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

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).


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.


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