linux-kernel - Re: RFC: remove __read

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Message-Id: <20071217023339.ce3da56c.akpm@linux-foundation.org>
Date:	Mon, 17 Dec 2007 02:33:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Kyle McMartin <kyle@...artin.ca>, Adrian Bunk <bunk@...nel.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: RFC: remove __read_mostly

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

> Kyle McMartin <kyle@...artin.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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