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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()
Date: Mon, 2 Mar 2009 17:18:11 +1100
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Cc: Arjan van de Ven <arjan@infradead.org>, Andi Kleen <andi@firstfloor.org>,
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On Monday 02 March 2009 15:46:03 H. Peter Anvin wrote:
> Nick Piggin wrote:
> > I would expect any high performance CPU these days to combine entries
> > in the store queue, even for normal store instructions (especially for
> > linear memcpy patterns). Isn't this likely to be the case?
>
> Actually, that is often not the case simply because it doesn't buy that
> much.  The big win comes when you don't read a whole cache line in from
> memory, but that is a property of the cache, not the store queue.

Hm, maybe I'm confused. As far as I thought, you could avoid the
RMW write allocate behaviour by bypassing the cache on a store
miss, or combining stores into cacheline blocks before they leave
the store queue.