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Date: Tue, 27 Jun 2017 16:00:08 +0200
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To: Yang Zhang <yang.zhang.wz@gmail.com>
Cc: Wanpeng Li <kernellwp@gmail.com>, Thomas Gleixner <tglx@linutronix.de>,
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Subject: Re: [PATCH 0/2] x86/idle: add halt poll support
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2017-06-23 14:49+0800, Yang Zhang:
> On 2017/6/23 12:35, Wanpeng Li wrote:
> > 2017-06-23 12:08 GMT+08:00 Yang Zhang <yang.zhang.wz@gmail.com>:
> > > On 2017/6/22 19:50, Wanpeng Li wrote:
> > > > 
> > > > 2017-06-22 19:22 GMT+08:00 root <yang.zhang.wz@gmail.com>:
> > > > > 
> > > > > From: Yang Zhang <yang.zhang.wz@gmail.com>
> > > > > 
> > > > > Some latency-intensive workload will see 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.
> > > > > 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.
> > > > > 
> > > > > Here is the data i get when running benchmark contextswitch
> > > > > (https://github.com/tsuna/contextswitch)
> > > > > before patch:
> > > > > 2000000 process context switches in 4822613801ns (2411.3ns/ctxsw)
> > > > > after patch:
> > > > > 2000000 process context switches in 3584098241ns (1792.0ns/ctxsw)
> > > > 
> > > > 
> > > > If you test this after disabling the adaptive halt-polling in kvm?
> > > > What's the performance data of w/ this patchset and w/o the adaptive
> > > > halt-polling in kvm, and w/o this patchset and w/ the adaptive
> > > > halt-polling in kvm? In addition, both linux and windows guests can
> > > > get benefit as we have already done this in kvm.
> > > 
> > > 
> > > I will provide more data in next version. But it doesn't conflict with
> > 
> > Another case I can think of is w/ both this patchset and the adaptive
> > halt-polling in kvm.
> > 
> > > current halt polling inside kvm. This is just another enhancement.
> > 
> > I didn't look close to the patchset, however, maybe there is another
> > poll in the kvm part again sometimes if you fails the poll in the
> > guest. In addition, the adaptive halt-polling in kvm has performance
> > penalty when the pCPU is heavily overcommitted though there is a
> > single_task_running() in my testing, it is hard to accurately aware
> > whether there are other tasks waiting on the pCPU in the guest which
> > will make it worser. Depending on vcpu_is_preempted() or steal time
> > maybe not accurately or directly.
> > 
> > So I'm not sure how much sense it makes by adaptive halt-polling in
> > both guest and kvm. I prefer to just keep adaptive halt-polling in
> > kvm(then both linux/windows or other guests can get benefit) and avoid
> > to churn the core x86 path.
> 
> This mechanism is not specific to KVM. It is a kernel feature which can
> benefit guest when running inside X86 virtualization environment. The guest
> includes KVM,Xen,VMWARE,Hyper-v. Administrator can control KVM to use
> adaptive halt poll but he cannot control the user to use halt polling inside
> guest. Lots of user set idle=poll inside guest to improve performance which
> occupy more CPU cycles. This mechanism is a enhancement to it not to KVM
> halt polling.

Users of idle=poll shouln't overcommit, so the goal seems to be energy
savings without crippling the guest performance too much ...

Wouldn't switching to idle=mwait work as well?

Thanks.