Re: [Xen-devel] [PATCH] cpuidle: Improve perf for certain workloads

[Ross Lagerwall <rosslagerwall@xxxxxxxxxx>]

On 06/13/2014 12:37 PM, Jan Beulich wrote:
> > > > Ross Lagerwall <ross.lagerwall@xxxxxxxxxx> 06/13/14 12:23 PM >>>
> > The existing mechanism of using interrupt frequency as a heuristic does
> > not work well for certain workloads.  As an example, synchronous dd on a
> > small block size uses deep C-states because much of the time is spent
> > doing processing so the interrupt frequency is not too high, but when an
> > IOP is submitted, the interrupt occurs soon after going idle.  This
> > causes exit latency to be a significant factor.
> >
> > To fix this, add a new factor which limits the exit latency to be no
> > more than 10% of the decaying measured idle time.  This improves
> > performance for workloads with a medium interrupt frequency but a short
> > idle duration.
> Does this have a Linux counterpart (after all the code here is a clone from
> Linux'es)? If so, adding a cross reference would be appreciated. If not, I'd
> expect you to explain why Xen needs what Linux doesn't need.
No it does not have a Linux counterpart. The Linux equivalent code for 
exit latency uses a combination of the PM QOS interface, the load 
average of the system and the number of processes in IO wait state on 
that CPU. If a process is in IO wait state, it compares the exit latency 
with the predicted residency reduced by a factor of 10, which is 
somewhat similar to what this patch does.
The use of average interrupt frequency was introduced by Keir Fraser in 
353533232730 ("cpuidle: fix the menu governor to enhance IO 
performance") when porting a Linux patch to Xen.
> > In the workload given previously, throughput improves by 20% with this
> > patch.
> This is the positive side. Did you also check for no negative effects?
From a performance perspective, this patch will never cause the machine 
to go in a deeper C-state than it would have previously.
From a power perspective, I don't have a power meter to do actual 
measurements but watching xenpm on an idle system shows that almost all 
of the time is spent in the deepest C-state.
> > A side effect of this patch is to fix the use of MAX_INTERESTING.
> What does this sentence refer to?
I will make that more explicit in the commit message.

> > @@ -88,6 +89,10 @@
>    >* the average interrupt interval is, the smaller C state latency should be
>    >* and thus the less likely a busy CPU will hit such a deep C state.
>    >*
> > + * As an additional rule to reduce the performance impact, menu tries to
> > + * limit the exit latency duration to be no more than 10% of the decaying
> > + * measured idle time.
> > + *
>    >*/
>
> Even if previously there was a blank comment line at the end, please avoid
> retaining such when you add further text anyway.
OK.

Thanks for the review,
--
Ross Lagerwall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel