,6 +379,7 @@ gistchoose(Relation r, Page p, IndexTuple it,	/* it has compressed entry */
 	GISTENTRY	entry,
 identry[INDEX_MAX_KEYS];
 	bool		isnull[INDEX_MAX_KEYS];
+	int			look_further_on_equal = -1;
 
 	Assert(!GistPageIsLeaf(p));
 
@@ -446,6 +447,8 @@ gistchoose(Relation r, Page p, IndexTuple it,	/* it has compressed entry */
 
 if (j  r-rd_att-natts - 1)
 	best_penalty[j + 1] = -1;
+
+look_further_on_equal = -1;
 			}
 			else if (best_penalty[j] == usize)
 			{
@@ -468,12 +471,46 @@ 

Re: [HACKERS] gistchoose vs. bloat

[Tom Lane]

Heikki Linnakangas hlinnakan...@vmware.com writes:
 I did some experimenting with that. I used the same test case Alexander 
 did, with geonames data, and compared unpatched version, the original 
 patch, and the attached patch that biases the first best tuple found, 
 but still sometimes chooses the other equally good ones.

  testname| initsize | finalsize | idx_blks_read | idx_blks_hit
 +--+---+---+--
   patched-10-4mb | 75497472 |  90202112 |   5853604 | 10178331
   unpatched-4mb  | 75145216 |  94863360 |   5880676 | 10185647
   unpatched-4mb  | 75587584 |  97165312 |   5903107 | 10183759
   patched-2-4mb  | 74768384 |  81403904 |   5768124 | 10193738
   origpatch-4mb  | 74883072 |  82182144 |   5783412 | 10185373

 I think the conclusion is that all of these patches are effective. The 
 1/10 variant is less effective, as expected, as it's closer in behavior 
 to the unpatched behavior than the others. The 1/2 variant seems as good 
 as the original patch.

At least on this example, it seems a tad better, if you look at index
size.

 A table full of duplicates isn't very realistic, but overall, I'm 
 leaning towards my version of this patch (gistchoose-2.patch). It has 
 less potential for causing a regression in existing applications, but is 
 just as effective in the original scenario of repeated delete+insert.

+1 for this patch, but I think the comments could use more work.  I was
convinced it was wrong on first examination, mainly because it's hard to
follow the underdocumented look_further_on_equal logic.  I propose the
attached, which is the same logic with better comments (I also chose to
rename and invert the sense of the state variable, because it seemed
easier to follow this way ... YMMV on that though).

regards, tom lane

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 8b60774f50349222ee04fef13613e3592120973b..a127627334e80704edeed38d3522e4000a695abb 100644
*** a/src/backend/access/gist/gistutil.c
--- b/src/backend/access/gist/gistutil.c
*** gistchoose(Relation r, Page p, IndexTupl
*** 379,384 
--- 379,385 
  	GISTENTRY	entry,
  identry[INDEX_MAX_KEYS];
  	bool		isnull[INDEX_MAX_KEYS];
+ 	int			keep_current_best;
  
  	Assert(!GistPageIsLeaf(p));
  
