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[Xen-devel] Re: Power aware credit scheduler

  • To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
  • From: Emmanuel Ackaouy <ackaouy@xxxxxxxxx>
  • Date: Thu, 19 Jun 2008 16:38:28 +0200
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Wei, Gang" <gang.wei@xxxxxxxxx>, "Yu, Ke" <ke.yu@xxxxxxxxx>
  • Delivery-date: Thu, 19 Jun 2008 07:38:59 -0700
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On Jun 19, 2008, at 15:32 , Tian, Kevin wrote:
Regardless of any new knobs, a good default behavior might be
to only take a package out of C-state when another non-idle
package has had more than one VCPU active on it over some
reasonable amount of time.

By default, putting multiple VCPUs on the same physical package
when other packages are idle is obviously not always going to
be optimal. Maybe it's not a bad default for VCPUs that are
related (same VM or qemu)? I think Ian P hinted at this. But it
frightens me that you would always do this by default for any set
of VCPUs. Power saving is good but so is memory bandwidth

To enable this feature depends on a control command from system
adminstrator, who knows the tradeoff. From absolute performance
P.O.V, I believe it's not optimal. However if looking from the
performance/watt, i.e. power efficiency angle, power saving due to
package level idle may overwhelm performance impact by keeping
activity in other package. Of course finally memory latency should
be also considered in NUMA system, as you mentioned.

I'm saying something can be done to improve power saving in
the current system without adding a knob. Perhaps you can give
the admin even more power saving abilities with a knob, but it
makes sense to save power when performance is not impacted,
regardless of any knob position.

Also, note I mentioned memory BANDWIDTH and not latency.
It's not the same thing. And I wasn't just thinking about NUMA

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