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

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Date:   Mon, 20 Nov 2017 15:18:10 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Quan Xu <quan.xu03@...il.com>
Cc:     kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [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 <yang.zhang.wz@...il.com>
>>
>> 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.


Quan
Alibaba Cloud
>
>