Re: [Xen-devel] [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment

[Waiman Long <waiman.long@xxxxxx>]

On 02/27/2014 07:28 AM, David Vrabel wrote:
> On 26/02/14 15:14, Waiman Long wrote:
> > Locking is always an issue in a virtualized environment as the virtual
> > CPU that is waiting on a lock may get scheduled out and hence block
> > any progress in lock acquisition even when the lock has been freed.
> >
> > One solution to this problem is to allow unfair lock in a
> > para-virtualized environment. In this case, a new lock acquirer can
> > come and steal the lock if the next-in-line CPU to get the lock is
> > scheduled out. Unfair lock in a native environment is generally not a
> > good idea as there is a possibility of lock starvation for a heavily
> > contended lock.
> I'm not sure I'm keen on losing the fairness in PV environment.  I'm
> concerned that on an over-committed host, the lock starvation problem
> will be particularly bad.
>
> But I'll have to revist this once a non-broken PV qspinlock
> implementation exists (or someone explains how the proposed one works).
>
> David
On second thought, the unfair qspinlock may not be as bad as other 
unfair locks. Basically, a task gets one chance to steal the lock. If it 
can't, it will go to the back of the queue and wait for its turn. So 
unless a single CPU can monopolize a lock by acquiring it again 
immediately after release, all the tasks queuing up will eventually has 
its chance of getting the lock.
-Longman

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