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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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

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