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

[Quan Xu]

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 <yang.zhang.wz@xxxxxxxxx>
> >
> > 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?
ÂÂ 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?
ÂÂÂ 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.


Quan
Alibaba Cloud
>