# Compilation time with \`@time\` wrong with multi-threading?

**URL:** <https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978>\
**Category:** General Usage\
**Created:** [January 18, 2024, 5:48pm UTC](https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978 "2024-01-18T17:48:56Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [January 18, 2024, 5:48pm UTC](https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978/1 "2024-01-18T17:48:56Z")

</div>

It seems that compilation time is not properly computed when a function is run multi-threaded, apparently increasing with the number of threads. For example, take the standard sum example of the docs:

```julia-repl
julia> function sum_single(a)
           s = 0
           for i in a
               s += i
           end
           s
       end
sum_single (generic function with 1 method)

julia> function sum_multi_good(a)
           chunks = Iterators.partition(a, length(a) ÷ Threads.nthreads())
           tasks = map(chunks) do chunk
               Threads.@spawn sum_single(chunk)
           end
           chunk_sums = fetch.(tasks)
           return sum_single(chunk_sums)
       end
sum_multi_good (generic function with 1 method)

```

If I start Julia with 4 threads, I get:

```julia-repl
julia> @time sum_multi_good(1:4_000_000)
  0.040086 seconds (49.33 k allocations: 3.509 MiB, 148.52% compilation time)
8000002000000

julia> @time sum_multi_good(1:4_000_000)
  0.000185 seconds (34 allocations: 2.516 KiB)
8000002000000

```

Now if I start with 8 threads, but increase the workload by 2, I get:

```julia
julia> @time sum_multi_good(1:8_000_000)
  0.040237 seconds (49.86 k allocations: 3.537 MiB, 335.67% compilation time)
32000004000000

julia> @time sum_multi_good(1:8_000_000)
  0.000079 seconds (58 allocations: 4.641 KiB)
32000004000000

```

And now with 16 threads:

```julia
julia> @time sum_multi_good(1:16_000_000)
  0.040438 seconds (50.94 k allocations: 3.592 MiB, 484.09% compilation time)
128000008000000

julia> @time sum_multi_good(1:16_000_000)
  0.000252 seconds (106 allocations: 8.891 KiB)
128000008000000

```

Note how the excution time is similar for the first run in all cases, but reported compilation time is increasing.

With 32 threads I get:

```julia
julia> @time sum_multi_good(1:32_000_000)
  0.105830 seconds (53.08 k allocations: 3.703 MiB, 1804.13% compilation time)
512000016000000

```

(my computer does not have such many cores).

These compilation times do not seem to make sense. Do they?

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [January 18, 2024, 6:31pm UTC](https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978/2 "2024-01-18T18:31:17Z")

</div>

they do not. tracking compile time requires knowing when you are entering and exiting the compiler and when multiple threads call the compiler you can sometimes end up with double-counting. This is a bug.

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<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [January 18, 2024, 6:34pm UTC](https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978/3 "2024-01-18T18:34:15Z")

</div>

Is it known? Or should I report it? (done here:[compilation time reported incorrectly with @time and multithreading · Issue #53016 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/53016))

---

<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [January 18, 2024, 6:45pm UTC](https://discourse.julialang.org/t/compilation-time-with-time-wrong-with-multi-threading/108978/4 "2024-01-18T18:45:58Z")

</div>

Reporting it would be good probably. I think the general issue is known but good reproducers are always nice.