Re: MultiXact\SLRU buffers configuration

From: "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru>
To: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: MultiXact\SLRU buffers configuration
Date: 2020-05-13 18:08:37
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> 11 мая 2020 г., в 16:17, Andrey M. Borodin <x4mmm(at)yandex-team(dot)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

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.

What am I missing?

Best regards, Andrey Borodin.

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