linux-kernel - Re: Specific support for Intel Atom architecture

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Message-ID: <20090514172901.GH10933@one.firstfloor.org>
Date:	Thu, 14 May 2009 19:29:01 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ulrich Drepper <drepper@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Harvey Harrison <harvey.harrison@...il.com>,
	Tobias Doerffel <tobias.doerffel@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Specific support for Intel Atom architecture

On Thu, May 14, 2009 at 09:19:38AM -0700, Ulrich Drepper wrote:
> On Thu, May 14, 2009 at 7:01 AM, Andi Kleen <andi@...stfloor.org> wrote:
> > How would you test that?
> 
> Compare runtimes with mov+bswap for some simple code which uses the
> value after the conversion (e.g., just add to something).
> 
> Or in your case: get the Atom designers to comment.

Don't really need Atom designers; you can prove or disprove my theory
(that they generate the same uops sequence) by checking the uops performance
counter for a micro benchmark.

However even if that was not the case I have some doubts the
kernel is doing enough endian conversions that it really matters.

For example the network stack is doing maybe 4-5 endian conversions
(very conservative estimate) per packet and processing a packet
takes tens of thousands of cycles. But at best you could save 1-2 cycles
this way, so even if you save a few cycles this way it will be very likely 
in the noise.

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