*** gistchoose(Relation r, Page p, IndexTupl
*** 402,407 
--- 403,433 
  	best_penalty[0] = -1;
  
  	/*
+ 	 * If we find a tuple that's exactly as good as the currently best one, we
+ 	 * could use either one.  When inserting a lot of tuples with the same or
+ 	 * similar keys, it's preferable to descend down the same path when
+ 	 * possible, as that's more cache-friendly.  On the other hand, if all
+ 	 * inserts land on the same leaf page after a split, we're never going to
+ 	 * insert anything to the other half of the split, and will end up using
+ 	 * only 50% of the available space.  Distributing the inserts evenly would
+ 	 * lead to better space usage, but that hurts cache-locality during
+ 	 * insertion.  To get the best of both worlds, when we find a tuple that's
+ 	 * exactly as good as the previous best, choose randomly whether to stick
+ 	 * to the old best, or use the new one.  Once we decide to stick to the
+ 	 * old best, we keep sticking to it for any subsequent equally good tuples
+ 	 * we might find.  This favors tuples with low offsets, but still allows
+ 	 * some inserts to go to other equally-good subtrees.
+ 	 *
+ 	 * keep_current_best is -1 if we haven't yet had to make a random choice
+ 	 * whether to keep the current best tuple.  If we have done so, and
+ 	 * decided to keep it, keep_current_best is 1; if we've decided to
+ 	 * replace, keep_current_best is 0.  (This state will be reset to -1 as
+ 	 * soon as we've made the replacement, but sometimes we make the choice in
+ 	 * advance of actually finding a replacement best tuple.)
+ 	 */
+ 	keep_current_best = -1;
+ 
+ 	/*
  	 * Loop over tuples on page.
  	 */
  	maxoff = PageGetMaxOffsetNumber(p);
*** gistchoose(Relation r, Page p, IndexTupl
*** 446,451 
--- 472,480 
  
  if (j  r-rd_att-natts - 1)
  	best_penalty[j + 1] = -1;
+ 
+ /* we have new best, so reset keep-it decision */
+ keep_current_best = -1;
  			}
  			else if (best_penalty[j] == usize)
  			{
*** gistchoose(Relation r, Page p, IndexTupl
*** 468,479 
  		}
  
  		/*
! 		 * If we find a tuple with zero penalty for all columns, there's no
! 		 * need to examine remaining tuples; just break out of the loop and
! 		 * return it.
  		 */
  		if (zero_penalty)
! 			break;
  	}
  
  	return result;
--- 497,537 
  		}
  
  		/*
! 		 * If we looped past the last column, and did not update result,
! 		 * then this tuple is exactly as good as the prior best tuple.
! 		 */
! 		if (j == r-rd_att-natts  result != i)
! 		{
! 			if 

Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Thu, Jan 24, 2013 at 11:26 PM, Heikki Linnakangas 
hlinnakan...@vmware.com wrote:

 On 21.01.2013 15:19, Heikki Linnakangas wrote:

 On 21.01.2013 15:06, Tom Lane wrote:

 Jeff Davispg...@j-davis.com writes:

 On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:

 I looked at this patch. ISTM we should not have the option at all but
 just do it always. I cannot believe that always-go-left is ever a
 preferable strategy in the long run; the resulting imbalance in the
 index will surely kill any possible benefit. Even if there are some
 cases where it miraculously fails to lose, how many users are going to
 realize that applies to their case and make use of the option?


  Sounds good to me.


  If I remember correctly, there was also an argument that it may be
 useful for repeatable test results. That's a little questionable for
 performance (except in those cases where few penalties are identical
 anyway), but could plausibly be useful for a crash report or something.


 Meh. There's already a random decision, in the equivalent place and for
 a comparable reason, in btree (cf _bt_findinsertloc). Nobody's ever
 complained about that being indeterminate, so I'm unconvinced that
 there's a market for it with gist.


 I wonder if it would work for gist to do something similar to
 _bt_findinsertloc, and have a bias towards the left page, but sometimes
 descend to one of the pages to the right. You would get the cache
 locality of usually descending down the same subtree, but also fill the
 pages to the right. Jeff / Alexander, want to give that a shot?


 I did some experimenting with that. I used the same test case Alexander
 did, with geonames data, and compared unpatched version, the original
 patch, and the attached patch that biases the first best tuple found, but
 still sometimes chooses the other equally good ones.

 testname| initsize | finalsize | idx_blks_read | idx_blks_hit
 +--+--**-+---+**--
  patched-10-4mb | 75497472 |  90202112 |   5853604 | 10178331
  unpatched-4mb  | 75145216 |  94863360 |   5880676 | 10185647
  unpatched-4mb  | 75587584 |  97165312 |   5903107 | 10183759
  patched-2-4mb  | 74768384 |  81403904 |   5768124 | 10193738
  origpatch-4mb  | 74883072 |  82182144 |   5783412 | 10185373

