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List:       linux-kernel
Subject:    Re: [patch] Prefetching file_read_actor()
From:       Ryan Cumming <bodnar42 () phalynx ! dhs ! org>
Date:       2001-12-31 20:40:19
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On December 31, 2001 11:47, Dave Jones wrote:
> Completly puzzled right now. Moving the prefetching to copy_to_user
> (and doing the tlb preload & prefetching the whole chunk to be copied
> (or cachesize if smaller)) results in a performance drop instead of a win.
>
> My initial guess is that some of the callers of copy_to_user are
> doing something that is harmed the prefetching.
> (Maybe they are doing additional prefetch() calls)

Maybe syscalls that only have to move a very small chunk of data 
(gettimeofday(2), for instance), are hurt because of the wasted bytes they 
are prefetching after the intended data? Also, for sizes greater than 512, 
copy_to_user will call mmx_copy_user, which might call mmx_memcpy, which does 
prefetching already on x86 CPUs that support it.

-Ryan
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