Re: Tasking for Mandelbrot program

[jonathan <johnscpg@googlemail.com>]

On Sep 27, 10:27 pm, Georg Bauhaus <rm.tsoh.plus-
bug.bauh...@maps.futureapps.de> wrote:

> - writing the image bytes with Stream_IO removes 6% running time
>   when compared to GNAT.IO.Put.
>   This adds standard Ada but also adds about 10 lines of code for
>   the Put procedure and a Stdout variable. Is it worth it?

Speeding up IO would give you a very detectable improvement
in the multi-core benchmark, since the present program
parallelizes the computation well, and the remaining
problem is a small but irritating IO overhead that can't
be parallelized.

Here are a few timings on 8 cores.
Perfect parallelization would give speed-up factor = 8.

With Output enabled:

  No_Of_Workers (tasks) =  8, speed-up factor = 4.45
  No_Of_Workers (tasks) = 16, speed-up factor = 6.30
  No_Of_Workers (tasks) = 24, speed-up factor = 6.66
  No_Of_Workers (tasks) = 32, speed-up factor = 6.86

With Output disabled, it is nearer the optimal
speed-up factor of 8:

  No_Of_Workers (tasks) = 32, speed-up factor = 7.66

The actual benchmark uses 4 cores, so I suspect that the
present standard setting of No_Of_Workers = 16 is
good.

For those who are interested in this problem as
much as I am, a few more words of explanation ...
The difficulty  with mandelbrot is that if you
parallelize it by breaking it up into
work-segments (break up the outer loop into
segments of equal length), then some work-segments
finish quick, some slow, so we have a load balancing
problem.  The solution Georg came up with breaks
the problem into a number of independent tasks four
time greater in number than the number of cores.
The operating system successfully distributes the
tasks over the cores in such a way that the cores
do comparable amounts of work.
(Hope my description is accurate.)

Jonathan