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

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Date:	Fri, 8 Feb 2008 23:44:27 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Arjan van de Ven <arjan@...ux.intel.com>
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

On Fri, Feb 08, 2008 at 07:09:07AM -0800, Arjan van de Ven wrote:
> David Miller wrote:
> >From: Linus Torvalds <torvalds@...ux-foundation.org>
> >Date: Thu, 7 Feb 2008 09:42:56 -0800 (PST)
> >
> >>Can we please just stop doing these one-by-one assignments, and just do 
> >>something like
> >>
> >>	memset(rq, 0, sizeof(*rq));
> >>	rq->q = q;
> >>	rq->ref_count = 1;
> >>	INIT_HLIST_NODE(&rq->hash);
> >>	RB_CLEAR_NODE(&rq->rb_node);
> >>
> >>instead?
> >>
> >>The memset() is likely faster and smaller than one-by-one assignments 
> >>anyway, even if the one-by-ones can avoid initializing some field or 
> >>there ends up being a double initialization..
> >
> >The problem is store buffer compression.  At least a few years
> >ago this made a huge difference in sk_buff initialization in the
> >networking.
> >
> >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?

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