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

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Date:	Thu, 08 Mar 2007 11:57:25 -0500
From:	Valdis.Kletnieks@...edu
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>,
	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 ...

On Thu, 08 Mar 2007 08:24:04 +0100, Eric Dumazet said:
>
> 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.

You avoid those two, but add the very high likelyhood that a statement of the
form 'if (!x) {....}' will creep back in and bug out.  I doubt that changing
the form of a basic C idiom to save a few cycles is worth it, especially when
the (!x) form can be tested without a memory fetch, but (x != nil) may require
fetching 'nil' (remember - the x86 is a very popular chipset, but register
starved - if the loop is any size at all, 'nil' may require a reload each
time around, costing you the win you thought you had....

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