linux-kernel - Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

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Date:	Thu, 13 Nov 2008 17:20:08 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, lguest@...abs.org,
	jeremy@...source.com, Steven Rostedt <srostedt@...hat.com>,
	Mike Travis <travis@....com>
Subject: Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

Nick Piggin wrote:
> 
> I heard from an Intel hardware engineer that Nehalem has some
> really fancy logic in it to make locked instructions "free", that
> was nacked from earlier CPUs because it was too costly. So obviously
> it is taking a fair whack of transistors or power for them to do it.
> And even then it is far from free, but still seems to be one or two
> orders of magnitude more expensive than a regular instruction.
> 

Last I heard it was still a dozen-ish cycles even on Nehalem.

> 
> IMO, we shouldn't stop bothering about LOCK prefix in the forseeable
> future.
> 

Even if a CPU came out *today* that had zero-cost locks we'd have to
worry about it for at least another 5-10 years.  The good news is that
we're doing pretty good with it for now, but I don't believe in general
we can avoid the fact that improving LOCK performance helps everything
when you're dealing with large numbers of cores/threads.

	-hpa

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