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Date: Thu, 13 Jun 2002 18:38:22 +0200
From: Gabriel Paubert <paubert@iram.es>
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To: Linus Torvalds <torvalds@transmeta.com>
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Subject: Re: [PATCH] Futex Asynchronous Interface
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Linus Torvalds wrote:

> Right now the kernel does _zero_ work for a lock that isn't contended. It
> doesn't know _anything_ about the process that got the lock initially.
> 
> Any amount of tracking would be _extremely_ expensive. Right now getting
> an uncontended lock is about 15 CPU cycles in user space.
> 
> Tryin to tell the kernel about gettign that lock takes about 1us on a P4
> (system call overhead), ie we're talking 18000 cycles. 18 THOUSAND cycles
> minimum. Compared to the current 15 cycles. That's more than three orders
> of magnitude slower than the current code, and you just lost the whole
> point of doing this all in user space in the first place.


Please tell us where you got your 18GHz CPU (18000 cycles/microsecond) ;-)

I want one, badly!

	Gabriel