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Message-ID: <CALzav=ez6DqQNDCCB0t4w0Fu2JMcCZRUT0YAfvfj-J_VCGRMkA@mail.gmail.com> Date: Mon, 23 May 2016 18:16:54 -0700 From: David Matlack <dmatlack@...gle.com> To: Yang Zhang <yang.zhang.wz@...il.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 Mon, May 23, 2016 at 6:13 PM, Yang Zhang <yang.zhang.wz@...il.com> wrote: > 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. Sorry for the confusion, my question was about the host, not the guest. Halt-polling only polls if there are no other runnable threads on the host CPU (see the check for single_task_running() in kvm_vcpu_block()). > > >> >>> 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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