# What is julia doing with your threads?

**URL:** https://discourse.julialang.org/t/what-is-julia-doing-with-your-threads/110052
**Category:** General Usage
**Created:** [February 11, 2024, 12:07am UTC](https://discourse.julialang.org/t/what-is-julia-doing-with-your-threads/110052 "2024-02-11T00:07:10Z")
**Posts on this page:** 1
**Showing post:** 20

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### Author: ![abraemer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraemer/32/51403_2.png) [@abraemer](https://discourse.julialang.org/u/abraemer)
#### Post date: [February 12, 2024, 9:45am UTC](https://discourse.julialang.org/t/what-is-julia-doing-with-your-threads/110052/20 "2024-02-12T09:45:48Z")

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I can replicate this as well on my laptop with Julia 1.10.0 and a AMD Ryzen 7 4800H

- `-t 5` gives `extrema(ts) = (1.126322503, 2.474583519)` → Sometimes slow and no interactive threads
- `-t 5,1` gives `extrema(ts) = (1.130685362, 1.339060755)` → never slow and 1 interactive thread
- `-t auto` (equivalent to `-t 16,0` for me) gives:  
`extrema(ts) = (1.148305388, 2.280251291)` → sometimes slow and no interactive threads

I wonder if this is loosely related to [Bug in sleep() function - main thread work affecting sleep duration on running tasks](https://discourse.julialang.org/t/bug-in-sleep-function-main-thread-work-affecting-sleep-duration-on-running-tasks/101911).  
Here is a wild guess (as I really don’t anything about the implementation of Tasks and scheduling): Julia needs to schedule the task in some thread. If there are no interactive threads, Julia’s “main thread” is in the same threadpool that works on the tasks. So maybe sometimes a Task gets scheduled on the main thread and starts running before all Tasks where scheduled and so some Tasks are scheduled late. This would not happen if there is at least a single `interactive` thread because the main thread is always in the `interactive` pool\[1\] and the tasks are scheduled to run in the `:default` pool.  
I modified the example above to schedule the tasks on the `:interactive` pool instead and then the slowdown again occurs. For `julia -t 5,5` (so 5 interactive threads and 5 normal ones) I get again `extrema(ts) = (1.117211121, 2.510383934)`.

EDIT: Found another thread that notices (likely) this problem: [With julia-1.9, should the main task block :interactive tasks? - #2 by samtkaplan](https://discourse.julialang.org/t/with-julia-1-9-should-the-main-task-block-interactive-tasks/92809/2)

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1. This can be verified by starting a session with or without interactive threads and just running `Threads.threadpool()` to see the current threadpool. It is `:interactive` if there are interactive threads and `:default` otherwise.

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