linux-kernel - Re: [patch] epoll use a single inode ...

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Message-ID: <Pine.LNX.4.64.0703071453080.5963@woody.linux-foundation.org>
Date:	Wed, 7 Mar 2007 14:57:27 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anton Blanchard <anton@...ba.org>
cc:	Eric Dumazet <dada1@...mosbay.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Avi Kivity <avi@...o.co.il>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [patch] epoll use a single inode ...



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
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