linux-kernel - Re: [rfc patch] i386: don't save eflags on task switch

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Message-ID: <20061105035556.GQ9057@kvack.org>
Date:	Sat, 4 Nov 2006 22:55:56 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Zachary Amsden <zach@...are.com>
Cc:	Chuck Ebbert <76306.1226@...puserve.com>,
	Linus Torvalds <torvalds@...l.org>, Andi Kleen <ak@...e.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [rfc patch] i386: don't save eflags on task switch

On Sat, Nov 04, 2006 at 11:09:42AM -0800, Zachary Amsden wrote:
> Every processor I've ever measured it on, popf is slower.  On P4, for 
> example, pushf is 6 cycles, and popf is 54.  On Opteron, it is 2 / 12.  
> On Xeon, it is 7 / 91.

pushf has to wait until all flag dependancies can be resolved.  On the 
P4 with >100 instructions in flight, that can take a long time.  Popf 
on the other hand has no dependancies on outstanding instructions as it 
resets the machine state.

		-ben
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