From owner-freebsd-current@FreeBSD.ORG  Sun Jan 30 18:59:58 2005
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id 9153816A4CE; Sun, 30 Jan 2005 18:59:58 +0000 (GMT)
Received: from cyrus.watson.org (cyrus.watson.org [204.156.12.53])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 6029B43D1D; Sun, 30 Jan 2005 18:59:58 +0000 (GMT)
	(envelope-from robert@fledge.watson.org)
Received: from fledge.watson.org (fledge.watson.org [204.156.12.50])
	by cyrus.watson.org (Postfix) with SMTP id B846646B0D;
	Sun, 30 Jan 2005 13:59:57 -0500 (EST)
Date: Sun, 30 Jan 2005 18:59:20 +0000 (GMT)
From: Robert Watson <rwatson@FreeBSD.org>
X-Sender: robert@fledge.watson.org
To: Colin Percival <cperciva@freebsd.org>
In-Reply-To: <41FD24C2.5070700@freebsd.org>
Message-ID: <Pine.NEB.3.96L.1050130185829.15336L-100000@fledge.watson.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
cc: Poul-Henning Kamp <phk@phk.freebsd.dk>
cc: silby@freebsd.org
cc: current@freebsd.org
Subject: Re: tcp_isn_tick() / dummynet() callout madness ?
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sun, 30 Jan 2005 18:59:58 -0000

On Sun, 30 Jan 2005, Colin Percival wrote:

> Robert Watson wrote:
> > since the callout_reset() is one of the more
> > expensive parts of this code, Colin has been looking at some locking
> > optimizations to lower the cost.
> 
> To elaborate somewhat: I think I can avoid the spinlock cost when
> callouts reset themselves (which is the case here).  However, while this
> will reduce the time spent in the callouts themselves, it's really only
> a 50% solution -- softclock locks and unlocks the callout spin lock each
> time it launches a callout.  If we're spending 5% of our cpu time in
> these two callouts, then they're actually responsible for using 10% of
> our cpu time; I think I can cut that in half, but in the end we can't
> avoid the cost of a mtx_lock_spin / mtx_unlock_spin pair (in softclock)
> for each callout. 

On some further iteration, it transpired that Poul-Henning's configuration
included WITNESS, and without that things look a bit more reasonable.  We
should still run the callout less often, if we can, and I think the
optimization is useful.

Robert N M Watson