linux-kernel - Re: [PATCH RFC 2/2] [x86] Optimize copy_page by re-arranging instruction sequence and saving register

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Date:	Fri, 12 Oct 2012 08:18:13 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"Ma, Ling" <ling.ma@...el.com>
Cc:	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, "hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/2] [x86] Optimize copy_page by re-arranging
 instruction sequence and saving register

On Fri, Oct 12, 2012 at 03:37:50AM +0000, Ma, Ling wrote:
> > > Load and write operation occupy about 35% and 10% respectively for
> > > most industry benchmarks. Fetched 16-aligned bytes code include about
> > > 4 instructions, implying 1.34(0.35 * 4) load, 0.4 write.
> > > Modern CPU support 2 load and 1 write per cycle, so throughput from
> > > write is bottleneck for memcpy or copy_page, and some slight CPU only
> > > support one mem operation per cycle. So it is enough to issue one
> > read
> > > and write instruction per cycle, and we can save registers.
> > 
> > So is that also true for AMD CPUs?
> Although Bulldozer put 32byte instruction into decoupled 16byte entry buffers,
> it still decode 4 instructions per cycle, so 4 instructions will be fed into execution unit and
> 2 loads ,1 write will be issued per cycle.

I'd be very interested with what benchmarks are you seeing that perf
improvement on Atom and who knows, maybe I could find time to run them
on Bulldozer and see how your patch behaves there :-).

Thanks.

-- 
Regards/Gruss,
    Boris.
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