Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 07/12/2012 01:45 PM, Avi Kivity wrote:
> On 07/11/2012 05:01 PM, Raghavendra K T wrote:
>> On 07/11/2012 07:29 PM, Raghavendra K T wrote:
>>> On 07/11/2012 02:30 PM, Avi Kivity wrote:
>>>> On 07/10/2012 12:47 AM, Andrew Theurer wrote:
>>>>>
>>>>> For the cpu threads in the host that are actually active (in this case
>>>>> 1/2 of them), ~50% of their time is in kernel and ~43% in guest. This
>>>>> is for a no-IO workload, so that's just incredible to see so much cpu
>>>>> wasted. I feel that 2 important areas to tackle are a more scalable
>>>>> yield_to() and reducing the number of pause exits itself (hopefully by
>>>>> just tuning ple_window for the latter).
>>>>
>>>> One thing we can do is autotune ple_window. If a ple exit fails to wake
>>>> anybody (because all vcpus are either running, sleeping, or in ple
>>>> exits) then we deduce we are not overcommitted and we can increase the
>>>> ple window. There's the question of how to decrease it again though.
>>>>
>>>
>>> I see some problem here, If I interpret situation correctly. What
>>> happens if we have two guests with one VM having no over-commit and
>>> other with high over-commit. (except when we have gang scheduling).
>>>
>> Sorry, I meant less load and high load inside the guest.
>>
>>> Rather we should have something tied to VM rather than rigid PLE
>>> window.
>
> The problem occurs even with no overcommit at all.  One vcpu is in a
> legitimately long pause loop.  All those exits accomplish nothing, since
> all vcpus are scheduled.  Better to let it spin in guest mode.
>

I agree. One idea is we can have a scan_window to limit the scan of all
n vcpus each time we enter vcpu_spin, to say 2*log n initially;

then algorithm would be like;

if (yield fails)
    increase ple_window , increase scan_window

if (yield succeeds)
    decrease ple_window , decrease scan_window


and we have to set limit on what is max and min scan window and max and
min ple_window.