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

Maksym Planeta mplaneta at os.inf.tu-dresden.de
Thu Sep 29 09:35:45 CDT 2016


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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Maksym Planeta

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