On Mon, May 23, 2016 at 6:13 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote: > On 2016/5/24 2:04, David Matlack wrote: >> >> On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> >> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
