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

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Message-ID: <20080126001128.GB24559@elte.hu>
Date:	Sat, 26 Jan 2008 01:11:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Keir Fraser <Keir.Fraser@...cam.ac.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	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()

* 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)

	Ingo
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