Re: [HACKERS] Restricting maximum keep segments by repslots

On 2020-Apr-28, Alvaro Herrera wrote:

> On 2020-Apr-28, Kyotaro Horiguchi wrote:
>
> > > Anyway I think this patch should fix it also -- instead of adding a new
> > > flag, we just rely on the existing flags (since do_checkpoint must have
> > > been set correctly from the flags earlier in that block.)
> >
> > Since the added (!do_checkpoint) check is reached with
> > do_checkpoint=false at server start and at archive_timeout intervals,
> > the patch makes checkpointer run a busy-loop at that timings, and that
> > loop lasts until a checkpoint is actually executed.
> >
> > What we need to do here is not forgetting the fact that the latch has
> > been set even if the latch itself gets reset before reaching to
> > WaitLatch.
>
> After a few more false starts :-) I think one easy thing we can do
> without the additional boolean flag is to call SetLatch there in the
> main loop if we see that ckpt_flags is nonzero.

I went back to "continue" instead of SetLatch, because it seems less
wasteful, but I changed the previously "do_checkpoint" condition to
rechecking ckpt_flags. We would not get in the busy loop in that case,
because the condition is true when the next loop would take action and
false otherwise. So I think this should fix the problem without causing
any other issues. But if you do see problems with this, please let us
know.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services