netdev - Re: [PATCH net-2.6.26] fib_trie: RCU optimizations

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Date:	Fri, 21 Mar 2008 15:49:33 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	shemminger@...tta.com, paulmck@...ux.vnet.ibm.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-2.6.26] fib_trie: RCU optimizations

From: Eric Dumazet <dada1@...mosbay.com>
Date: Fri, 21 Mar 2008 18:44:33 +0100

> Loop is so small that prefetches hints are throwed away by CPU, or the 
> cost to setup the prefetch(register + offset) is too expensive...

That's been my experience as well.

For this reason I consider the unconditional prefetches in the
linux/list.h list iteration macros troublesome.

Most loops are small so by default those macros should not prefetch,
and when you know you have a large enough loop (and thus enough work
to make the prefetch actually matter) you use a specific specially
named variant of the list traversal macro.

Oh yes, and you have to provide performance improvement proof before
submitting a patch that uses the prefetch-using list macro variants.
:-)

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