# Task scheduling puzzle

**URL:** <https://discourse.julialang.org/t/task-scheduling-puzzle/112876>\
**Category:** Performance\
**Tags:** parallel\
**Created:** [April 12, 2024, 12:58pm UTC](https://discourse.julialang.org/t/task-scheduling-puzzle/112876 "2024-04-12T12:58:41Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![sgaure](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sgaure/32/14779_2.png) [@sgaure](https://discourse.julialang.org/u/sgaure)\
**Post date:** [April 12, 2024, 12:58pm UTC](https://discourse.julialang.org/t/task-scheduling-puzzle/112876/1 "2024-04-12T12:58:41Z")

</div>

I have a somewhat puzzling behaviour with task scheduling. I’ve this example:

```julia
using BenchmarkTools
using Base.Threads

function work(n)
    x = pi/4
    for i in 1:n
        x *= 4*(1-x)
        (i & 0x3ff == 0) && yield()
    end
    return x
end

function run()
    @sync for i in 1:nthreads()
        @spawn work(10_000_000)
    end
end

@btime run()

```

I run it with varying number of threads on a 12-core AMD:

```julia
$ for i in $(seq 1 12); do echo -n $i' '; JULIA_EXCLUSIVE=1 julia -t $i threads.jl; done
1 23.743 ms (14 allocations: 928 bytes)
2 24.579 ms (19 allocations: 1.39 KiB)
3 25.206 ms (24 allocations: 1.88 KiB)
4 25.668 ms (29 allocations: 2.36 KiB)
5 26.727 ms (34 allocations: 2.84 KiB)
6 27.717 ms (39 allocations: 3.33 KiB)
7 52.496 ms (44 allocations: 3.81 KiB)
8 144.130 ms (49 allocations: 4.30 KiB)
9 190.985 ms (55 allocations: 5.14 KiB)
10 252.950 ms (60 allocations: 5.62 KiB)
11 318.004 ms (65 allocations: 6.11 KiB)
12 402.408 ms (70 allocations: 6.59 KiB)

```

I can understand some increased time with more threads, but this is quite massive. Without the `yield()`, the time is constant, around 20 ms. So, what is going on here?

On 1.10.2 I get the following output:

```julia
$ for i in $(seq 1 12); do echo -n $i' '; JULIA_EXCLUSIVE=1 /snap/bin/julia -t $i threads.jl; done
1 23.372 ms (14 allocations: 928 bytes)
2 24.328 ms (19 allocations: 1.39 KiB)
3 25.185 ms (24 allocations: 1.88 KiB)
4 25.810 ms (29 allocations: 2.36 KiB)
5 26.423 ms (34 allocations: 2.84 KiB)
6 27.644 ms (39 allocations: 3.33 KiB)
7 41.494 ms (44 allocations: 3.81 KiB)
8 123.716 ms (49 allocations: 4.30 KiB)
9 50.254 ms (55 allocations: 5.11 KiB)
10 55.592 ms (60 allocations: 5.59 KiB)
11 60.642 ms (65 allocations: 6.08 KiB)
12 74.447 ms (70 allocations: 6.56 KiB)

```

I’ve run this several times, it’s roughly the same every time, the behaviour is not due to other activity in the box.

```julia
Julia Version 1.12.0-DEV.317
Commit 0e28cf6abf* (2024-04-08 10:46 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 24 × AMD Ryzen Threadripper PRO 5945WX 12-Cores
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 24 default, 0 interactive, 24 GC (on 24 virtual cores)

```