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SubjectRe: [Xen-devel] [PATCH RFC v3 0/6] x86/idle: add halt poll support

On 2017-11-16 05:31, Konrad Rzeszutek Wilk wrote:
> On Mon, Nov 13, 2017 at 06:05:59PM +0800, Quan Xu wrote:
>> From: Yang Zhang <>
>> Some latency-intensive workload have seen obviously performance
>> drop when running inside VM. The main reason is that the overhead
>> is amplified when running inside VM. The most cost I have seen is
>> inside idle path.
> Meaning an VMEXIT b/c it is an 'halt' operation ? And then going
> back in guest (VMRESUME) takes time. And hence your latency gets
> all whacked b/c of this?
   Konrad, I can't follow 'b/c' here.. sorry.

> So if I understand - you want to use your _full_ timeslice (of the guest)
> without ever (or as much as possible) to go in the hypervisor?
    as much as possible.

> Which means in effect you don't care about power-saving or CPUfreq
> savings, you just want to eat the full CPU for snack?
  actually, we  care about power-saving. The poll duration is
self-tuning, otherwise it is almost as the same as
  'halt=poll'. Also we always sent out with CPU usage of benchmark
netperf/ctxsw. We got much more
  performance with limited promotion of CPU usage.

>> This patch introduces a new mechanism to poll for a while before
>> entering idle state. If schedule is needed during poll, then we
>> don't need to goes through the heavy overhead path.
> Schedule of what? The guest or the host?
  rescheduled of guest scheduler..
  it is the guest.

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