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From: Rob Wultsch <wultsch@gmail.com>
Date: Sun, 5 Dec 2010 19:47:33 -0700
Message-ID: <AANLkTikOHMnjTZh94AT2nkeDGCyDyAzOhuDpQFzm9OSi@mail.gmail.com>
Subject: Re: Group commit and commit delay/siblings
To: Jignesh Shah <jkshah@gmail.com>
Cc: postgres performance list <pgsql-performance@postgresql.org>
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On Sun, Dec 5, 2010 at 7:30 PM, Jignesh Shah <jkshah@gmail.com> wrote:
> The commit_siblings =3D 5 basically checks that it sleeps only when that
> many backends are active. This I think is a very expensive check and I
> would rather make commit_siblings=3D0 (which the current code does not
> support.. it only supports minimum of 1) The check is expensive
> irrespective of the settings .. But anyway here is the real kicker.
> In all the tests I did with recent verions 8.4 and version 9.0 , it
> seems that the default behavior handles the load well enough and one
> does not have to use commit_delay at all. Since when the load is very
> high all of them are basically in sync phase and the desired thing
> happens anyway.
>
> Infact using commit_delay will actually add the cost of doing
> commit_siblings check and can hurt the performance by increasing CPU
> consumption.. Doing commit_siblings check for every transaction is a
> killer since it does not return after meeting the minimum backends and
> goes through every backend to calculate the total number before
> comparing with the minimum. This is probably why most people see a
> drop in performance when using commit_delay compared to the default.
>
> Anyway =A0I would recommended right now to stick with the default and
> not really use it. It does the sync absorbtion well if you have two
> many users (though not perfect).

Sounds like this setting should go away unless there is a very good
reason to keep it.


--=20
Rob Wultsch
wultsch@gmail.com