Re: Move PinBuffer and UnpinBuffer to atomics

On Mon, Apr 11, 2016 at 7:33 PM, Alexander Korotkov <
a(dot)korotkov(at)postgrespro(dot)ru> wrote:

> On Sun, Apr 10, 2016 at 2:24 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)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?
>
>

As already pointed out by Andres, that this is mainly due to shared memory
alignment issues. We have observed that changing some shared memory
arrangement (structures) some times causes huge differences in
performance. I guess that is the reason why with Cache the snapshot patch,
I am seeing the performance gets restored (mainly because it is changing
shared memory structures). I think the right way to fix this is to find
which shared structure/structure's needs padding, so that we don't see such
fluctuations every time we change some thing is shared memory.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com