ThreadTools.jl

Some tools for working with threads in Julia
Author baggepinnen
Popularity
23 Stars
Updated Last
1 Year Ago
Started In
September 2019

Build Status codecov

ThreadTools

This package implements some utilities for using threads in Julia v1.3+

The utilities provided are:

@spawnatmost n for i in ...

Spawn at most n threads to carry out for-loop

tmap(f, args...)
tmap(f, nthreads::Int, args...)
tmap1(f, args...)
tmap1(f, nthreads::Int,args...)

Threaded map. The optional argument nthreads limits the number of threads used in parallel. tmap1 is the same as tmap, but falls back to a regular map if julia only has access to one thread.

The tools to limit the number of threads used are useful when doing, e.g., threaded IO, where the disk might get overloaded if you try to access it from too many threads at the same time.

Examples

using ThreadTools

julia> l = SpinLock();
julia> times = [];
julia> @spawnatmost 3 for i = 1:10 # This will use only three parallel threads, even if more are avilable
            lock(l) do # We protect the access to the array using a lock
                push!(times, time())
            end
            println(i)
            sleep(1)
       end

julia> round.(diff(times), digits=2)
9-element Array{Float64,1}:
    0.0
    0.0
    1.0
    0.0
    0.0
    1.0
    0.0
    0.0
    1.0

julia> tmap(_->threadid(), 1:5) # A threaded version of map
5-element Array{Int64,1}:
 2
 6
 3
 4
 5

 julia> times = tmap(_->(t=time();sleep(0.3);t), 3, 1:10); # The second argument limits the number of threads used
 julia> round.(diff(times), digits=2)
 9-element Array{Float64,1}:
  0.0
  0.0
  0.3
  0.0
  0.0
  0.3
  0.0
  0.0
  0.3

Benchmark

All benchmarks are done on a machine with 6 physical cores. The following function will be used for benchmarking

function fib(n)
    if n <= 1 return 1 end
    return fib(n - 1) + fib(n - 2)
end

Light workload

fib(20) 720 times

workload = fill(20, factorial(6))
times = map(1:6) do nt
    nt == 1 && return @belapsed map(fib, workload)
    @belapsed tmap(fib, $nt, workload)
end
t720 = @belapsed tmap(fib, workload)
plot(1:6, times, xlabel="Number of threads", ylabel="Time [s]")
hline!([t720], label="720 tasks", ylims=(0,Inf))

window

Conclusion: for light loads, the overhead is large. You may want to consider KissThreading.jl for a more advanced approach.

Heavy workload

fib(30) 720 times

workload = fill(30, factorial(6))
times = map(1:6) do nt
    nt == 1 && return @belapsed map(fib, workload)
    @belapsed tmap(fib, $nt, workload)
end
t720 = @belapsed tmap(fib, workload)
plot(1:6, times, xlabel="Number of threads", ylabel="Time [s]")
hline!([t720], label="720 tasks", ylims=(0,Inf))

window

Conclusion: tmap is effective when the computational load is heavy.

Required Packages

No packages found.