Re: LogwrtResult contended spinlock

Hi,

On August 31, 2020 11:21:56 AM PDT, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>Jaime Casanova recently reported a situation where pglogical
>replicating
>from 64 POS sites to a single central (64-core) node, each with two
>replication sets, causes XLog's info_lck to become highly contended
>because of frequently reading LogwrtResult.  We tested the simple fix
>of
>adding a new LWLock that protects LogwrtResult and LogwrtRqst; that
>seems to solve the problem easily enough.
>
>At first I wanted to make the new LWLock cover only LogwrtResult
>proper,
>and leave LogwrtRqst alone.  However on doing it, it seemed that that
>might change the locking protocol in a nontrivial way.  So I decided to
>make it cover both and call it a day.  We did verify that the patch
>solves the reported problem, at any rate.

Wouldn't the better fix here be to allow reading of individual members without a lock? E.g. by wrapping each in a 64bit
atomic.

Andres
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