Re: [HACKERS] Restricting maximum keep segments by repslots

Hi,

On 2020-05-16 22:51:50 -0400, Alvaro Herrera wrote:
> On 2020-May-16, Andres Freund wrote:
>
> > I, independent of this patch, added a few additional paths in which
> > checkpointer's latch is reset, and I found a few shutdowns in regression
> > tests to be extremely slow / timing out. The reason for that is that
> > the only check for interrupts is at the top of the loop. So if
> > checkpointer gets SIGUSR2 we don't see ShutdownRequestPending until we
> > decide to do a checkpoint for other reasons.
>
> Ah, yeah, this seems a genuine bug.
>
> > I also suspect that it could have harmful consequences to not do a
> > AbsorbSyncRequests() if something "ate" the set latch.
>
> I traced through this when looking over the previous fix, and given that
> checkpoint execution itself calls AbsorbSyncRequests frequently, I
> don't think this one qualifies as a bug.

There's no AbsorbSyncRequests() after CheckPointBuffers(), I think. And
e.g. CheckPointTwoPhase() could take a while. Which then would mean that
we'd potentially not AbsorbSyncRequests() until checkpoint_timeout
causes us to wake up. Am I missing something?

> > One way to do that would be to WaitLatch() call to much earlier, and
> > only do a WaitLatch() if do_checkpoint is false. Roughly like in the
> > attached.
>
> Hm. I'd do "WaitLatch() / continue" in the "!do_checkpoint" block, and
> put the checpkoint code not in the else block; seems easier to read to
> me.

Yea, that'd probably be better. I was also pondering if we shouldn't
just move the checkpoint code into, gasp, it's own function ;)

Greetings,

Andres Freund