Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptivespinning

From: Avi Kivity
Date: Tue Apr 06 2010 - 12:11:09 EST

On 04/06/2010 05:09 PM, Peter Zijlstra wrote:
On Tue, 2010-04-06 at 16:41 +0300, Avi Kivity wrote:
On 04/06/2010 04:35 PM, Peter Zijlstra wrote:
On Tue, 2010-04-06 at 16:28 +0300, Avi Kivity wrote:

Yes, but that's the best case for spinning. You could simply use a
userspace spinlock in this case.

Userspace spinlocks are evil.. they should _never_ be used.

But in this case they're fastest. If we don't provide a non-evil
alternative, people will use them.

That's what FUTEX_LOCK is about.

That works for the uncontended case. For the contended case, the waiter and the owner have to go into the kernel and back out to transfer ownership. In the non-adaptive case you have to switch to the idle task and back as well, and send an IPI. That's a lot of latency if the unlock happened just after the waiter started the descent into the kernel.

error compiling committee.c: too many arguments to function

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