Re: [Xen-devel] Performance overhead of paravirt_ops on nativeidentified

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-05-14 at 09:05 +0100, Jan Beulich wrote:
> Wouldn't a third solution be to use ticket spinlocks everywhere, i.e. eliminate
> the current indirection, and replace it by an indirection for just the contention
> case? As I view it, the problem for Xen aren't really the ticket locks by
> themselves, but rather the extra spinning involved, which is of concern only
> if a lock is contended. We're using ticket locks quite happily in our kernels,
> with directed instead of global wakeup from the unlock path. The only open
> issue we currently have is that while for native keeping interrupts disabled
> while spinning may be acceptable (though I'm not sure how -rt folks are
> viewing this), in a pv environment one should really re-enable interrupts
> here due to the potentially much higher latency.

the -rt folks don't nearly have as many spinlocks, and for those we do
like ticket locks, because they are much fairer and give better worst
case contention behaviour.

Also, for the -rt folks, preempt disable is about as bad as irq disable.