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

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Message-ID: <45EFBA14.9090307@cosmosbay.com>
Date:	Thu, 08 Mar 2007 08:24:04 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Kyle Moffett <mrmacman_g4@....com>
CC:	"Michael K. Edwards" <medwards.linux@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Anton Blanchard <anton@...ba.org>,
	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 ...

Kyle Moffett a écrit :
> 
> Prefetching is also fairly critical on a Power4 or G5 PowerPC system as 
> they have a long memory latency; an L2-cache miss can cost 200+ cycles.  
> On such systems the "dcbt" prefetch instruction brings in a single 
> 128-byte cacheline and has no serializing effects whatsoever, making it 
> ideal for use in a linked-list-traversal inner loop.

OK, 200 cycles...

But what is the cost of the conditional branch you added in prefetch(x) ?

if (!x) return;

(correctly predicted or not, but do powerPC have a BTB ?)

About the NULL 'potential problem', maybe we could use a dummy nil (but 
mapped) object, and use its address in lists, ie compare for &nil instead of 
NULL. This would avoid :

- The conditional test in some prefetch() implementations
- The potential TLB problem with the NULL value.




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