Re: RFC: remove __read_mostly [LWN.net]

From:		Andrew Morton <akpm-AT-linux-foundation.org>
To:		Andi Kleen <andi-AT-firstfloor.org>
Subject:		Re: RFC: remove __read_mostly
Date:		Mon, 17 Dec 2007 02:33:39 -0800
Message-ID:		<20071217023339.ce3da56c.akpm@linux-foundation.org>
Cc:		Kyle McMartin <kyle-AT-mcmartin.ca>, Adrian Bunk <bunk-AT-kernel.org>, linux-kernel-AT-vger.kernel.org, linux-arch-AT-vger.kernel.org
Archive-link:		Article, Thread

On Fri, 14 Dec 2007 01:33:45 +0100 Andi Kleen <andi@firstfloor.org> wrote:

> Kyle McMartin <kyle@mcmartin.ca> writes:
> 
> > I'd bet, in the __read_mostly case at least, that there's no
> > improvement in almost all cases.
> 
> I bet you're wrong. Cache line behaviour is critical, much more
> than pipeline behaviour (which unlikely affects). That is because
> if you eat a cache miss it gets really expensive, which e.g.
> a mispredicted jump is relatively cheap in comparison. We're talking
> one or more orders of magnitude.

So...  once we've moved all read-mostly variables into __read_mostly, what
is left behind in bss?

All the write-often variables.  All optimally packed together to nicely
maximise cacheline sharing.
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