Re: LogwrtResult contended spinlock

On 2020-Aug-31, Andres Freund wrote:

> Hi,
>
> On August 31, 2020 11:21:56 AM PDT, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

> >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.

Heh, Simon said the same. It's not clear to me due to the lack of
general availability of 64-bit atomics. If they are spinlock-protected
when emulated, I think that would make the problem worse.

IIRC Thomas wanted to start relying on atomic 64-bit vars in some patch,
but I don't remember what it was.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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