Subject: Re: [PATCH RFC 0/7] kvm pvtimer
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Quan Xu <quan.xu0@gmail.com>
Cc: Radim Krcmar <rkrcmar@redhat.com>,
        Yang Zhang <yang.zhang.wz@gmail.com>, kvm <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, Ben Luo <bn0418@gmail.com>
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 <20171213162800.GS10097@char.us.oracle.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <78ebabd2-cc38-2694-b104-c5e0230aba15@redhat.com>
Date: Thu, 14 Dec 2017 12:56:54 +0100
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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