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

[Avi Kivity]

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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