Re: Question regarding "Make archiver process an auxiliary process. commit"

At Tue, 6 Dec 2022 17:23:50 +0530, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> wrote in 
> Thanks. +1 for fixing this.
> 
> I would like to quote recent discussions on reducing the useless
> wakeups or increasing the sleep/hibernation times in various processes
> to reduce the power savings [1] [2] [3] [4] [5]. With that in context,
> does the archiver need to wake up every 60 sec at all when its latch
> gets set (PgArchWakeup()) whenever the server switches to a new WAL
> file? What happens if we get rid of PGARCH_AUTOWAKE_INTERVAL and rely
> on its latch being set? If required, we can spread PgArchWakeup() to
> more places, no?

I thought so first, but archiving may be interrupted for various
reasons (disk full I think is the most common one). So, only
intentional wakeups aren't sufficient.

> Before even answering the above questions, I think we need to see if
> there're any cases where a process can miss SetLatch() calls (I don't
> have an answer for that).

I read a recent Thomas' mail that says something like "should we
consider the case latch sets are missed?".  It is triggered by SIGURG
or SetEvent().  I'm not sure but I believe the former is now reliable
and the latter was born reliable.

> Or do we want to stick to what the below comment says?
> 
>     /*
>      * There shouldn't be anything for the archiver to do except to wait for a
>      * signal ... however, the archiver exists to protect our data, so she
>      * wakes up occasionally to allow herself to be proactive.
>      */

So I think this is still valid.  If we want to eliminate useless
wakeups, archiver needs to remember whether the last iteration was
fully done or not. But it seems to be a race condition is involved.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center