linux-kernel - Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Date:	Tue, 24 May 2016 09:13:59 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	David Matlack <dmatlack@...gle.com>
Cc:	Wanpeng Li <kernellwp@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will
 fire soon

On 2016/5/24 2:04, David Matlack wrote:
> On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@...il.com> wrote:
>> On 2016/5/21 2:37, David Matlack wrote:
>>>
>>> It's not obvious to me why polling for a timer interrupt would improve
>>> context switch latency. Can you explain a bit more?
>>
>>
>> We have a workload which using high resolution timer(less than 1ms) inside
>> guest. It rely on the timer to wakeup itself. Sometimes the timer is
>> expected to fired just after the VCPU is blocked due to execute halt
>> instruction. But the thread who is running in the CPU will turn off the
>> hardware interrupt for long time due to disk access. This will cause the
>> timer interrupt been blocked until the interrupt is re-open.
>
> Does this happen on the idle thread (swapper)? If not, halt-polling
> may not help; it only polls if there are no other runnable threads.

Yes, there is no runnable task inside guest.

>
>> For optimization, we let VCPU to poll for a while if the next timer will
>> arrive soon before schedule out. And the result shows good when running
>> several workloads inside guest.
>
> Thanks for the explanation, I appreciate it.
>
>>
>> --
>> best regards
>> yang


-- 
best regards
yang