Re: Move PinBuffer and UnpinBuffer to atomics

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

> On Sun, Apr 10, 2016 at 11:33 AM, Alexander Korotkov <
> a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>
>> On Sun, Apr 10, 2016 at 8:36 AM, Alexander Korotkov <
>> a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>>
>>> On Sat, Apr 9, 2016 at 10:49 PM, Andres Freund <andres(at)anarazel(dot)de>
>>> wrote:
>>>
>>>>
>>>>
>>>> On April 9, 2016 12:43:03 PM PDT, Andres Freund <andres(at)anarazel(dot)de>
>>>> wrote:
>>>> >On 2016-04-09 22:38:31 +0300, Alexander Korotkov wrote:
>>>> >> There are results with 5364b357 reverted.
>>>> >
>>>> >Crazy that this has such a negative impact. Amit, can you reproduce
>>>> >that? Alexander, I guess for r/w workload 5364b357 is a benefit on that
>>>> >machine as well?
>>>>
>>>> How sure are you about these measurements?
>>>
>>>
>>> I'm pretty sure. I've retried it multiple times by hand before re-run
>>> the script.
>>>
>>>
>>>> Because there really shouldn't be clog lookups one a steady state is
>>>> reached...
>>>>
>>>
>>> Hm... I'm also surprised. There shouldn't be clog lookups once hint bits
>>> are set.
>>>
>>
>> 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?

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company