prefetch

From: Linus Torvalds <[email protected]>
Newsgroups: fa.linux.kernel
Subject: Re: [patch] epoll use a single inode ...
Date: Wed, 07 Mar 2007 22:58:57 UTC
Message-ID: <fa.xioENCdH8/[email protected]>

On Wed, 7 Mar 2007, Anton Blanchard wrote:
>
> Funny you mention this. We found some noticeable ppc64 regressions when
> moving the dcache to standard list macros and had to do this to fix it
> up:
>
> static inline void prefetch(const void *x)
> {
>         if (unlikely(!x))
>                 return;
>
>         __asm__ __volatile__ ("dcbt 0,%0" : : "r" (x));
> }
>
> Urgh :)

Yeah, I'm not at all surprised. Any implementation of "prefetch" that
doesn't just turn into a no-op if the TLB entry doesn't exist (which makes
them weaker for *actual* prefetching) will generally have a hard time with
a NULL pointer. Exactly because it will try to do a totally unnecessary
TLB fill - and since most CPU's will not cache negative TLB entries, that
unnecessary TLB fill will be done over and over and over again..

In general, using software prefetching is just a stupid idea, unless

 - the prefetch really is very strict (ie for a linked list you do exactly
   the above kinds of things to make sure that you don't try to prefetch
   the non-existent end entry)
AND
 - the CPU is stupid (in-order in particular).

I think Intel even suggests in their optimization manuals to *not* do
software prefetching, because hw can usually simply do better without it.

		Linus