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SubjectRe: [PATCH v7 0/3] KVM: Dynamic Halt-Polling
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Am 03.09.2015 um 16:07 schrieb Wanpeng Li:
> v6 -> v7:
> * explicit signal (set a bool)
> * fix the tracepoint
>
> v5 -> v6:
> * fix wait_ns and poll_ns
>
> v4 -> v5:
> * set base case 10us and max poll time 500us
> * handle short/long halt, idea from David, many thanks David
>
> v3 -> v4:
> * bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
> when idle VCPU is detected
>
> v2 -> v3:
> * grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or /halt_poll_ns_shrink
> * drop the macros and hard coding the numbers in the param definitions
> * update the comments "5-7 us"
> * remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns time,
> vcpu->halt_poll_ns start at zero
> * drop the wrappers
> * move the grow/shrink logic before "out:" w/ "if (waited)"
>
> v1 -> v2:
> * change kvm_vcpu_block to read halt_poll_ns from the vcpu instead of
> the module parameter
> * use the shrink/grow matrix which is suggested by David
> * set halt_poll_ns_max to 2ms
>
> There is a downside of always-poll since poll is still happened for idle
> vCPUs which can waste cpu usage. This patchset add the ability to adjust
> halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,
> and to shrink halt_poll_ns when long halt is detected.
>
> There are two new kernel parameters for changing the halt_poll_ns:
> halt_poll_ns_grow and halt_poll_ns_shrink.
>
> no-poll always-poll dynamic-poll
> -----------------------------------------------------------------------
> Idle (nohz) vCPU %c0 0.15% 0.3% 0.2%
> Idle (250HZ) vCPU %c0 1.1% 4.6%~14% 1.2%
> TCP_RR latency 34us 27us 26.7us
>
> "Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
> c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
> guest was tickless. (250HZ) means the guest was ticking at 250HZ.
>
> The big win is with ticking operating systems. Running the linux guest
> with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close
> to no-polling overhead levels by using the dynamic-poll. The savings
> should be even higher for higher frequency ticks.
>
> Wanpeng Li (3):
> KVM: make halt_poll_ns per-vCPU
> KVM: dynamic halt-polling
> KVM: trace kvm_halt_poll_ns grow/shrink
>
> include/linux/kvm_host.h | 1 +
> include/trace/events/kvm.h | 30 +++++++++++++++++++
> virt/kvm/kvm_main.c | 72 ++++++++++++++++++++++++++++++++++++++++++----
> 3 files changed, 97 insertions(+), 6 deletions(-)
>

I get some nice improvements for uperf between 2 guests, but there is one "bug":
If there is already some polling ongoing, its impossible to disable the polling,
even a
echo 0 > /sys/module/kvm/parameters/halt_poll_ns
is ignored. (as vcpu->halt_poll_ns is used to check)



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