Re: [PATCH v7 0/3] KVM: Dynamic Halt-Polling

[Christian Borntraeger <borntraeger@de.ibm.com>]

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)