Re: 9.4 regression

On 7 August 2013 17:54, Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> wrote:
> On Wed, Aug 7, 2013 at 11:21 AM, Thom Brown <thom(at)linux(dot)com> wrote:
>> Hi all,
>>
>> I recently tried a simple benchmark to see how far 9.4 had come since
>> 8.4, but I discovered that I couldn't get 9.4 to even touch 8.4 for
>> performance. After checking 9.2 and 9.3 (as per Kevin Grittner's
>> suggestion), I found that those were fine, so the issue must be in
>> 9.4devel. I used identical configurations for each test, and used
>> 9.1's pgbench to ensure changes in pgbench didn't affect the outcome.
>> The common config changes were:
>
> ...
>
>> 8.4: 812.482108
>> 9.4 HEAD: 355.397658
>> 9.4 e5592c (9th July): 356.485625
>> 9.4 537227 (7th July): 365.992518
>> 9.4 9b2543 (7th July): 362.587339
>> 9.4 269e78 (5th July): 359.439143
>> 9.4 8800d8 (5th July): 821.933082
>> 9.4 568d41 (2nd July): 822.991160
>>
>> 269e78 was the commit immediately after 8800d8, so it appears that
>> introduced the regression.
>>
>> "Use posix_fallocate() for new WAL files, where available."
>>
>> Ironically, that was intended to be a performance improvement.
>
> Would it be possible for you to download, compile, and run the test
> program as described and located in this email:
>
> http://www.postgresql.org/message-id/CAKuK5J1AcML-1cGBhnRzED-vh4oG+8HkmFhy2ECa-8JBJ-6qbQ@mail.gmail.com

I shall do after the 2 x 1 hour benchmarks are complete.

> I also wonder if there is a problem with the 3.8.0 kernel specifically.

Well my laptop has the same kernel (and also 64-bit Linux Mint), so
took 3 quick sample benchmarks on those two commits, and I get the
following (all without --enable-cassert):

269e78: 1162.593665 / 1158.644302 / 1153.955611
8800d8: 2446.087618 / 2443.797252 / 2321.876431

And running your test program gives the following (again, just on my laptop):

for i in 1 2 5 10 100; do ./test_fallocate foo $i 1; done
method: classic. 1 open/close iterations, 1 rewrite in 0.6380s
method: posix_fallocate. 1 open/close iterations, 1 rewrite in 0.3204s
method: glibc emulation. 1 open/close iterations, 1 rewrite in 0.6274s
method: classic. 2 open/close iterations, 1 rewrite in 1.2908s
method: posix_fallocate. 2 open/close iterations, 1 rewrite in 0.6596s
method: glibc emulation. 2 open/close iterations, 1 rewrite in 1.2666s
method: classic. 5 open/close iterations, 1 rewrite in 3.1419s
method: posix_fallocate. 5 open/close iterations, 1 rewrite in 1.5930s
method: glibc emulation. 5 open/close iterations, 1 rewrite in 3.1516s
method: classic. 10 open/close iterations, 1 rewrite in 6.2912s
method: posix_fallocate. 10 open/close iterations, 1 rewrite in 3.2626s
method: glibc emulation. 10 open/close iterations, 1 rewrite in 6.3667s
method: classic. 100 open/close iterations, 1 rewrite in 67.4174s
method: posix_fallocate. 100 open/close iterations, 1 rewrite in 37.8788s
method: glibc emulation. 100 open/close iterations, 1 rewrite in 55.0714s

--
Thom