Horton

Re: MultiXact\SLRU buffers configuration

Kyotaro Horiguchi <horikyota.ntt@gmail.com>

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From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
To: x4mmm@yandex-team.ru
Cc: pgsql-hackers@postgresql.org
Date: 2020-05-14T01:25:26Z
Lists: pgsql-hackers 
At Wed, 13 May 2020 23:08:37 +0500, "Andrey M. Borodin" <x4mmm@yandex-team.ru> wrote in 
> 
> 
> > 11 мая 2020 г., в 16:17, Andrey M. Borodin <x4mmm@yandex-team.ru> написал(а):
> > 
> > I've went ahead and created 3 patches:
> > 1. Configurable SLRU buffer sizes for MultiXacOffsets and MultiXactMembers
> > 2. Reduce locking level to shared on read of MultiXactId members
> > 3. Configurable cache size
> 
> I'm looking more at MultiXact and it seems to me that we have a race condition there.
> 
> When we create a new MultiXact we do:
> 1. Generate new MultiXactId under MultiXactGenLock
> 2. Record new mxid with members and offset to WAL
> 3. Write offset to SLRU under MultiXactOffsetControlLock
> 4. Write members to SLRU under MultiXactMemberControlLock

But, don't we hold exclusive lock on the buffer through all the steps
above?

> When we read MultiXact we do:
> 1. Retrieve offset by mxid from SLRU under MultiXactOffsetControlLock
> 2. If offset is 0 - it's not filled in at step 4 of previous algorithm, we sleep and goto 1
> 3. Retrieve members from SLRU under MultiXactMemberControlLock
> 4. ..... what we do if there are just zeroes because step 4 is not executed yet? Nothing, return empty members list.

So transactions never see such incomplete mxids, I believe.

> What am I missing?

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Commits

  1. Rework new SLRU test with injection points

  2. injection_point: Add injection_points.stats

  3. injection_points: Add initialization of shmem state when loading module

  4. Add injection-point test for new multixact CV usage

  5. Use conditional variable to wait for next MultiXact offset

  6. Improve performance of subsystems on top of SLRU

  7. Adjust VACUUM hastup LP_REDIRECT comments.

  8. Test replay of regression tests, attempt II.