Multicore computations in Julia language.

julia_parallel_multicore.jl

# start julia specifying how many cores to make available
##$ julia -p 4

# julia multicore start command in a shell script
##$ julia -p $(nproc)
# http://stackoverflow.com/questions/2314750/how-to-assign-the-output-of-a-shell-command-to-a-variable

# alternatively add local cores dynamically (e.g. after starting julia single-core)
##$ julia
addprocs_local(3)

# check the number of cores made available
nprocs()

# total number of cores (with hyperthreading)
#  http://stackoverflow.com/questions/27931026/obtain-the-number-of-cpu-cores-in-julia
CPU_CORES

# make functions from a source file available to all processes
require("source-name.jl")
# altermnatively: (e.g. in interactive work)
# @everywhere macro executes a statement on all processes
@everywhere id = myid()
@everywhere function name(arg)
  # do something
end


# @parallel for
# use for small iterations, reduce-step at the calling node
@parallel for i=1:4
  run(`hostname`)
end
# IMPORTANT: allocate variables before the loop scope
# do not mutate on some process as the change is not visible to others

# comprehension syntax
@parallel [funname(i) for i=1:4]


# pmap
# use when each call does lots of computations
# multiple argument function examples:
#  http://stackoverflow.com/questions/24637064/julia-using-pmap-correctly
@everywhere f(s,count)=(println("process id = $(myid()) s = $s count = $count");repeat(s,count))
pmap((a1,a2)->f(a1,a2),{"a","b","c"},{2,1,3})
# or:
pmap((args)->f(args...),{{"a",2},{"b",1},{"c",3}})


# loop parallelization with timing
tic()
# NOTE: (+) here is reduce-step
nheads = @parallel (+) for i=1:200000000
  int(randbool())
end
s=toc()
println("Number of Heads: $nheads in $s seconds")


# further reading
# http://julia.readthedocs.org/en/latest/manual/parallel-computing/
# http://julialang.org/blog/2013/04/distributed-numerical-optimization/
# http://www.admin-magazine.com/HPC/Articles/Parallel-Julia-Jumping-Right-In
# http://www.blog.juliaferraioli.com/2014/02/julia-on-google-compute-engine-parallel.html

julia_parallel_timeout.sh

#!/bin/bash

# setting a hard limit on the code runtime
timeout 600 julia -p $(nproc) juliacode.jl >>results.log 2>&1

# clean up remaining processes if any
killall -9 -v julia >>cleanup.log 2>&1
### does this cover all possible cases?