[mlpack] Introduction of ExecutionPolicy

[Ankit Aggarwal]

"""Thanks for the tip, Ankit.

I measured, but couldn't find a large difference on my machine. My
benchmark.
<https://gist.github.com/shikharbhardwaj/6f5a48075ae964c294b775a6c153a6bd>

Anyways, this is just a rough indicator of the execution time, not a
benchmark for measuring the performance."""

Attached is the code showing the difference between the execution, also
note the time difference is in microseconds.

@Ryan I have emailed you an idea on how to implement the Fast k-centre
algorithm.Please do have a look on that so that I can approach forward on
the implementation phase.

Ankit Aggarwal



On Tue, May 16, 2017 at 10:52 PM, Ryan Curtin <ryan at ratml.org> wrote:

> On Tue, May 16, 2017 at 09:30:52PM +0530, Ankit Aggarwal wrote:
> > Hi Shikhar
> >
> > Just a tip
> >
> > Please avoid using *"auto t0 = std::chrono::high_resolution_
> clock::now();*"
> > and rather use* gettimeofday* for any kind of benchmarking as if you
> > measure the performance of these two function you will find out that
> > high_resolution_clock to be atleast 2-3x slower than gettimeofday.I know
> > you are taking the difference but it will help if you require some kind
> of
> > book-keeping of time events to avoid latency.
>
> There's also Timer::Start() and Timer::Stop() which might be a little
> more convenient, if you want to drag the rest of mlpack in too. :)
> Personally I like to use those when I am benchmarking for simplicity.
>
> It seems to me like the idea of the policy is to determine how many
> cores to use, but I think maybe OpenMP already has some support for
> this.  For instance, if I write an OpenMP-ized program called 'program',
> I can specify the number of cores it should use as an environment
> variable:
>
> $ OMP_NUM_THREADS=2 ./program
>
> By default the number of threads used will be the maximum on the
> machine, but this environment variable gives the user some nice
> standardized control over the way the programs are executed.  I think
> most users who see software that's parallelized with OpenMP interpret
> this as the standard way to control its behavior.
>
> You can also use omp_set_num_threads(...):
>
> // The number of cores we want to use.
> #define N 4
>
> int main()
> {
>   omp_set_num_threads(N);
>
>   // the rest of the program
>   // ...
> }
>
> So I think maybe some of the support you need is already there.  Let me
> know what you think.
>
> Thanks,
>
> Ryan
>
> --
> Ryan Curtin    | "The mighty hand of vengeance, sent down to strike
> ryan at ratml.org | the unroadworthy!"  - The Nightrider
>
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