Hi Paul,
> 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
I agree with you that on POWER the dcbz is probably not helping.
On PowerPC my experience is different.
>From what I have seen DCBZ help enormously on 970,PA-Semi and CELL.
Cheers
Gunnar
Paul Mackerras
<[EMAIL PROTECTED]
> To
Gunnar von
24/06/2008 01:49 Boehn/Germany/Contr/[EMAIL PROTECTED]
cc
[EMAIL PROTECTED], Mark Nelson
<[EMAIL PROTECTED]>,
[email protected], Michael
Ellerman <[EMAIL PROTECTED]>,
[EMAIL PROTECTED], Arnd
Bergmann <[EMAIL PROTECTED]>
Subject
Re: [RFC 1/3] powerpc:
__copy_tofrom_user tweaked for Cell
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.
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