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

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

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.