netdev - Re: [PATCH] netfilter: unfold two critical loops in ip_packet

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Message-ID: <20090130172705.GB18453@one.firstfloor.org>
Date:	Fri, 30 Jan 2009 18:27:05 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: [PATCH] netfilter: unfold two critical loops in ip_packet_match()

On Fri, Jan 30, 2009 at 05:54:10PM +0100, Eric Dumazet wrote:
> Andi Kleen a écrit :
> > Eric Dumazet <dada1@...mosbay.com> writes:
> > 
> >> While doing oprofile tests I noticed two loops are not properly unrolled by gcc
> > 
> > That's because nobody passed -funroll-loops. Did you try that for
> > that file? Likely will need -O2 too
> 
> I dont want to unroll all loops, only those two :)

gcc 4.4 will have a way to do that per function, but earlier
you would need to move it to a separate file and specify
the option only for that.

Doing so would be still a good idea compared to your
patch because the code will be cleaner and might
be more adaptable to future architectures
(such manual tunings tend to outdate)

> I wish gcc (4.3.2 here) was litle bit smarter :(

It cannot do much without profile feedback because
it has no clue which loops are hot and which are not.

> (By the way, I still use the patch on arch/x86/oprofile/op_model_ppro.c
> to have a working oprofile on my dev machine...)

Yes I know, sorry for that.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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