 All these tests were performed with shared_buffers=4MB, so that the index
 won't fit completely in shared buffers, and I could use the
 idx_blk_read/hit ratio as a measure of cache-friendliness. The origpath
 test was with a simplified version of Alexander's patch, see attached. It's
 the same as the original, but with the randomization-option and gist-build
 related code removed. The patched-10 and patched-2 tests are two variants
 with my patch, with 1/10 and 1/2 chance of moving to the next equal tuple,
 respectively. The differences in cache hit ratio might be just a result of
 different index sizes. I included two unpatched runs above to show that
 there's a fair amount of noise in these tests. That's because of the random
 nature of the test case; it picks rows to delete and insert at random.

 I think the conclusion is that all of these patches are effective. The
 1/10 variant is less effective, as expected, as it's closer in behavior to
 the unpatched behavior than the others. The 1/2 variant seems as good as
 the original patch.


Does two unpatched-4mb lines are different by coincidence? If so then
variance is significant and we need more experiments to actually compare
patched-2-4mb and origpatch-4mb lines.
There is another cause of overhead when use randomization in gistchoose:
extra penalty calls. It could be significant when index fits to cache. In
order evade it I especially change behaviour of my patch from look
sequentially and choose random to look in random order. I think we need
to include comparison of CPU time.

--
With best regards,
Alexander Korotkov.

Re: [HACKERS] gistchoose vs. bloat

[Tom Lane]

Alexander Korotkov aekorot...@gmail.com writes:
 There is another cause of overhead when use randomization in gistchoose:
 extra penalty calls. It could be significant when index fits to cache. In
 order evade it I especially change behaviour of my patch from look
 sequentially and choose random to look in random order. I think we need
 to include comparison of CPU time.

Hmm ... actually, isn't that an argument in favor of Heikki's method?
The way he's doing it, we can exit without making additional penalty
calls once we've found a zero-penalty tuple and decided not to look
further (which is something with a pretty high probability).

regards, tom lane


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Re: [HACKERS] gistchoose vs. bloat

[Heikki Linnakangas]

On 24.01.2013 22:35, Tom Lane wrote:

Alexander Korotkovaekorot...@gmail.com  writes:

There is another cause of overhead when use randomization in gistchoose:
extra penalty calls. It could be significant when index fits to cache. In
order evade it I especially change behaviour of my patch from look
sequentially and choose random to look in random order. I think we need
to include comparison of CPU time.


Hmm ... actually, isn't that an argument in favor of Heikki's method?
The way he's doing it, we can exit without making additional penalty
calls once we've found a zero-penalty tuple and decided not to look
further (which is something with a pretty high probability).


No, I think Alexander is right, although I believe the difference is 
minimal in practice.


If we assume that the there are no zero-penalty tuples on the page, with 
both approaches you have to call penalty on every tuple to know which is 
best. If there are zero-penalty tuples, then there is a small 
difference. With Alexander's method, you can stop looking as soon as you 
find a zero-penalty tuple (the order you check the tuples is random). 
With my method, you can stop looking as soon as you find a zero-penalty 
tuple, *and* you decide to not look further. With the 1/2 probability to 
stop looking further, you give up on average after 2 tuples.


So if I'm doing my math right, my patch does on average between 1x - 2x 
as many penalty calls as Alexander's, depending on how many zero-penalty 
tuples there are. The 2x case is when the page is full of zero-penalty 
tuples, in which case the difference isn't big in absolute terms; 2 
penalty calls per page versus 1.


BTW, one thing that I wondered about this: How expensive is random()? 
I'm assuming not very, but I don't really know. Alexander's patch called 
random() for every tuple on the page, while I call it only once for each 
equal-penalty tuple. If it's really cheap, I think my/Tom's patch could 
be slightly simplified by always initializing keep_current_best with 
random() at top of the function, and eliminating the -1 haven't decided 
yet state. OTOH, if random() is expensive, I note that we only need one 
bit of randomness, so we could avoid calling random() so often if we 
utilized all the bits from each random() call.


- Heikki


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Re: [HACKERS] gistchoose vs. bloat

[Tom Lane]

Heikki Linnakangas hlinnakan...@vmware.com writes:
 BTW, one thing that I wondered about this: How expensive is random()? 
 I'm assuming not very, but I don't really know. Alexander's patch called 
 random() for every tuple on the page, while I call it only once for each 
 equal-penalty tuple. If it's really cheap, I think my/Tom's patch could 
 be slightly simplified by always initializing keep_current_best with 
 random() at top of the function, and eliminating the -1 haven't decided 
 yet state.

I thought about that too, and concluded that random() is probably
expensive enough that we don't want to call it unnecessarily.

 OTOH, if random() is expensive, I note that we only need one 
 bit of randomness, so we could avoid calling random() so often if we 
 utilized all the bits from each random() call.

Meh.  That would hard-wire the decision that the probability of keeping
a best tuple is exactly 0.5.  I'd rather keep the flexibility to tune it
later.  The way your patch is set up, it seems unlikely that it will
call random() very many times per page, so I'm not that concerned about
minimizing the number of calls further.  (Also, in the probably-common
case that there are no exactly equally good alternatives, this would
actually be a net loss since it would add one unnecessary call.)

So basically, Alexander's method requires more random() calls and fewer
penalty() calls than yours, at least in typical cases.  It's hard to say
which is faster without benchmarking, and it would matter which opclass
you were testing on.

regards, tom lane


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Re: [HACKERS] gistchoose vs. bloat

[Tom Lane]

Jeff Davis pg...@j-davis.com writes:
 On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:
 I looked at this patch.  ISTM we should not have the option at all but
 just do it always.  I cannot believe that always-go-left is ever a
 preferable strategy in the long run; the resulting imbalance in the
 index will surely kill any possible benefit.  Even if there are some
 cases where it miraculously fails to lose, how many users are going to
 realize that applies to their case and make use of the option?

 Sounds good to me.

 If I remember correctly, there was also an argument that it may be
 useful for repeatable test results. That's a little questionable for
 performance (except in those cases where few penalties are identical
 anyway), but could plausibly be useful for a crash report or something.

Meh.  There's already a random decision, in the equivalent place and for
a comparable reason, in btree (cf _bt_findinsertloc).  Nobody's ever
complained about that being indeterminate, so I'm unconvinced that
there's a market for it with gist.

regards, tom lane


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Re: [HACKERS] gistchoose vs. bloat

[Heikki Linnakangas]

On 21.01.2013 15:06, Tom Lane wrote:

Jeff Davispg...@j-davis.com  writes:

On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:

I looked at this patch.  ISTM we should not have the option at all but
just do it always.  I cannot believe that always-go-left is ever a
preferable strategy in the long run; the resulting imbalance in the
index will surely kill any possible benefit.  Even if there are some
cases where it miraculously fails to lose, how many users are going to
realize that applies to their case and make use of the option?



Sounds good to me.



If I remember correctly, there was also an argument that it may be
useful for repeatable test results. That's a little questionable for
performance (except in those cases where few penalties are identical
anyway), but could plausibly be useful for a crash report or something.


Meh.  There's already a random decision, in the equivalent place and for
a comparable reason, in btree (cf _bt_findinsertloc).  Nobody's ever
complained about that being indeterminate, so I'm unconvinced that
there's a market for it with gist.


I wonder if it would work for gist to do something similar to 
_bt_findinsertloc, and have a bias towards the left page, but sometimes 
descend to one of the pages to the right. You would get the cache 
locality of usually descending down the same subtree, but also fill the 
pages to the right. Jeff / Alexander, want to give that a shot?


When building an index from scratch, using the new buffered index build, 
you could do a lot better than fill each page like with regular inserts 
and split when one fills up. You could e.g buffer 10 pages worth of 
tuples, and perform a 10-way split of all the buffered tuples together, 
distributing them equally to 10 pages (or 10+something, to leave some 
room for updates). But that's clearly a separate and much larger patch.


