# Threading two consecutive double loops

**URL:** https://discourse.julialang.org/t/threading-two-consecutive-double-loops/41931
**Category:** Julia at Scale
**Tags:** performance, multithreading
**Created:** [June 23, 2020, 11:35am UTC](https://discourse.julialang.org/t/threading-two-consecutive-double-loops/41931 "2020-06-23T11:35:08Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![jcook](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jcook/32/18211_2.png) [@jcook](https://discourse.julialang.org/u/jcook)
#### Post date: [June 23, 2020, 11:35am UTC](https://discourse.julialang.org/t/threading-two-consecutive-double-loops/41931/1 "2020-06-23T11:35:08Z")

</div>

Hi All,

I’m trying to improve the threaded performance of some code: I’ve written up the associated [MWE](https://gist.github.com/jwscook/2a9f080a96a01d62a805a9f0f2b64244).

Essentially we have a matrix `m`, which is mutated in two consecutive double for loops.

1. The first loop:  
The first for loop iterates over a subset of rows, and all the columns. For any given row of the subset of rows any column of of the matrix may be mutated (based on a random draw for the column index).

2. The second loop:  
This is more straightforward in that it iterates over all rows and columns: for all rows `i` and columns `j` mutate `m[i,j]` in a way that depends on the value of `m[i,j]` at the end of the first loop.

The MWE runs the code via the straightforward `method1`, which has two consecutive nested double for loops, and then by `method2` which `@spawn`s individual iterations of the first loop, which are `wait`ed for in the second loop.

If anyone can beat the performance of this, let me know!

On my machine (1st time printed is `method1`, second time is `method2`)

```bash
$for i in 1 2 4 8;do echo $i;JULIA_NUM_THREADS=$i julia ThreadProblemMWE.jl;done
1
 79.999939 seconds (201.69 k allocations: 5.648 MiB)
 61.961386 seconds (201.78 k allocations: 5.657 MiB)
2
 39.999903 seconds (201.70 k allocations: 5.649 MiB)
 31.959529 seconds (201.78 k allocations: 5.658 MiB)
4
 20.999882 seconds (201.72 k allocations: 5.652 MiB)
 16.960056 seconds (201.79 k allocations: 5.657 MiB)
8
 11.504882 seconds (201.76 k allocations: 5.657 MiB)
  9.460095 seconds (201.80 k allocations: 5.660 MiB)

```