Re: [Xen-devel] [PATCH] switch rangeset's lock to rwlock

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.09.14 at 15:02, <tim@xxxxxxx> wrote:
> At 13:15 +0100 on 18 Sep (1411042524), Jan Beulich wrote:
>> >>> On 18.09.14 at 12:43, <tim@xxxxxxx> wrote:
>> > At 13:55 +0100 on 12 Sep (1410526507), Jan Beulich wrote:
>> >> As a general library routine, it should behave as efficiently as
>> >> possible, even if at present no significant contention is known here.
>> >> 
>> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> >> ---
>> >> With the widened use of rangesets I'd like to re-suggest this change
>> >> which I had posted already a couple of years back.
>> > 
>> > Is this addressing an actual (measured) problem or just seems like a
>> > good idea?  If the latter maybe keep it until after 4.5?
>> 
>> The latter. And yes, I have no problem keeping it until after 4.5,
>> it's just that the multi-ioreq-server's extended use of rangesets
>> (as said elsewhere) would seem to make this a reasonable fit for
>> 4.5.
> 
> Well, I think it's a question for the release manager, then. :)

Konrad?

> I wouldn't be inclined to tinker with them unless we had a measurement
> justifying the change.  After all, the rangeset operations are not
> long-running ones and we'd be adding some overhead.

Well, that's a pretty weak argument: The vHPET lock doesn't
protect long running operations either, yet its conversion to rwlock
did yield a significant improvement. But yes, unless rangesets
participate in something that may get used heavily by a guest,
changing the lock kind would likely not have a significant effect.
Otoh figuring out that lock contention is a problem involves a non-
eglectible amount of work, preemption of which seems at least
desirable.

In fact, in the latest runs with those many-vCPU Windows guests I
see signs of contention even on vpt's pl_time_lock (8 out of 64 vCPU-s
racing for it).

Jan


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