linux-kernel - Re: x86 memcpy performance

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Message-ID: <CAObL_7HMTDE5hYBs_C2esuOExOvK1akhxvn4MCaUyK6vPTmHJg@mail.gmail.com>
Date:	Mon, 15 Aug 2011 16:08:19 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Borislav Petkov <bp@...en8.de>, Andrew Lutomirski <luto@....edu>,
	melwyn lobo <linux.melwyn@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Mon, Aug 15, 2011 at 4:05 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Aug 15, 2011 at 03:11:40PM -0400, Andrew Lutomirski wrote:
>> > Well, copy_from_user... does a bunch of rep; movsq - if the SSE version
>> > shows reasonable speedup there, we might need to make those work too.
>>
>> I'm a little surprised that SSE beats fast string operations, but I
>> guess benchmarking always wins.
>
> If by fast string operations you mean X86_FEATURE_ERMS, then that's
> Intel-only and that actually would need to be benchmarked separately.
> Currently, I see speedup for large(r) buffers only vs rep; movsq. But I
> dunno about rep; movsb's enhanced rep string tricks Intel does.

I meant X86_FEATURE_REP_GOOD.  (That may also be Intel-only, but it
sounds like rep;movsq might move whole cachelines on cpus at least a
few generations back.)  I don't know if any ERMS cpus exist yet.
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