Re: [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest

[David Vrabel <david.vrabel@citrix.com>]

On 12/03/14 18:54, 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 do not think this is a good idea -- the problems with unfair locks are
worse in a virtualized guest.  If a waiting VCPU deschedules and has to
be kicked to grab a lock then it is very likely to lose a race with
another running VCPU trying to take a lock (since it takes time for the
VCPU to be rescheduled).

> With the unfair locking activated on bare metal 4-socket Westmere-EX
> box, the execution times (in ms) of a spinlock micro-benchmark were
> as follows:
> 
>   # of    Ticket       Fair	    Unfair
>   tasks    lock     queue lock    queue lock
>   ------  -------   ----------    ----------
>     1       135        135	     137
>     2      1045       1120	     747
>     3      1827       2345     	    1084
>     4      2689       2934	    1438
>     5      3736       3658	    1722
>     6      4942       4434	    2092
>     7      6304       5176          2245
>     8      7736       5955          2388

Are these figures with or without the later PV support patches?

David