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Message-ID: <1f925ff3-b654-e22e-9b60-23a26694aa89@gmail.com> 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
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