Re: [PATCH V3 RFC 2/2] kvm: Handle yield_to failure return code for potential undercommit case

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

On 12/04/2012 01:26 AM, Marcelo Tosatti wrote:
> On Wed, Nov 28, 2012 at 10:40:56AM +0530, Raghavendra K T wrote:
>> On 11/28/2012 06:42 AM, Marcelo Tosatti wrote:
>>>
>>> Don't understand the reasoning behind why 3 is a good choice.
>>
>> Here is where I came from. (explaining from scratch for
>> completeness, forgive me :))
>> In moderate overcommits, we can falsely exit from ple handler even when
>> we have preempted task of same VM waiting on other cpus. To reduce this
>> problem, we try few times before exiting.
>> The problem boils down to:
>> what is the probability that we exit ple handler even when we have more
>> than 1 task in other cpus. Theoretical worst case should be around 1.5x
>> overcommit (As also pointed by Andrew Theurer). [But practical
>> worstcase may be around 2x,3x overcommits as indicated by the results
>> for the patch series]
>>
>> So if p is the probability of finding rq length one on a particular cpu,
>> and if we do n tries, then probability of exiting ple handler is:
>>
>>   p^(n+1) [ because we would have come across one source with rq length
>> 1 and n target cpu rqs  with length 1 ]
>>
>> so
>> num tries:         probability of aborting ple handler (1.5x overcommit)
>>   1                 1/4
>>   2                 1/8
>>   3                 1/16
>>
>> We can increase this probability with more tries, but the problem is
>> the overhead.
>> Also, If we have tried three times that means we would have iterated
>> over 3 good eligible vcpus along with many non-eligible candidates. In
>> worst case if we iterate all the vcpus, we reduce 1x performance and
>> overcommit performance get hit. [ as in results ].
>>
>> I have tried num_tries = 1,2,3 and n already ( not 4 yet). So I
>> concluded 3 is enough.
>>
>> Infact I have also run kernbench and hackbench which are giving 5-20%
>> improvement.
>>
>> [ As a side note , I also thought how about having num_tries = f(n) =
>> ceil ( log(num_online_cpus)/2 ) But I thought calculation is too much
>> overhead and also there is no point in probably making it dependent on
>> online cpus ]
>>
>> Please let me know if you are happy with this rationale/ or correct me
>> if you foresee some problem. (Infact Avi, Rik's concern about false
>> exiting made me arrive at 'try' logic which I did not have earlier).
>>
>> I am currently trying out the result for 1.5x overcommit will post the
>> result.
>
> Raghavendra
>
> Makes sense to me. Thanks.
>

Hi Marcelo, Thanks for looking into patches.