There are bunch of ways to address these issues. One can save power while still spinning. See e.g. http://mug.mvapich.cse.ohio-state.edu/static/media/mug/presentations/2015/mu.... LLNL has done something similar. Certain types of passive waiting may not work with shared-memory communication. monitor-mwait would be interesting if not for the ring-0 issue. Jeff On Thu, Sep 29, 2016 at 8:08 AM, Rob Latham <[email protected]> wrote:

Your proposal introduces latency. Data centers want power efficiency, but several large classes of applications care about message rate.

you don't need usleeep. There are several event-based system calls you could use: sleect(), poll() -- which does not actually poll in most implementations.

Also, there are non-spinning channels you can use: e.g. ch3:sock

It's been something we've been thinking about for a while! http://trac.mpich.org/projects/mpich/ticket/79

==rob

On 09/29/2016 09:07 AM, 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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