Re: [PERFORM] Cpu usage 100% on slave. s_lock problem.

On Thu, Nov 21, 2013 at 9:02 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2013-11-21 16:25:02 +0200, Heikki Linnakangas wrote:
>> Hmm. All callers of RecoveryInProgress() must be prepared to handle the case
>> that RecoveryInProgress() returns true, but the system is no longer in
>> recovery. No matter what locking we do in RecoveryInProgress(), the startup
>> process might finish recovery just after RecoveryInProgress() has returned.
>
> True.
>
>> What about the attached? It reads the shared variable without a lock or
>> barrier. If it returns 'true', but the system in fact just exited recovery,
>> that's OK. As explained above, all the callers must tolerate that anyway.
>> But if it returns 'false', then it performs a full memory barrier, which
>> should ensure that it sees any other shared variables as it is after the
>> startup process cleared SharedRecoveryInProgress (notably,
>> XLogCtl->ThisTimeLineID).
>
> I'd argue that we should also remove the spinlock in StartupXLOG and
> replace it with a write barrier. Obviously not for performance reasons,
> but because somebody might add more code to run under that spinlock.
>
> Looks good otherwise, although a read memory barrier ought to suffice.

This code is in a very hot code path.  Are we *sure* that the read
barrier is fast enough that we don't want to provide an alternate
function that only returns the local flag?  I don't know enough about
them to say either way.

merlin