- Heikki


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Re: [HACKERS] gistchoose vs. bloat

[Tom Lane]

Jeff Davis pg...@j-davis.com writes:
 On Fri, 2012-12-14 at 18:36 +0200, Heikki Linnakangas wrote:
 BTW, I don't much like the option name randomization. It's not clear 
 what's been randomized. I'd prefer something like 
 distribute_on_equal_penalty, although that's really long. Better ideas?

 I agree that randomization is vague, but I can't think of anything
 better.

I looked at this patch.  ISTM we should not have the option at all but
just do it always.  I cannot believe that always-go-left is ever a
preferable strategy in the long run; the resulting imbalance in the
index will surely kill any possible benefit.  Even if there are some
cases where it miraculously fails to lose, how many users are going to
realize that applies to their case and make use of the option?

regards, tom lane


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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Mon, 2013-01-21 at 00:48 -0500, Tom Lane wrote:
 I looked at this patch.  ISTM we should not have the option at all but
 just do it always.  I cannot believe that always-go-left is ever a
 preferable strategy in the long run; the resulting imbalance in the
 index will surely kill any possible benefit.  Even if there are some
 cases where it miraculously fails to lose, how many users are going to
 realize that applies to their case and make use of the option?

Sounds good to me.

If I remember correctly, there was also an argument that it may be
useful for repeatable test results. That's a little questionable for
performance (except in those cases where few penalties are identical
anyway), but could plausibly be useful for a crash report or something.

Regards,
Jeff Davis



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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Fri, 2012-12-14 at 01:03 +0400, Alexander Korotkov wrote:
 Hi!
 
 On Sat, Dec 8, 2012 at 7:05 PM, Andres Freund and...@2ndquadrant.com
 wrote:
 I notice there's no documentation about the new reloption at
 all?
 
 
 Thanks for notice! I've added small description to docs in the
 attached patch.

Here is an edited version of the documentation note. Please review to
see if you like my version.

Also, I fixed a compiler warning.

My tests showed a significant reduction in the size of a gist index with
many of the same penalty values. The run times showed mixed results,
however, and I didn't dig in much further because you've already done
significant testing.

Marking this one ready again.

Regards,
Jeff Davis



gist_choose_bloat-0.5A.patch.gz
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Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Fri, Dec 14, 2012 at 12:46 PM, Jeff Davis pg...@j-davis.com wrote:

  Thanks for notice! I've added small description to docs in the
  attached patch.

 Here is an edited version of the documentation note. Please review to
 see if you like my version.


Edited version looks good for me.


 Also, I fixed a compiler warning.


Thanks!

--
With best regards,
Alexander Korotkov.

Re: [HACKERS] gistchoose vs. bloat

[Heikki Linnakangas]

One question: does the randomization ever help when building a new 
index? In the original test case, you repeatedly delete and insert 
tuples, and I can see how the index can get bloated in that case. But I 
don't see how bloat would occur when building the index from scratch.


BTW, I don't much like the option name randomization. It's not clear 
what's been randomized. I'd prefer something like 
distribute_on_equal_penalty, although that's really long. Better ideas?


- Heikki


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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Fri, 2012-12-14 at 18:36 +0200, Heikki Linnakangas wrote:
 One question: does the randomization ever help when building a new 
 index? In the original test case, you repeatedly delete and insert 
 tuples, and I can see how the index can get bloated in that case. But I 
 don't see how bloat would occur when building the index from scratch.

When building an index on a bunch of identical int4range values (in my
test, [1,10) ), the resulting index was about 17% smaller.

If the current algorithm always chooses to insert on the left-most page,
then it seems like there would be a half-filled right page for every
split that occurs. Is that reasoning correct?

However, I'm having some second thoughts about the run time for index
builds. Maybe we should have a few more tests to determine if this
should really be the default or just an option?

 BTW, I don't much like the option name randomization. It's not clear 
 what's been randomized. I'd prefer something like 
 distribute_on_equal_penalty, although that's really long. Better ideas?

I agree that randomization is vague, but I can't think of anything
better.

Regards,
Jeff Davis



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Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

Hi!

On Sat, Dec 8, 2012 at 7:05 PM, Andres Freund and...@2ndquadrant.comwrote:

 I notice there's no documentation about the new reloption at all?


Thanks for notice! I've added small description to docs in the attached
patch.

--
With best regards,
Alexander Korotkov.


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Re: [HACKERS] gistchoose vs. bloat

[Andres Freund]

Hi,

On 2012-11-02 12:54:33 +0400, Alexander Korotkov wrote:
 On Sun, Oct 21, 2012 at 11:03 AM, Jeff Davis pg...@j-davis.com wrote:

  On Thu, 2012-10-18 at 15:09 -0300, Alvaro Herrera wrote:
   Jeff, do you think we need more review of this patch?
 
  In the patch, it refers to rd_options without checking for NULL first,
  which needs to be fixed.
 
  There's actually still one place where it says id rather than is.
  Just a nitpick.
 
  Regarding my point 4 from the previous email, I mildly disagree with the
  style, but I don't see a correctness problem there.
 
  If the first two items are fixed, then the patch is fine with me.
 

 First two items are fixed in attached version of the patch.

So the patch is ready for committer now?

I notice there's no documentation about the new reloption at all?

Greetings,

Andres Freund

--
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 PostgreSQL Development, 24x7 Support, Training  Services


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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Thu, 2012-10-18 at 15:09 -0300, Alvaro Herrera wrote:
 Jeff, do you think we need more review of this patch?

In the patch, it refers to rd_options without checking for NULL first,
which needs to be fixed.

There's actually still one place where it says id rather than is.
Just a nitpick.

Regarding my point 4 from the previous email, I mildly disagree with the
style, but I don't see a correctness problem there.

If the first two items are fixed, then the patch is fine with me.

Regards,
Jeff Davis



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Re: [HACKERS] gistchoose vs. bloat

[Alvaro Herrera]

Alexander Korotkov escribió:

  4. It looks like the randomization is happening while trying to compare
  the penalties. I think it may be more readable to separate those two
  steps; e.g.
 
/* create a mapping whether randomization is on or not */
for (i = FirstOffsetNumber; i = maxoff; i = OffsetNumberNext(i))
offsets[i - FirstOffsetNumber] = i;
 
if (randomization)
/* randomize offsets array */
 
for (i = 0; i  maxoff; i++)
{
   offset = offsets[i];
   ...
}
 
  That's just an idea; if you think it's more readable as-is (or if I am
  misunderstanding) then let me know.
 
 Actually, current implementation comes from idea of creating possible less
 overhead when randomization is off. I'll try to measure overhead in worst
 case. If it is low enough then you proposal looks reasonable to me.

Were you able to do these measurements?  If not, I'll defer to your and
Jeff's judgement on what's the best next step here.

Jeff, do you think we need more review of this patch?

-- 
Álvaro Herrerahttp://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training  Services


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Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Mon, Oct 1, 2012 at 5:15 AM, Jeff Davis pg...@j-davis.com wrote:

 On Tue, 2012-09-04 at 19:21 +0400, Alexander Korotkov wrote:

  New version of patch is attached. Parameter randomization was
  introduced. It controls whether to randomize choose. Choose algorithm
  was rewritten.
 
 Review comments:

 1. Comment above while loop in gistRelocateBuildBuffersOnSplit needs to
 be updated.


Actually, I didn't realize what exact comment you expect. Check if added
commend meets you expectations.



 2. Typo in two places: if randomization id required.

 3. In gistRelocateBuildBuffersOnSplit, shouldn't that be:
  splitPageInfo = relocationBuffersInfos[bufferIndex];
not:
  splitPageInfo = relocationBuffersInfos[i];


Fixed.


 4. It looks like the randomization is happening while trying to compare
 the penalties. I think it may be more readable to separate those two
 steps; e.g.

