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SubjectRe: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks
Raghu, thanks for you input. I'm more than glad to work together with
you to make this idea work better.

-Jiannan

On Thu, Jun 6, 2013 at 11:15 PM, Raghavendra K T
<raghavendra.kt@linux.vnet.ibm.com> wrote:
> On 06/03/2013 11:51 AM, Raghavendra K T wrote:
>>
>> On 06/03/2013 07:10 AM, Raghavendra K T wrote:
>>>
>>> On 06/02/2013 09:50 PM, Jiannan Ouyang wrote:
>>>>
>>>> On Sun, Jun 2, 2013 at 1:07 AM, Gleb Natapov <gleb@redhat.com> wrote:
>>>>
>>>>> High level question here. We have a big hope for "Preemptable Ticket
>>>>> Spinlock" patch series by Jiannan Ouyang to solve most, if not all,
>>>>> ticketing spinlocks in overcommit scenarios problem without need for
>>>>> PV.
>>>>> So how this patch series compares with his patches on PLE enabled
>>>>> processors?
>>>>>
>>>>
>>>> No experiment results yet.
>>>>
>>>> An error is reported on a 20 core VM. I'm during an internship
>>>> relocation, and will start work on it next week.
>>>
>>>
>>> Preemptable spinlocks' testing update:
>>> I hit the same softlockup problem while testing on 32 core machine with
>>> 32 guest vcpus that Andrew had reported.
>>>
>>> After that i started tuning TIMEOUT_UNIT, and when I went till (1<<8),
>>> things seemed to be manageable for undercommit cases.
>>> But I still see degradation for undercommit w.r.t baseline itself on 32
>>> core machine (after tuning).
>>>
>>> (37.5% degradation w.r.t base line).
>>> I can give the full report after the all tests complete.
>>>
>>> For over-commit cases, I again started hitting softlockups (and
>>> degradation is worse). But as I said in the preemptable thread, the
>>> concept of preemptable locks looks promising (though I am still not a
>>> fan of embedded TIMEOUT mechanism)
>>>
>>> Here is my opinion of TODOs for preemptable locks to make it better ( I
>>> think I need to paste in the preemptable thread also)
>>>
>>> 1. Current TIMEOUT UNIT seem to be on higher side and also it does not
>>> scale well with large guests and also overcommit. we need to have a
>>> sort of adaptive mechanism and better is sort of different TIMEOUT_UNITS
>>> for different types of lock too. The hashing mechanism that was used in
>>> Rik's spinlock backoff series fits better probably.
>>>
>>> 2. I do not think TIMEOUT_UNIT itself would work great when we have a
>>> big queue (for large guests / overcommits) for lock.
>>> one way is to add a PV hook that does yield hypercall immediately for
>>> the waiters above some THRESHOLD so that they don't burn the CPU.
>>> ( I can do POC to check if that idea works in improving situation
>>> at some later point of time)
>>>
>>
>> Preemptable-lock results from my run with 2^8 TIMEOUT:
>>
>> +-----------+-----------+-----------+------------+-----------+
>> ebizzy (records/sec) higher is better
>> +-----------+-----------+-----------+------------+-----------+
>> base stdev patched stdev %improvement
>> +-----------+-----------+-----------+------------+-----------+
>> 1x 5574.9000 237.4997 3484.2000 113.4449 -37.50202
>> 2x 2741.5000 561.3090 351.5000 140.5420 -87.17855
>> 3x 2146.2500 216.7718 194.8333 85.0303 -90.92215
>> 4x 1663.0000 141.9235 101.0000 57.7853 -93.92664
>> +-----------+-----------+-----------+------------+-----------+
>> +-----------+-----------+-----------+------------+-----------+
>> dbench (Throughput) higher is better
>> +-----------+-----------+-----------+------------+-----------+
>> base stdev patched stdev %improvement
>> +-----------+-----------+-----------+------------+-----------+
>> 1x 14111.5600 754.4525 3930.1602 2547.2369 -72.14936
>> 2x 2481.6270 71.2665 181.1816 89.5368 -92.69908
>> 3x 1510.2483 31.8634 104.7243 53.2470 -93.06576
>> 4x 1029.4875 16.9166 72.3738 38.2432 -92.96992
>> +-----------+-----------+-----------+------------+-----------+
>>
>> Note we can not trust on overcommit results because of softlock-ups
>>
>
> Hi, I tried
> (1) TIMEOUT=(2^7)
>
> (2) having yield hypercall that uses kvm_vcpu_on_spin() to do directed yield
> to other vCPUs.
>
> Now I do not see any soft-lockup in overcommit cases and results are better
> now (except ebizzy 1x). and for dbench I see now it is closer to base and
> even improvement in 4x
>
>
> +-----------+-----------+-----------+------------+-----------+
> ebizzy (records/sec) higher is better
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improvement
> +-----------+-----------+-----------+------------+-----------+
> 5574.9000 237.4997 523.7000 1.4181 -90.60611
> 2741.5000 561.3090 597.8000 34.9755 -78.19442
> 2146.2500 216.7718 902.6667 82.4228 -57.94215
> 1663.0000 141.9235 1245.0000 67.2989 -25.13530
>
> +-----------+-----------+-----------+------------+-----------+
> +-----------+-----------+-----------+------------+-----------+
> dbench (Throughput) higher is better
> +-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improvement
> +-----------+-----------+-----------+------------+-----------+
> 14111.5600 754.4525 884.9051 24.4723 -93.72922
> 2481.6270 71.2665 2383.5700 333.2435 -3.95132
> 1510.2483 31.8634 1477.7358 50.5126 -2.15279
> 1029.4875 16.9166 1075.9225 13.9911 4.51050
> +-----------+-----------+-----------+------------+-----------+
>
>
> IMO hash based timeout is worth a try further.
> I think little more tuning will get more better results.
>
> Jiannan, When you start working on this, I can also help
> to get best of preemptable lock idea if you wish and share
> the patches I tried.
>
>
>


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