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

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> 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.

>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?

>A side effect of this patch is to fix the use of MAX_INTERESTING.

What does this sentence refer to?

>@@ -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.

Jan

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