   /* create a mapping whether randomization is on or not */
   for (i = FirstOffsetNumber; i = maxoff; i = OffsetNumberNext(i))
   offsets[i - FirstOffsetNumber] = i;

   if (randomization)
   /* randomize offsets array */

   for (i = 0; i  maxoff; i++)
   {
  offset = offsets[i];
  ...
   }

 That's just an idea; if you think it's more readable as-is (or if I am
 misunderstanding) then let me know.


Actually, current implementation comes from idea of creating possible less
overhead when randomization is off. I'll try to measure overhead in worst
case. If it is low enough then you proposal looks reasonable to me.

--
With best regards,
Alexander Korotkov.


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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Tue, 2012-09-04 at 19:21 +0400, Alexander Korotkov wrote:

 New version of patch is attached. Parameter randomization was
 introduced. It controls whether to randomize choose. Choose algorithm
 was rewritten.
 
Review comments:

1. Comment above while loop in gistRelocateBuildBuffersOnSplit needs to
be updated.

2. Typo in two places: if randomization id required.

3. In gistRelocateBuildBuffersOnSplit, shouldn't that be:
 splitPageInfo = relocationBuffersInfos[bufferIndex];
   not:
 splitPageInfo = relocationBuffersInfos[i];

4. It looks like the randomization is happening while trying to compare
the penalties. I think it may be more readable to separate those two
steps; e.g.

  /* create a mapping whether randomization is on or not */
  for (i = FirstOffsetNumber; i = maxoff; i = OffsetNumberNext(i))
  offsets[i - FirstOffsetNumber] = i;

  if (randomization)
  /* randomize offsets array */

  for (i = 0; i  maxoff; i++)
  {
 offset = offsets[i];
 ...
  }

That's just an idea; if you think it's more readable as-is (or if I am
misunderstanding) then let me know.

Regards,
Jeff Davis



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Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Tue, 2012-09-04 at 19:21 +0400, Alexander Korotkov wrote:

 New version of patch is attached. Parameter randomization was
 introduced. It controls whether to randomize choose. Choose algorithm
 was rewritten.
 
Do you expect it to be bad in any reasonable situations? I'm inclined to
just make it always randomize if it's better. I think it would be hard
for a user to guess when it's better and when not.

Maybe it's useful to turn randomization off for testing purposes, e.g.
to ensure determinism?

Regards,
Jeff Davis




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Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Tue, Sep 11, 2012 at 10:35 AM, Jeff Davis pg...@j-davis.com wrote:

 On Tue, 2012-09-04 at 19:21 +0400, Alexander Korotkov wrote:

  New version of patch is attached. Parameter randomization was
  introduced. It controls whether to randomize choose. Choose algorithm
  was rewritten.
 
 Do you expect it to be bad in any reasonable situations? I'm inclined to
 just make it always randomize if it's better. I think it would be hard
 for a user to guess when it's better and when not.


Randomization should increase IO when index doesn't entirely fit to cache.
Without randomization only fraction of the tree would be used for actual
insertions. While with randomization whole tree would be potentially used
for insertions.


 Maybe it's useful to turn randomization off for testing purposes, e.g.
 to ensure determinism?


Yes, that's another good point. For example, randomization impede
reproducing of bugs.

--
With best regards,
Alexander Korotkov.

Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Mon, Aug 20, 2012 at 9:13 PM, Alexander Korotkov aekorot...@gmail.comwrote:

 Current gistchoose code has a bug. I've started separate thread about it.
 http://archives.postgresql.org/pgsql-hackers/2012-08/msg00544.php
 Also, it obviously needs more comments.

 Current state of patch is more proof of concept than something ready. I'm
 going to change it in following ways:
 1) We don't know how expensive user penalty function is. So, I'm going to
 change randomization algorithm so that it doesn't increase number of
 penalty calls in average.
 2) Since, randomization could produce additional IO, there are probably no
 optimal solution for all the cases. We could introduce user-visible option
 which enables or disables randomization. However, default value of this
 option is another question.


 Also, I think you should use random() rather than rand().


 Thanks, will fix.


New version of patch is attached. Parameter randomization was introduced.
It controls whether to randomize choose. Choose algorithm was rewritten.

--
With best regards,
Alexander Korotkov.


