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

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Message-ID: <53430f19-2fd3-78cd-d45d-0ebb0a67e226@redhat.com>
Date:   Thu, 14 Dec 2017 14:00:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Quan Xu <quan.xu0@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.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 14/12/2017 13:06, Quan Xu wrote:
> 
> 
> On 2017/12/14 19:56, Paolo Bonzini wrote:
>> 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.
> 
> Paolo, could you share the Amazon's patch or the LML link? thanks.

Here: https://www.spinics.net/lists/kvm/msg159356.html

Paolo