To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Michael Fuhr <mike@fuhr.org>,
	Merlin Moncure <merlin.moncure@rcsonline.com>,
	Carlos Benkendorf <carlosbenkendorf@yahoo.com.br>,
	PostgreSQL-patches <pgsql-patches@postgresql.org>
Subject: Re: Stats collector performance improvement 
In-reply-to: <200601021840.k02Ieed21704@candle.pha.pa.us> 
References: <200601021840.k02Ieed21704@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Mon, 02 Jan 2006 13:40:40 -0500"
Date: Mon, 02 Jan 2006 13:45:21 -0500
Message-ID: <7838.1136227521@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> I found if I put a pg_usleep(100) in the buffer process the backend
> speed was good, but packets were lost.  What I found worked well was to
> do multiple recv() calls in a loop.  The previous code did a select(),
> then perhaps a recv() and pipe write() based on the results of the
> select().  This caused many small packets to be written to the pipe and
> the pipe write overhead seems fairly large.  The best fix I found was to
> loop over the recv() call at most 25 times, collecting a group of
> packets that can then be sent to the collector in one pipe write.  The
> recv() socket is non-blocking, so a zero return indicates there are no
> more packets available.  Patch attached.

This seems incredibly OS-specific.  How many platforms did you test it
on?

A more serious objection is that it will cause the stats machinery to
work very poorly if there isn't a steady stream of incoming messages.
You can't just sit on 24 messages until the 25th one arrives next week.

			regards, tom lane