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Re: [HACKERS] gistchoose vs. bloat

[Alexander Korotkov]

On Mon, Aug 20, 2012 at 7:13 AM, Jeff Davis pg...@j-davis.com wrote:

 I took a look at this patch. The surrounding code is pretty messy (not
 necessarily because of your patch). A few comments would go a long way.

 The 'which_grow' array is initialized as it goes, first using pointer
 notations (*which_grows = -1.0) and then using subscript notation. As
 far as I can tell, the first r-rd_att-natts of the array (the only
 elements that matter) need to be written the first time through anyway.
 Why not just replace which_grow[j]  0 with i == FirstOffsetNumber
 and add a comment that we're initializing the penalties with the first
 index tuple?

 The 'sum_grow' didn't make any sense, thank you for getting rid of that.

 Also, we should document that the earlier attributes always take
 precedence, which is why we break out of the inner loop as soon as we
 encounter an attribute with a higher penalty.

 Please add a comment indicating why you are randomly choosing among the
 equal penalties.

 I think that there might be a problem with the logic, as well. Let's say
 you have two attributes and there are two index tuples, it1 and it2;
 with penalties [10,10] and [10,100] respectively. The second time
 through the outer loop, with i = 2, you might (P=0.5) assign 2 to the
 'which' variable in the first iteration of the inner loop, before it
 realizes that it2 actually has a higher penalty. I think you need to
 finish out the inner loop and have a flag that indicates that all
 attributes are equal before you do the probabilistic replacement.


Current gistchoose code has a bug. I've started separate thread about it.
http://archives.postgresql.org/pgsql-hackers/2012-08/msg00544.php
Also, it obviously needs more comments.

Current state of patch is more proof of concept than something ready. I'm
going to change it in following ways:
1) We don't know how expensive user penalty function is. So, I'm going to
change randomization algorithm so that it doesn't increase number of
penalty calls in average.
2) Since, randomization could produce additional IO, there are probably no
optimal solution for all the cases. We could introduce user-visible option
which enables or disables randomization. However, default value of this
option is another question.


 Also, I think you should use random() rather than rand().


Thanks, will fix.

--
With best regards,
Alexander Korotkov.

Re: [HACKERS] gistchoose vs. bloat

[Jeff Davis]

On Mon, 2012-06-18 at 15:12 +0400, Alexander Korotkov wrote:
 Hackers,
 
 
 While experimenting with gistchoose I achieve interesting results
 about relation of gistchoose behaviour and gist index bloat.

...
 
 Current implementation of gistchoose select first index tuple which
 have minimal penalty. It is possible for several tuples to have same
 minimal penalty. I've created simple patch which selects random from
 them. I then I've following results for same testcase.
 
I took a look at this patch. The surrounding code is pretty messy (not
necessarily because of your patch). A few comments would go a long way.

The 'which_grow' array is initialized as it goes, first using pointer
notations (*which_grows = -1.0) and then using subscript notation. As
far as I can tell, the first r-rd_att-natts of the array (the only
elements that matter) need to be written the first time through anyway.
Why not just replace which_grow[j]  0 with i == FirstOffsetNumber
and add a comment that we're initializing the penalties with the first
index tuple?

The 'sum_grow' didn't make any sense, thank you for getting rid of that.

Also, we should document that the earlier attributes always take
precedence, which is why we break out of the inner loop as soon as we
encounter an attribute with a higher penalty.

Please add a comment indicating why you are randomly choosing among the
equal penalties.

I think that there might be a problem with the logic, as well. Let's say
you have two attributes and there are two index tuples, it1 and it2;
with penalties [10,10] and [10,100] respectively. The second time
through the outer loop, with i = 2, you might (P=0.5) assign 2 to the
'which' variable in the first iteration of the inner loop, before it
realizes that it2 actually has a higher penalty. I think you need to
finish out the inner loop and have a flag that indicates that all
attributes are equal before you do the probabilistic replacement.

Also, I think you should use random() rather than rand().

Regards,
Jeff Davis



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