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

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Date:   Thu, 14 Dec 2017 12:56:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Quan Xu <quan.xu0@...il.com>
Cc:     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 13/12/2017 17:28, Konrad Rzeszutek Wilk wrote:
> 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?

Even high-CPU usage, as long as there is a small idle time.  The cost of
setting the TSC deadline timer twice is about 3000 cycles.

However, I think Amazon's approach of not intercepting HLT/MWAIT/PAUSE
can recover most of the performance and it's way less intrusive.

Thanks,

Paolo