Date: Wed, 29 Apr 2020 18:58:16 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: jgdr@dalibo.com, andres@anarazel.de, michael@paquier.xyz,
	sawada.mshk@gmail.com, peter.eisentraut@2ndquadrant.com,
	pgsql-hackers@lists.postgresql.org, thomas.munro@enterprisedb.com,
	sk@zsrv.org, michael.paquier@gmail.com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
Message-ID: <20200429225816.GA18918@alvherre.pgsql>
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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