From owner-freebsd-stable@FreeBSD.ORG  Thu May  1 05:15:56 2008
Return-Path: <owner-freebsd-stable@FreeBSD.ORG>
Delivered-To: stable@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 263CF10656D1;
	Thu,  1 May 2008 05:15:56 +0000 (UTC)
	(envelope-from bright@elvis.mu.org)
Received: from elvis.mu.org (elvis.mu.org [192.203.228.196])
	by mx1.freebsd.org (Postfix) with ESMTP id 128768FC1D;
	Thu,  1 May 2008 05:15:55 +0000 (UTC)
	(envelope-from bright@elvis.mu.org)
Received: by elvis.mu.org (Postfix, from userid 1192)
	id 325491A4D82; Wed, 30 Apr 2008 21:57:36 -0700 (PDT)
Date: Wed, 30 Apr 2008 21:57:36 -0700
From: Alfred Perlstein <alfred@freebsd.org>
To: gnn@freebsd.org
Message-ID: <20080501045736.GU30325@elvis.mu.org>
References: <m2lk2ultc1.wl%gnn@neville-neil.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <m2lk2ultc1.wl%gnn@neville-neil.com>
User-Agent: Mutt/1.4.2.3i
Cc: stable@freebsd.org
Subject: Re: Some odd behaviour of vmstat and vmtotal...
X-BeenThere: freebsd-stable@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Production branch of FreeBSD source code <freebsd-stable.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-stable>, 
	<mailto:freebsd-stable-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-stable>
List-Post: <mailto:freebsd-stable@freebsd.org>
List-Help: <mailto:freebsd-stable-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-stable>,
	<mailto:freebsd-stable-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 01 May 2008 05:15:56 -0000

* gnn@freebsd.org <gnn@freebsd.org> [080430 18:13] wrote:
> Howdy,
> 
> In deploying 7.0 at work we were finding a persistent problem when
> running "vmstat 1" on systems.  The problem shows up as a 10ms "pause"
> in processing, usually packet stamping and forwarding by a user level
> process.  This is due to the fact that vmstat calls the vmtotal()
> routine which in turn does a whole lot of locking.  The vmtotal call
> locks and walks the VM object list twice in a mark and sweep
> operation.
> 
> So, the question is, "What is the right way to fix this?"  I could
> remove the locking since the O_ACTIVE bit is not used by any other
> routine besides vmtotal, but I'm not too happy about that.
> 
> The relevant code can be found in src/sys/vm/vm_meter.c in vmtotal().
> 
> Thoughts?

Can you _really_ remove the locking?  I don't think that would be safe
if the list is being manipulated, but I haven't checked.

Typically one uses a sentinal to get around such problems.  Basically
insert a dummy object that no one should touch other than you, then
drop the lock periodically and insert it into the list, then resume.

-- 
- Alfred Perlstein