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Subject: Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will
 fire soon
To: David Matlack <dmatlack@google.com>
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Cc: Wanpeng Li <kernellwp@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>, Wanpeng Li <wanpeng.li@hotmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
From: Yang Zhang <yang.zhang.wz@gmail.com>
Message-ID: <1f925ff3-b654-e22e-9b60-23a26694aa89@gmail.com>
Date: Tue, 24 May 2016 09:13:59 +0800
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On 2016/5/24 2:04, David Matlack wrote:
> On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@gmail.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