linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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Message-ID: <20090609145736.GA31535@elte.hu>
Date:	Tue, 9 Jun 2009 16:57:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native
	kernels


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> I was benchmarking btrfs on my little EeePC. There, kmap overhead 
> was 25% of file access time. Part of it is that people have been 
> taught to use "kmap_atomic()", which is usable under spinlocks and 
> people have been told that it's "fast". It's not fast. The whole 
> TLB thing is slow as hell.

yeah. I noticed it some time ago that INVLPG is unreasonably slow.

My theory is that in the CPU it's perhaps a loop (in microcode?) 
over _all_ TLBs - so as TLB caches get larger, INVLPG gets slower 
and slower ...

	Ingo
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