Re: Slow standby snapshot

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Michail Nikolaev <michail(dot)nikolaev(at)gmail(dot)com>
Cc: Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Kirill Reshke <reshkekirill(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Slow standby snapshot
Date: 2022-02-20 17:56:08
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> On 22 Nov 2021, at 14:05, Michail Nikolaev <michail(dot)nikolaev(at)gmail(dot)com> wrote:
>> Write barrier must be issued after write, not before.
>> Don't we need to issue read barrier too?
> The write barrier is issued after the changes to KnownAssignedXidsNext
> and KnownAssignedXidsValid arrays and before the update of
> headKnownAssignedXids.
> So, it seems to be correct. We make sure once the CPU sees changes of
> headKnownAssignedXids - underlying arrays contain all the required
> data.

Patch on barrier seems too complicated to me right now. I’d propose to focus on KnowAssignedXidsNext patch: it’s clean, simple and effective.

I’ve rebased the patch so that it does not depend on previous step. Please check out it’s current state, if you are OK with it - let’s mark the patch Ready for Committer. Just maybe slightly better commit message would make the patch easier to understand.

Thanks! Best regards, Andrey Borodin.

Attachment Content-Type Size
v4-0001-Use-linked-list-to-improve-KnownAssignedXids-perf.patch application/octet-stream 6.1 KB

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