# Why other calculation will effect the time consumed by Threads.@threads

**URL:** <https://discourse.julialang.org/t/why-other-calculation-will-effect-the-time-consumed-by-threads-threads/83602>\
**Category:** Performance\
**Tags:** threads\
**Created:** [July 1, 2022, 8:17am UTC](https://discourse.julialang.org/t/why-other-calculation-will-effect-the-time-consumed-by-threads-threads/83602 "2022-07-01T08:17:48Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![wly](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wly/32/49517_2.png) [@wly](https://discourse.julialang.org/u/wly)\
**Post date:** [July 1, 2022, 8:17am UTC](https://discourse.julialang.org/t/why-other-calculation-will-effect-the-time-consumed-by-threads-threads/83602/1 "2022-07-01T08:17:49Z")

</div>

When I test my code, I found that the time consumed by `Threads.@threads` in actual environment is bigger than the test environment. And I think the reason is that other calculation in the actual environment effects the performance of `Threads.@threads`, so I write a demo to show that:

```julia
using TimerOutputs
using LinearAlgebra
using Polyester

@show Threads.nthreads()
LinearAlgebra.BLAS.set_num_threads(1)

function testmul0(A,B,C)
    for i in 1:1000
        mul!(C,A,B)
    end
end
function testmul(A,B,C)
    for i in 1:100
        mul!(C[i],A,B)
    end
end
function testmul_thread(A,B,C)
    Threads.@threads for i in 1:100
        mul!(C[i],A,B)
    end
end

function testmuls(A,B,C)
    for i in 1:20
        @timeit "testmul0" testmul0(A,B,C[1])
    end
    for i in 1:20
        @timeit "testmul_th" testmul_thread(A,B,C)
    end
end

function testmuls2(A,B,C)
    for i in 1:20
        # @timeit "testmul" testmul(A,B,C)
        @timeit "testmul0" testmul0(A,B,C[1])
        @timeit "testmul_th" testmul_thread(A,B,C)
    end
end

A = rand(100,100)
B = rand(100,100)
C = [rand(100,100) for i in 1:100]
testmuls(A,B,C); # first time run
testmuls2(A,B,C);

function bar()
    A = rand(100,100)
    B = rand(100,100)
    C = [rand(100,100) for i in 1:100]
    reset_timer!()
    testmuls(A,B,C)
    show(TimerOutputs.get_defaulttimer());
    reset_timer!()
    testmuls2(A,B,C)
    show(TimerOutputs.get_defaulttimer());
end
bar()

```

result:

```julia
Threads.nthreads() = 5
 ───────────────────────────────────────────────────────────────────────
                               Time Allocations      
                      ─────────────────────── ────────────────────────
   Tot / % measured: 6.24s / 100.0% 55.4KiB / 96.7%

 Section ncalls time %tot avg alloc %tot avg
 ───────────────────────────────────────────────────────────────────────
 testmul0 20 6.10s 97.9% 305ms 0.00B 0.0% 0.00B
 testmul_th 20 131ms 2.1% 6.57ms 53.6KiB 100.0% 2.68KiB
 ─────────────────────────────────────────────────────────────────────── 
───────────────────────────────────────────────────────────────────────
                               Time Allocations      
                      ─────────────────────── ────────────────────────
   Tot / % measured: 6.32s / 100.0% 55.8KiB / 96.8%

 Section ncalls time %tot avg alloc %tot avg
 ───────────────────────────────────────────────────────────────────────
 testmul0 20 6.10s 96.6% 305ms 0.00B 0.0% 0.00B
 testmul_th 20 218ms 3.4% 10.9ms 54.0KiB 100.0% 2.70KiB
 ───────────────────────────────────────────────────────────────────────

```

`testmul0` is just a time consuming calculation.  
In function `testmuls`, I repeat call `testmul0` and `testmul_th` in two for loops, and the time consumed by `testmul_th` is only `6.57`ms. But when I repeat call `testmul0` and `testmul_th` in one for loop, the time consumed by `testmul_th` increases to `10.9`ms.  
So why the calculation of `testmul0` will effect the time of `testmul_th`?

Thank you very much.