linux-kernel - Re: follow_page() performance regressions

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Message-ID: <alpine.LFD.1.10.0807010837510.20870@hp.linux-foundation.org>
Date:	Tue, 1 Jul 2008 08:43:26 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	Hideo Saito <saito@...san.co.jp>, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: follow_page() performance regressions



On Tue, 1 Jul 2008, Paul Mundt wrote:
> 
> So yes, the ZERO_PAGE handling in follow_page() is causing the slow-down here.

I suspect it could be any of:

 - actual code generation differences in follow_page() (some SH person 
   needs to check that)

 - some cache issue on SH. ZERO_PAGE is a single page at a fixed virtual 
   address, while it used to populate the page tables with individual 
   pages. Normally, this should be *better* for caching, but maybe there 
   is some conflict? What kind of caches does SH have (virtually indexed?)

 - hackbench relying on follow_page() to populate the page tables, which 
   it no longer does for anonymous areas (using ZERO_PAGE directly 
   instead).

   Again, normally this would speed things up (fewer TLB misses etc), but 
   if it then causes a new page fault that used to have been covered by 
   follow_page(), who knows?

What does hackbench actually do? Anybody?

			Linus
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