linux-kernel - Re: [tip:x86/urgent] x86 idle: Repair large-server 50-watt idle-power regression

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Message-ID: <1395102010.2474.21.camel@buesod1.americas.hpqcorp.net>
Date:	Mon, 17 Mar 2014 17:20:10 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, hpa@...ux.intel.com, len.brown@...el.com,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86 idle: Repair large-server 50-watt
 idle-power regression

On Thu, 2013-12-19 at 11:51 -0800, tip-bot for Len Brown wrote:
> Commit-ID:  40e2d7f9b5dae048789c64672bf3027fbb663ffa
> Gitweb:     http://git.kernel.org/tip/40e2d7f9b5dae048789c64672bf3027fbb663ffa
> Author:     Len Brown <len.brown@...el.com>
> AuthorDate: Wed, 18 Dec 2013 16:44:57 -0500
> Committer:  H. Peter Anvin <hpa@...ux.intel.com>
> CommitDate: Thu, 19 Dec 2013 11:47:39 -0800
> 
> x86 idle: Repair large-server 50-watt idle-power regression

FYI this commit can cause some non trivial performance regressions for
larger core count systems. While not surprising because of the nature of
the change, having intel_idle do more cacheline invalidations, I still
wanted to let you guys know. For instance, on a 160 core Westmere
system, aim7 throughput can go down in a number of tests, anywhere from
-10% to -25%.

I guess it comes down to one of those performance vs energy things. And
sure, max_cstate can be set to overcome this, but it's still something
that was previously taken for granted.

Thanks,
Davidlohr

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