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

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

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