linux-kernel - Re: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

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Date:	Wed, 2 Sep 2015 06:30:10 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	David Matlack <dmatlack@...gle.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

On 9/2/15 5:45 AM, David Matlack wrote:
> On Thu, Aug 27, 2015 at 2:47 AM, Wanpeng Li <wanpeng.li@...mail.com> wrote:
>> 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)"
> I posted a patchset which adds dynamic poll toggling (on/off switch). I think
> this gives you a good place to build your dynamic growth patch on top. The
> toggling patch has close to zero overhead for idle VMs and equivalent
> performance VMs doing message passing as always-poll. It's a patch that's been
> in my queue for a few weeks but just haven't had the time to send out. We can
> win even more with your patchset by only polling as much as we need (via
> dynamic growth/shrink). It also gives us a better place to stand for choosing
> a default for halt_poll_ns. (We can run experiments and see how high
> vcpu->halt_poll_ns tends to grow.)
>
> The reason I posted a separate patch for toggling is because it adds timers
> to kvm_vcpu_block and deals with a weird edge case (kvm_vcpu_block can get
> called multiple times for one halt). To do dynamic poll adjustment correctly,
> we have to time the length of each halt. Otherwise we hit some bad edge cases:
>
>    v3: v3 had lots of idle overhead. It's because vcpu->halt_poll_ns grew every
>    time we had a long halt. So idle VMs looked like: 0 us -> 500 us -> 1 ms ->
>    2 ms -> 4 ms -> 0 us. Ideally vcpu->halt_poll_ns should just stay at 0 when
>    the halts are long.
>
>    v4: v4 fixed the idle overhead problem but broke dynamic growth for message
>    passing VMs. Every time a VM did a short halt, vcpu->halt_poll_ns would grow.
>    That means vcpu->halt_poll_ns will always be maxed out, even when the halt
>    time is much less than the max.
>
> I think we can fix both edge cases if we make grow/shrink decisions based on
> the length of kvm_vcpu_block rather than the arrival of a guest interrupt
> during polling.
>
> Some thoughts for dynamic growth:
>    * Given Windows 10 timer tick (1 ms), let's set the maximum poll time to
>      less than 1ms. 200 us has been a good value for always-poll. We can
>      probably go a bit higher once we have your patch. Maybe 500 us?
>
>    * The base case of dynamic growth (the first grow() after being at 0) should
>      be small. 500 us is too big. When I run TCP_RR in my guest I see poll times
>      of < 10 us. TCP_RR is on the lower-end of message passing workload latency,
>      so 10 us would be a good base case.

How to get your TCP_RR benchmark?

Regards,
Wanpeng Li

>> 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 halt_poll_ns since poll is still happen for idle
>> VCPU which can waste cpu usage. This patchset add the ability to adjust
>> halt_poll_ns dynamically, grows halt_poll_ns if an interrupt arrives and
>> shrinks halt_poll_ns when idle VCPU is detected.
>>
>> There are two new kernel parameters for changing the halt_poll_ns:
>> halt_poll_ns_grow and halt_poll_ns_shrink.
>>
>>
>> Test w/ high cpu overcommit ratio, pin vCPUs, and the halt_poll_ns of
>> halt-poll is the default 500000ns, the max halt_poll_ns of dynamic
>> halt-poll is 2ms. Then watch the %C0 in the dump of Powertop tool.
>> The test method is almost from David.
>>
>> +-----------------+----------------+-------------------+
>> |                 |                |                   |
>> |  w/o halt-poll  |  w/ halt-poll  | dynamic halt-poll |
>> +-----------------+----------------+-------------------+
>> |                 |                |                   |
>> |    ~0.9%        |    ~1.8%       |     ~1.2%         |
>> +-----------------+----------------+-------------------+
>>
>> The always halt-poll will increase ~0.9% cpu usage for idle vCPUs and the
>> dynamic halt-poll drop it to ~0.3% which means that reduce the 67% overhead
>> introduced by always halt-poll.
>>
>> Wanpeng Li (3):
>>    KVM: make halt_poll_ns per-VCPU
>>    KVM: dynamic halt_poll_ns adjustment
>>    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        | 50 +++++++++++++++++++++++++++++++++++++++++++---
>>   3 files changed, 78 insertions(+), 3 deletions(-)
>> --
>> 1.9.1
>>

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