Re: Move PinBuffer and UnpinBuffer to atomics - Mailing list pgsql-hackers

On Sun, Apr 10, 2016 at 2:24 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:

I also tried to run perf top during pgbench and get some interesting results.

Without 5364b357:

   5,69%  postgres                 [.] GetSnapshotData

   4,47%  postgres                 [.] LWLockAttemptLock

   3,81%  postgres                 [.] _bt_compare

   3,42%  postgres                 [.] hash_search_with_hash_value

   3,08%  postgres                 [.] LWLockRelease

   2,49%  postgres                 [.] PinBuffer.isra.3

   1,58%  postgres                 [.] AllocSetAlloc

   1,17%  [kernel]                 [k] __schedule

   1,15%  postgres                 [.] PostgresMain

   1,13%  libc-2.17.so             [.] vfprintf

   1,01%  libc-2.17.so             [.] __memcpy_ssse3_back

With 5364b357:

  18,54%  postgres                 [.] GetSnapshotData

   3,45%  postgres                 [.] LWLockRelease

   3,27%  postgres                 [.] LWLockAttemptLock

   3,21%  postgres                 [.] _bt_compare

   2,93%  postgres                 [.] hash_search_with_hash_value

   2,00%  postgres                 [.] PinBuffer.isra.3

   1,32%  postgres                 [.] AllocSetAlloc

   1,10%  libc-2.17.so             [.] vfprintf

Very surprising.  It appears that after 5364b357, GetSnapshotData consumes more time.  But I can't see anything depending on clog buffers in GetSnapshotData code...

There is a related fact presented by Mithun C Y as well [1] which suggests that Andres's idea of reducing the cost of snapshot shows noticeable gain after increasing the clog buffers.  If you read that thread you will notice that initially we didn't notice much gain by that idea, but with increased clog buffers, it started showing noticeable gain.  If by any chance, you can apply that patch and see the results (latest patch is at [2]).

[1] - http://www.postgresql.org/message-id/CAD__Ouic1Tvnwqm6Wf6j7Cz1Kk1DQgmy0isC7=OgX+3JtfGk9g@mail.gmail.com 

[2] - http://www.postgresql.org/message-id/CAD__OuiwEi5sHe2wwQCK36Ac9QMhvJuqG3CfPN+OFCMb7rdruQ@mail.gmail.com

I took a look at this thread but I still didn't get why number of clog buffers affects read-only benchmark.

Could you please explain it to me in more details?