From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755318Ab1ACOBu (ORCPT <rfc822;w@1wt.eu>);
	Mon, 3 Jan 2011 09:01:50 -0500
Received: from THUNK.ORG ([69.25.196.29]:56680 "EHLO thunker.thunk.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1754875Ab1ACOBt (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 3 Jan 2011 09:01:49 -0500
Date: Mon, 3 Jan 2011 08:58:15 -0500
From: "Ted Ts'o" <tytso@mit.edu>
To: Pekka Enberg <penberg@kernel.org>
Cc: Minchan Kim <minchan.kim@gmail.com>,
        Christoph Lameter <cl@linux-foundation.org>,
        Pekka Enberg <penberg@cs.helsinki.fi>, Matt Mackall <mpm@selenic.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>,
        David Rientjes <rientjes@google.com>, npiggin@kernel.dk
Subject: Re: Should we be using unlikely() around tests of GFP_ZERO?
Message-ID: <20110103135815.GA6024@thunk.org>
Mail-Followup-To: Ted Ts'o <tytso@mit.edu>,
	Pekka Enberg <penberg@kernel.org>,
	Minchan Kim <minchan.kim@gmail.com>,
	Christoph Lameter <cl@linux-foundation.org>,
	Pekka Enberg <penberg@cs.helsinki.fi>,
	Matt Mackall <mpm@selenic.com>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
	David Rientjes <rientjes@google.com>, npiggin@kernel.dk
References: <E1PZXeb-0004AV-2b@tytso-glaptop>
 <AANLkTi=9ZNk6w8PxvveWHy5+okfTyKUj3L2ywFOuFjoq@mail.gmail.com>
 <AANLkTinz52Ky5BhU-gHq8vx9=1uoN+iuDn1f0C8fnSjQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <AANLkTinz52Ky5BhU-gHq8vx9=1uoN+iuDn1f0C8fnSjQ@mail.gmail.com>
User-Agent: Mutt/1.5.20 (2009-06-14)
X-SA-Exim-Connect-IP: <locally generated>
X-SA-Exim-Mail-From: tytso@thunk.org
X-SA-Exim-Scanned: No (on thunker.thunk.org); SAEximRunCond expanded to false
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jan 03, 2011 at 09:40:57AM +0200, Pekka Enberg wrote:
> I guess the rationale here is that if you're going to take the hit of
> memset() you can take the hit of unlikely() as well. We're optimizing
> for hot call-sites that allocate a small amount of memory and
> initialize everything themselves. That said, I don't think the
> unlikely() annotation matters much either way and am for removing it
> unless people object to that.

I suspect for many slab caches, all of the slab allocations for a
given slab cache type will have the GFP_ZERO flag passed.  So maybe it
would be more efficient to zap the entire page when it is pressed into
service for a particular slab cache, so we can avoid needing to use
memset on a per-object basis?

						- Ted