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

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Message-ID: <20171213162800.GS10097@char.us.oracle.com>
Date:   Wed, 13 Dec 2017 11:28:00 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Quan Xu <quan.xu0@...il.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 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? 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?

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?

> 
> 
> 
> Quan
> Alibaba Cloud