Feedback-ID: ia2694551:Fastmail
Date: Thu, 5 Sep 2024 15:03:32 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, 
	pgsql-hackers@lists.postgresql.org
Subject: Re: per backend I/O statistics
Message-ID: <202409051303.qumelxccgzzn@alvherre.pgsql>
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On 2024-Sep-03, Bertrand Drouvot wrote:

> Cons is probably allocating shared memory space that might not be used (
> sizeof(PgStat_IO) is 16392 so that could be a concern for a high number of
> unused connection). OTOH, if a high number of connections is not used that might
> be worth to reduce the max_connections setting.

I was surprised by the size you mention so I went to look for the
definition of that struct:

typedef struct PgStat_IO
{
	TimestampTz stat_reset_timestamp;
	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
} PgStat_IO;

(It would be good to have more than zero comments here BTW)

So what's happening is that struct PgStat_IO stores the data for every
single process type, be it regular backends, backend-like auxiliary
processes, and all other potential postmaster children.  So it seems to
me that storing one of these struct for "my backend stats" is wrong: I
think you should be using PgStat_BktypeIO instead (or maybe another
struct which has a reset timestamp plus BktypeIO, if you care about the
reset time).  That struct is only 1024 bytes long, so it seems much more
reasonable to have one of those per backend.

Another way to think about this might be to say that the B_BACKEND
element of the PgStat_Snapshot.io array should really be spread out to
MaxBackends separate elements.  This would make it more difficult to
obtain a total value accumulating ops done by all backends (since it's
require to sum the values of each backend), but it allows you to obtain
backend-specific values, including those of remote backends rather than
only your own, as Kyotaro suggests.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/