Re: Server won't start with fallback setting by initdb.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Server won't start with fallback setting by initdb.
Date: 2018-03-05 06:06:35
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Michael Paquier <michael(at)paquier(dot)xyz> writes:
> On Sun, Mar 04, 2018 at 03:31:31PM -0500, Tom Lane wrote:
>> ... But what I would argue is that
>> of those three choices, the least defensible one is max_wal_senders = 10.
>> Where did that come from? What fraction of real-world installations will
>> need that? We don't choose defaults that overprovision small
>> installations by 5X or 10X anywhere else, so why here?

> Those numbers are coming from f6d6d29, which points to this thread at
> its root:

> The number of max_wal_senders came out as a consensus because those are
> cheap to enable, now the number came out by itself. I am not seeing on
> the thread any specific reason behind.

I don't doubt that the amount of shared memory involved is negligible,
but I'm less sure that there's no impact at all from overprovisioning
max_wal_senders. What I see in the code is a lot of places iterating
over each walsender slot and taking a spinlock on each slot, whether
or not the slot is used (or ever has been used). Are we sure that
none of those places are performance hot spots?

AFAICS from a quick spin through the above-mentioned thread, there
was little discussion of the exact value to set max_wal_senders to,
and no performance testing of the point.

regards, tom lane

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