[mpich-discuss] Why does MPICH perform busy wait?

[Michele De Stefano]

I suggest, to whoever will address this issue, to have a look at how C++ >=
2011 handles the wait on a condition variable.
I think the technique to be used should be analogous.

Best regards,

2016-09-29 16:35 GMT+02:00 Maksym Planeta <mplaneta at os.inf.tu-dresden.de>:

> Hi,
>
> have a look into this paper:
>
> A Case for Application-Oblivious Energy-Efficient MPI Runtime
> A. Venkatesh , A. Vishnu , K. Hamidouche , N. Tallent , D. K. Panda , D.
> Kerbyson , and A. Hoise
> Supercomputing 2015, Nov 2015
>
> That's about MVAPICH2, not MPICH, but they share a lot of code base.
>
>
> On 09/29/2016 04:07 PM, Tim Jammer wrote:
>
>> Hi
>>
>> Within my bachelor thesis I come across that the MPICH Implementation
>> implements a busy wait.
>> In our application, this leads to a waste of energy as all processes are
>> spinning in a barrier while one process performs I/O.
>>
>> Therefore I want to propse to not wait as busy as it is done now.
>> On the test system this saves 50% energy compared to busy wait.
>> My basic prototype implementation causes only a negligible additional
>> runtime overhead.
>>
>> My proposal is to sleep between polling if the communication has not
>> finished yet.
>> Within the first few iterations the implementation should not sleep, to
>> guarantee maximum performance.
>> But if the communication was not finished right away, the importance of
>> saving energy grows.
>> Therefore I propose to sleep and increase this sleeping time after every
>> polling
>> up to a maximum value in order to not waste much additional runtime on
>> waiting.
>> This will allow the CPU to power down and save energy.
>>
>> my basic prototype basically works as follows:
>>
>> int stime=0;
>>     while (/* Incoming data does not match the request*/)
>>     {
>> usleep(stime);
>> stime++;
>> // do the actual polling...
>>
>> I can imagine, that the user can choose to activate this behaviour (i.e.
>> with an environment variable) if he wants to save energy.
>>
>> I have attached the section of my thesis which describe it more in detail.
>>
>>
>> My question is: was something like this considered before and are there
>> any arguments why this was not implemented.
>>
>> Regards
>> Tim Jammer
>>
>>
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>>
> --
> Regards,
> Maksym Planeta
>
>
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-- 
Michele De Stefano
Linked In <http://it.linkedin.com/in/micheledestefano>
mds-utils: a general purpose Open Source library
<http://sourceforge.net/projects/mds-utils/>
Personal Web Site <http://www.micheledestefano.altervista.org>
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