Re: [RFC v2 PATCH 00/21] KVM: x86: CPU isolation and direct interrupts delivery to guests

Tomoki Sekiyama
2012-09-10T11:38:50Z

Hi Jan,

On 2012/09/07 17:26, Jan Kiszka wrote:

> On 2012-09-06 13:27, Tomoki Sekiyama wrote:
>> This RFC patch series provides facility to dedicate CPUs to KVM guests
>> and enable the guests to handle interrupts from passed-through PCI devices
>> directly (without VM exit and relay by the host).
>>
>> With this feature, we can improve throughput and response time of the device
>> and the host's CPU usage by reducing the overhead of interrupt handling.
>> This is good for the application using very high throughput/frequent
>> interrupt device (e.g. 10GbE NIC).
>> Real-time applicatoins also gets benefit from CPU isolation feature, which
>> reduces interfare from host kernel tasks and scheduling delay.
>>
>> The overview of this patch series is presented in CloudOpen 2012.
>> The slides are available at:
>> http://events.linuxfoundation.org/images/stories/pdf/lcna_co2012_sekiyama.pdf
>
> One question regarding your benchmarks: If you measured against standard
> KVM, were the vCPU thread running on an isolcpus core of its own as
> well? If not, your numbers about the impact of these patches on maximum,
> maybe also average latencies are likely too good. Also, using a non-RT
> host and possibly a non-prioritized vCPU thread for the standard
> scenario looks like an unfair comparison.

In the standard KVM benchmark, the vCPU thread is pinned down to its own
CPU core. In addition, the vCPU thread and irq/*-kvm threads are both set
to the max priority with SCHED_RR policy.

As you said, RT-host may result in better max latencies as below.
(But min/average latencies became worse, however, this might be our setup
issue.)
Min / Avg / Max latencies
Normal KVM
RT-host (3.4.4-rt14) 15us / 21us / 117us
non RT-host (3.5.0-rc6) 6us / 11us / 152us
KVM + Direct IRQ
non RT-host (3.5.0-rc6 +patch) 1us / 2us / 14us

Thanks,
--
Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>
Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory