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

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Message-ID: <20090112221023.GB10720@gambetta>
Date:	Mon, 12 Jan 2009 23:10:23 +0100
From:	Frederik Deweerdt <frederik.deweerdt@...og.eu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] tlb flush_data: replace per_cpu with an array

On Mon, Jan 12, 2009 at 10:57:02PM +0100, Andi Kleen wrote:
> On Mon, Jan 12, 2009 at 10:35:42PM +0100, Frederik Deweerdt wrote:
> > Hi,
> > 
> > On x86_64 flush tlb data is stored in per_cpu variables. This is
> > unnecessary because only the first NUM_INVALIDATE_TLB_VECTORS entries
> > are accessed.
> > This patch aims at making the code less confusing (there's nothing
> > really "per_cpu") by using a plain array. It also would save some memory
> > on most distros out there (Ubuntu x86_64 has NR_CPUS=64 by default).
> 
> Nope it doesn't save memory on most systems because per cpu is only allocated
> based on the CPUs that are actually there. And if you have more than 8
> cores you can likely afford a few bytes per CPU.
I did not understand that, thanks for clarifiying
> 
> You would need to cache line pad each entry then, otherwise you risk
> false sharing. That would make the array 1K on 128 bytes cache line 
> system.  This means on small systems this would actually waste
> much more memory.
> 
> per cpu avoids that problem completely.
It is also slower (or so percpu.h says), and confusing I'd say.

Regards,
Frederik
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