From: Rob Wultsch <wultsch@gmail.com>
To: Jignesh Shah <jkshah@gmail.com>
Cc: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Group commit and commit delay/siblings
Date: Sun, 5 Dec 2010 19:47:33 -0700
Message-ID: <AANLkTikOHMnjTZh94AT2nkeDGCyDyAzOhuDpQFzm9OSi@mail.gmail.com> (raw)
In-Reply-To: <AANLkTi=fUBnfR7K5uAQZkvaymuVHzUuCpeu4NOtLOMOU@mail.gmail.com>
References: <AANLkTik3dr076BMQ6MqCLZZ0x2-qxmQgWoqCR7c0ePoD@mail.gmail.com>
<AANLkTi=fUBnfR7K5uAQZkvaymuVHzUuCpeu4NOtLOMOU@mail.gmail.com>
On Sun, Dec 5, 2010 at 7:30 PM, Jignesh Shah <jkshah@gmail.com> wrote:
> The commit_siblings = 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=0 (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 I 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.
--
Rob Wultsch
wultsch@gmail.com
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To: pgsql-performance@postgresql.org
Cc: wultsch@gmail.com, jkshah@gmail.com
Subject: Re: Group commit and commit delay/siblings
In-Reply-To: <AANLkTikOHMnjTZh94AT2nkeDGCyDyAzOhuDpQFzm9OSi@mail.gmail.com>
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