linux-kernel - Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud

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Message-ID: <479A7CCB.7070501@zytor.com>
Date:	Fri, 25 Jan 2008 16:20:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Keir Fraser <Keir.Fraser@...cam.ac.uk>,
	LKML <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...e.de>,
	Jan Beulich <jbeulich@...ell.com>,
	Eduardo Pereira Habkost <ehabkost@...hat.com>,
	Ian Campbell <ijc@...lion.org.uk>,
	William Irwin <wli@...omorphy.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()

Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
>> Is there any guide about the tradeoff of when to use invlpg vs 
>> flushing the whole tlb?  1 page?  10?  90% of the tlb?
> 
> i made measurements some time ago and INVLPG was quite uniformly slow on 
> all important CPU types - on the order of 100+ cycles. It's probably 
> microcode. With a cr3 flush being on the order of 200-300 cycles (plus 
> any add-on TLB miss costs - but those are amortized quite well as long 
> as the pagetables are well cached - which they usually are on today's 
> 2MB-ish L2 caches), the high cost of INVLPG rarely makes it worthwile 
> for anything more than a few pages.
> 
> so INVLPG makes sense for pagetable fault realated single-address 
> flushes, but they rarely make sense for range flushes. (and that's how 
> Linux uses it)
> 

Incidentally, as far as I can tell, the main INVLPG is so slow is 
because of its painful behaviour with regards to large pages which may 
have been split by hardware.

	-hpa
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