linux-kernel - Re: Linux 3.1-rc9

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Message-ID: <20111017184916.GA5545@elte.hu>
Date:	Mon, 17 Oct 2011 20:49:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: Linux 3.1-rc9


* Thomas Gleixner <tglx@...utronix.de> wrote:

> > That simple time type could then trickle down as well: we could 
> > use it everywhere in kernel code and limit the hodge-podge of ABI 
> > time units to the syscall boundary. (and convert the internal 
> > time unit to whatever ABI unit there is close to the syscall 
> > boundary)
> > 
> > There's a point where micro-optimized 32-bit support related 
> > maintenance overhead (and the resulting loss of 
> > robustness/flexibility) becomes too expensive IMO.
> 
> That's not a micro optimization, it's a massive performance hit if 
> you force those 32bit archs to do 64/32 all over the place.

Do we have some hard data on this, which we could put into comments 
in include/linux/ktime.h and such? Older versions of GCC used to do a 
bad job of long long handling on 32-bit systems - that might be a 
factor in the performance figures.

But i suspect you are right that the cost is still very much there 
...

Thanks,

	Ingo
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