linux-kernel - Re: queued spinlock code and results

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Message-ID: <alpine.LFD.0.999.0707091250550.3412@woody.linux-foundation.org>
Date:	Mon, 9 Jul 2007 12:55:01 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: queued spinlock code and results

On Mon, 9 Jul 2007, Davide Libenzi wrote:
> 
> The always-lfence instruction in vadd-lock really is painfull though.
> If numbers are close, and given that spinlock size considering structure 
> alignments should not matter much, wouldn't it be better to use a double 
> short and remove the 256 CPUs cap?

On x86? No.

There are no issues with the 255-CPU cap on 32-bit x86. It's just not 
relevant to anybody. So the _only_ thing that matters is speed and to a 
secondary degree size.

On x86-64, things are slightly different, and we would want to have at 
least the _capability_ to do 16 bits. So there might be a (somewhat weak) 
argument in favor of trying to share code.

But even then, size and performance are really the only things that 
matter, and if the 8/16-bit version is no slower, then I'd pick that by 
default, and suggest the 16/32-bit one to be enabled by CONFIG_MAX_CPU's 
being >=256 (at which point you can share the code with x86 anyway, since 
that just becomes the <256 cpu case).

		Linus
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