linux-kernel - Re: [patch] tlb flush_data: replace per

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Date:	Mon, 12 Jan 2009 23:50:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Frederik Deweerdt <frederik.deweerdt@...og.eu>, tglx@...utronix.de,
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [patch] tlb flush_data: replace per_cpu with an array


* Andi Kleen <andi@...stfloor.org> wrote:

> > It is also slower (or so percpu.h says), and confusing I'd say.
> 
> Well it's something like 3 instructions versus one. [...]

That's enough - micro-optimizations are done like that, instruction by 
instruction.

> [...] You would have a hard time benchmarking it unless you run it in a 
> very tight loop. It will be lost in the noise compared to all the other 
> costs of the IPI.
> 
> Also why i don't like this patch is that on the typical small 
> single/dual core system running a 128 byte cache line distro kernel you 
> always pay the 1K cost now, while with per cpu it only needed one/two 
> entries.

4 or 8 cores is the norm these days - by the time this change hits real 
Linux computers en masse 8 cores will be quite common.

> Admittedly it could have been better commented.
> 
> Not that it matters now unfortunately it's already applied. Sometimes 
> wonder why I still bother to do patch review...

Whether patches are already applied or not has no relevance - patches can 
still be undone or reverted of course, should your review feedback be 
correct.

	Ingo
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