Date: Wed, 2 Feb 2005 22:06:20 -0500 (EST)
From: Rik van Riel <riel@redhat.com>
To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
cc: Christoph Lameter <clameter@sgi.com>,
       David Woodhouse <dwmw2@infradead.org>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, akpm@osdl.org
Subject: Re: A scrub daemon (prezeroing)
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On Wed, 2 Feb 2005, Marcelo Tosatti wrote:

> Someone should try implementing the zeroing driver for a fast x86 PCI 
> device. :)

I'm not convinced.  Zeroing a page takes 2000-4000 CPU
cycles, while faulting the page from RAM into cache takes
200-400 CPU cycles per cache line, or 6000-12000 CPU
cycles.

If the page is being used immediately after it is
allocated, it may be faster to prezero the page on
the fly.  On some CPUs these writes bypass the "read
from RAM" stage and allow things to just live in cache
completely.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan
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