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Date: Tue, 24 Jun 2008 09:49:33 +1000
From: Paul Mackerras <paulus@samba.org>
To: Gunnar von Boehn <VONBOEHN@de.ibm.com>
Subject: Re: [RFC 1/3] powerpc: __copy_tofrom_user tweaked for Cell
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Gunnar von Boehn writes:

> Interesting points.
> Can you help me to understand where the negative effect of DCBZ does come
> from?

In my experience, dcbz slows down the hot-cache case because it adds a
few cycles to the execution time of the inner loop, and on most 64-bit
PowerPC implementations, it doesn't actually help even in the
cold-cache case because the store queue does enough write combining
that the cache doesn't end up reading the line from memory.  I don't
know whether the Cell PPE can do that, but I could believe that it
can't.

Paul.