Re: MultiXact\SLRU buffers configuration

At Thu, 14 May 2020 11:44:01 +0500, "Andrey M. Borodin" <x4mmm(at)yandex-team(dot)ru> wrote in
> > GetMultiXactIdMembers believes that 4 is successfully done if 2
> > returned valid offset, but actually that is not obvious.
> >
> > If we add a single giant lock just to isolate ,say,
> > GetMultiXactIdMember and RecordNewMultiXact, it reduces concurrency
> > unnecessarily. Perhaps we need finer-grained locking-key for standby
> > that works similary to buffer lock on primary, that doesn't cause
> > confilicts between irrelevant mxids.
> >
> We can just replay members before offsets. If offset is already there - members are there too.
> But I'd be happy if we could mitigate those 1000us too - with a hint about last maixd state in a shared MX state, for example.

Generally in such cases, condition variables would work. In the
attached PoC, the reader side gets no penalty in the "likely" code
path. The writer side always calls ConditionVariableBroadcast but the
waiter list is empty in almost all cases. But I couldn't cause the
situation where the sleep 1000u is reached.

> Actually, if we read empty mxid array instead of something that is replayed just yet - it's not a problem of inconsistency, because transaction in this mxid could not commit before we started. ISTM.
> So instead of fix, we, probably, can just add a comment. If this reasoning is correct.

The step 4 of the reader side reads the members of the target mxid. It
is already written if the offset of the *next* mxid is filled-in.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center