linux-kernel - Re: [PATCH RFC 0/7] kvm pvtimer

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Message-ID: <462b7e38-da27-fcea-9f02-8f090d2d8603@gmail.com>
Date:   Thu, 14 Dec 2017 10:32:38 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Ben Luo <bn0418@...il.com>
Subject: Re: [PATCH RFC 0/7] kvm pvtimer



On 2017/12/14 00:28, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 13, 2017 at 11:25:13PM +0800, Quan Xu wrote:
>> On Fri, Dec 8, 2017 at 11:10 PM, Konrad Rzeszutek Wilk <
>> konrad.wilk@...cle.com> wrote:
>>
>>> On Fri, Dec 08, 2017 at 04:39:43PM +0800, Quan Xu wrote:
>>>> From: Ben Luo <bn0418@...il.com>
>>>>
>>>> This patchset introduces a new paravirtualized mechanism to reduce
>>> VM-exit
>>>> caused by guest timer accessing.
>>> And how bad is this blib in arming the timer?
>>>
>>> And how often do you get this timer to be armed? OR better yet - what
>>> are the workloads in which you found this VMExit to be painful?
>>>
>>> Thanks. Or better yet - what
>>> are the workloads in which you found this VMExit to be painful?
>>>
>> one painful point is from VM idle path..
>>   for some network req/resp services, or benchmark of  process context
>> switches..
> So:
>
> 1) VM idle path and network req/resp services:
>
> Does this go away if you don't hit the idle path? Meaning if you
> loop without hitting HLT/MWAIT?
   we still hit HLT.. we can use it with 
https://lkml.org/lkml/2017/8/29/279 ..
>   I am assuming the issue you are facing
> is the latency - that is first time the guest comes from HLT and
> responds to the packet the latency is much higher than without?
yes,
> And the arming of the timer?
> 2) process context switches.
>
> Is that related to the 1)? That is the 'schedule' call and the process
> going to sleep waiting for an interrupt or timer?
>
> This all sounds like issues with low-CPU usage workloads where you
> need low latency responses?
yes,  it is also helpful to some timer-intensive services.

Quan