linux-kernel - Re: [patch] block layer: kmemcheck fixes

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Date:	Fri, 08 Feb 2008 14:56:09 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Nick Piggin <npiggin@...e.de>
CC:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	mingo@...e.hu, jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	Alan.Brunelle@...com, dgc@....com, akpm@...ux-foundation.org,
	vegard.nossum@...il.com, penberg@...il.com
Subject: Re: [patch] block layer: kmemcheck fixes

Nick Piggin wrote:
>>> Maybe cpus these days have so much store bandwith that doing
>>> things like the above is OK, but I doubt it :-)
>> on modern x86 cpus the memset may even be faster if the memory isn't in 
>> cache;
>> the "explicit" method ends up doing Write Allocate on the cache lines
>> (so read them from memory) even though they then end up being written 
>> entirely.
>> With memset the CPU is told that the entire range is set to a new value, and
>> the WA can be avoided for the whole-cachelines in the range.
> 
> Don't you have write combining store buffers? Or is it still speculatively
> issuing the reads even before the whole cacheline is combined?

x86 memory order model doesn't allow that quite; and you need a "series" of at least 64 bytes
without any other memory accesses in between even if it would....
not happening in practice.
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