linux-kernel - Re: Should we be using unlikely() around tests of GFP

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Message-ID: <20110103135815.GA6024@thunk.org>
Date:	Mon, 3 Jan 2011 08:58:15 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	David Rientjes <rientjes@...gle.com>, npiggin@...nel.dk
Subject: Re: Should we be using unlikely() around tests of GFP_ZERO?

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
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