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Date: Thu, 9 Oct 2008 13:36:41 +1100
From: Paul Mackerras <paulus@samba.org>
To: Scott Wood <scottwood@freescale.com>
Subject: Re: performance: memcpy vs. __copy_tofrom_user
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Scott Wood writes:

> I'm not sure why we don't use dcbt in memcpy(), as it's just ignored if 
> the memory is cache-inhibited.

Both dcbt and dcbz tend to slow things down if the relevant block is
already in the cache.  Since the kernel memcpy is mostly used for
copies that are only 1 or a small number of cache lines long, it's not
clear that the benefit of dcbt and/or dcbz would outweigh the cost.
And anyway, I have yet to be convinced that optimizing memcpy would
provide a measureable benefit.

Paul.