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 03:37:50 +0000
From:	"Ma, Ling" <ling.ma@...el.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
CC:	"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

> > 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.

Thanks
Ling
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