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

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Message-ID: <4A2E860D.8070903@redhat.com>
Date:	Tue, 09 Jun 2009 18:55:57 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native
 kernels

Ingo Molnar wrote:
> * 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 ...
>   

The tlb already content-addresses entries when looking up translations, 
so it shouldn't be that bad.

invlpg does have to invalidate all the intermediate entries 
("paging-structure caches"), and it does (obviously) force a tlb reload.

I seem to recall 50 cycles for invlpg, what do you characterize as 
unreasonably slow?

-- 
error compiling committee.c: too many arguments to function

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