# Consistent 2x Slowdown with v1.2

**URL:** <https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535>\
**Category:** Performance\
**Created:** [October 5, 2019, 10:32pm UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535 "2019-10-05T22:32:34Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![swishmas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/swishmas/32/8803_2.png) [@swishmas](https://discourse.julialang.org/u/swishmas)\
**Post date:** [October 5, 2019, 10:32pm UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/1 "2019-10-05T22:32:34Z")

</div>

Hi there,

I have been working on some code (~4,000 lines) for a while, and I was excited about the upcoming changes to the parallelization in v1.3. I felt I had very efficient code on v1.1(.1), but when I run the code on v1.2 or v1.3, the code is slower by a factor of two in all instances and runs.

There’s nothing obvious on the release notes that would create this slow down. I do use parallelization (`Threads.@threads`) but I don’t think the change would make any difference and I still see the code is parallelized when running the code with `htop` in terminal.

I was wondering if I missed something from 1.1(.1) to v1.2 (or v1.3) with the parallelization or anything else that would cause a very consistent 2x slow-down. Have other people run into this? I can post a few examples in a little but I’m not even sure where to start looking in the code in the first place.

---

<div class="post-metadata">

**Author:** ![ssfrr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ssfrr/32/3736_2.png) [@ssfrr](https://discourse.julialang.org/u/ssfrr)\
**Post date:** [October 5, 2019, 11:24pm UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/2 "2019-10-05T23:24:10Z")

</div>

I would start with profiling the code on both 1.1 and 1.2 to see if you can pin down where the difference is. If you use Juno the `@profiler` macro is great and gives a nice interactive visualization to explore the code.

-s

---

<div class="post-metadata">

**Author:** ![swishmas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/swishmas/32/8803_2.png) [@swishmas](https://discourse.julialang.org/u/swishmas)\
**Post date:** [October 6, 2019, 12:54am UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/3 "2019-10-06T00:54:51Z")

</div>

Thank you for the response. Yes, I did profile the code (`@profile`, simply). I should have mentioned that. It didn’t pinpoint any function in particular that was longer by itself. Instead, a lot of functions appeared to be slower.

I chose one of the functions, placed `@time` around the sub-functions and found the allocations on a parallelized loop (using a lot of `let` statements and `Threads.@threads`) was tossing out a lot more allocations that it should have on v1.3.

Checking with `@code_warntype` didn’t show any type instability and the single function being called inside the loop showed no allocations. So, I figured it must be the parallel loop or something much deeper.

Edit 1: Commenting out the `let` statements didn’t help. I added the `let` into the loop in a previous version and it got rid of several type stability issues and made overall faster code with less allocations.

Edit 2: I actually tested between v1.1.1 and v1.3 if that makes any difference.

---

<div class="post-metadata">

**Author:** ![Ralph\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ralph_smith/32/10344_2.png) [@Ralph\_Smith](https://discourse.julialang.org/u/Ralph_Smith)\
**Post date:** [October 6, 2019, 1:17am UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/4 "2019-10-06T01:17:31Z")

</div>

[https://github.com/JuliaLang/julia/issues/32701](https://github.com/JuliaLang/julia/issues/32701)

The upshot is that after v1.1 the multithreading infrastructure was radically overhauled, and for v1.3 we should look for correctness rather than speed.

---

<div class="post-metadata">

**Author:** ![swishmas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/swishmas/32/8803_2.png) [@swishmas](https://discourse.julialang.org/u/swishmas)\
**Post date:** [October 6, 2019, 1:41am UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/5 "2019-10-06T01:41:51Z")

</div>

Oh my…what a surprise!

Ok, this puts my mind at ease. Thank you!

---

<div class="post-metadata">

**Author:** ![JesseSantoso](https://avatars.discourse-cdn.com/v4/letter/j/dfb087/32.png) [@JesseSantoso](https://discourse.julialang.org/u/JesseSantoso)\
**Post date:** [October 10, 2019, 5:07am UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/6 "2019-10-10T05:07:27Z")

</div>

I’ve had a very similar issue. My code relies on a parallelised inner loop which then gets called a bunch of times in a serialised outer loop. The threading overhead leads to a roughly 5-10x slowdown between 1.1 and 1.2/1.3 rc for my code. So I guess I’m just gonna be sticking to 1.1, as there seems to be no intention to fix this for the final 1.3 release version…

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 10, 2019, 5:55am UTC](https://discourse.julialang.org/t/consistent-2x-slowdown-with-v1-2/29535/7 "2019-10-10T05:55:54Z")

</div>

Won’t harm to try it when it comes out 😉 , also worth trying, depending on your use, is to remove the inner `@threads` and use `@spaw` – `fetch`, this can potentially speed up things if your inner loops don’t always finish at the same time and next outer loop doesn’t depend on a previous one. (probably uncommon)