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

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Message-ID: <20070309025102.GC24695@kryten>
Date:	Thu, 8 Mar 2007 20:51:02 -0600
From:	Anton Blanchard <anton@...ba.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Kyle Moffett <mrmacman_g4@....com>,
	"Michael K. Edwards" <medwards.linux@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.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 ...


> OK, 200 cycles...
> 
> But what is the cost of the conditional branch you added in prefetch(x) ?

Much less than the tablewalk. On ppc64 a tablewalk of an address that is
not populated in the hashtable will incur 2 cacheline lookups (primary
and secondary buckets). This plus the MMU state machine overhead adds up.

Cue Linus rant about PowerPC MMU :)

Anton
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