linux-kernel - Re: [LKP] [lkp] [x86/hweight] 65ea11ec6a: will-it-scale.per_process

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Message-ID: <ABE9E34B-8A07-4A82-B7A0-BA340FE2C6B2@zytor.com>
Date:	Wed, 17 Aug 2016 20:57:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...e.de>, "Huang, Ying" <ying.huang@...el.com>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>, lkp@...org,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Ville Syrjälä <ville.syrjala@...ux.intel.com>
Subject: Re: [LKP] [lkp] [x86/hweight] 65ea11ec6a: will-it-scale.per_process_ops 9.3% improvement

On August 17, 2016 8:45:13 PM PDT, Borislav Petkov <bp@...e.de> wrote:
>On Wed, Aug 17, 2016 at 03:29:04PM -0700, Huang, Ying wrote:
>> branch-miss-rate decreased from ~0.30% to ~0.043%.
>> 
>> So I guess there are some code alignment change, which caused
>decreased
>> branch miss rate.
>
>Hrrm, I still can't imagine how that would happen if the machine
>supports POPCNT and we never call the __sw_hweight* variants. Or does
>it?
>
>Can you paste /proc/cpuinfo from that Sandy Bridge-EP box?
>
>Thanks.

popcnt was introduced in Nehalem AFAIK, two generations before Sandy Bridge.
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