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SubjectRe: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks
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



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