linux-kernel - Re: [Xen-devel] [PATCH RFC v3 0/6] x86/idle: add halt poll support

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Date:   Wed, 15 Nov 2017 16:31:31 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Quan Xu <quan.xu03@...il.com>
Cc:     kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [Xen-devel] [PATCH RFC v3 0/6] x86/idle: add halt poll support

On Mon, Nov 13, 2017 at 06:05:59PM +0800, Quan Xu wrote:
> From: Yang Zhang <yang.zhang.wz@...il.com>
> 
> Some latency-intensive workload have seen obviously performance
> drop when running inside VM. The main reason is that the overhead
> is amplified when running inside VM. The most cost I have seen is
> inside idle path.

Meaning an VMEXIT b/c it is an 'halt' operation ? And then going
back in guest (VMRESUME) takes time. And hence your latency gets
all whacked b/c of this?

So if I understand - you want to use your _full_ timeslice (of the guest)
without ever (or as much as possible) to go in the hypervisor?

Which means in effect you don't care about power-saving or CPUfreq
savings, you just want to eat the full CPU for snack?

> 
> This patch introduces a new mechanism to poll for a while before
> entering idle state. If schedule is needed during poll, then we
> don't need to goes through the heavy overhead path.

Schedule of what? The guest or the host?