Parallelism Test

do_work.c

#include <math.h>
#include <stdlib.h>
#include <stdio.h>

static const int ITERATIONS = 2134113216;

static double do_work(int worker_num, int concurrency)
{
  int chunk_size = ITERATIONS / concurrency;
  int start_num = chunk_size * worker_num;
  int i;

  double sum = 0.0;

  for (i = start_num; i < start_num + chunk_size; i++)
  {
    sum += sqrt((double) i);
  }

  return sum;
}

int main (int argc, char const *argv[])
{
  int worker_num  = 0;
  int concurrency = 0;

  double result = 0.0;

  if (argc == 3)
  {
    worker_num  = atoi(argv[1]);
    concurrency = atoi(argv[2]);

    result = (worker_num < concurrency) ? do_work(worker_num, concurrency) : 0.0;

    printf("%0.3f\n", result);

    return 0;
  }
  else
  {
    (void) fputs("Pass exactly two arguments\n", stderr);
    return -1;
  }
}

parallel_test.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'parallel'

def seconds_elapsed_during(&blk)
  before = Time.now
  blk.call
  after = Time.now

  return after - before
end

proc_count = 32 # change to expected maximum

warn "Detected #{proc_count} processors..."

baseline = nil

speedups = 1.upto(proc_count).map do |concurrency|
  warn "Testing #{concurrency} active workers..."

  observed = seconds_elapsed_during do
    `seq 0 #{concurrency - 1} | xargs -L 1 -P #{concurrency} -I% ./a.out % #{concurrency}`
  end

  baseline ||= observed

  speedup = (baseline / observed) - 1

  warn ("%0.3f elapsed (%d%% speedup)" % [observed, speedup * 100])

  speedup
end

likely_count = speedups.each_with_index.find { |sp, i| i != sp.round }.last

puts likely_count

To use:

• Compile the C program — gcc do_work.c -lm
• Change 32 to the expected maximum possible processor count
• Run ./parallel_test.rb

See the Karp–Flatt Metric and related paper.