JuliaLang/julia

`JULIA_NUM_THREADS` ignored when embedding Julia on a Windows system

Open

#55,468 opened on Aug 12, 2024

View on GitHub
 (1 comment) (0 reactions) (0 assignees)Julia (48,709 stars) (5,773 forks)batch import
bughelp wantedsystem:windows

Description

The following c# code...

Environment.SetEnvironmentVariable("JULIA_NUM_THREADS", "4");
jl_init_with_image__threading(mBinPath, mSysImg);
jl_eval_string("println(\"Threads=$(Threads.nthreads())\")");
jl_eval_string("println(\"JULIA_NUM_THREADS=$(ENV[\"JULIA_NUM_THREADS\"])\")");

Produces the following output

Threads=1
JULIA_NUM_THREADS=4

In discussions with the Jeff and Keno it appears this happens in Windows because there are several environment stores in Windows, and the threads are created by reading with getenv (which access the machine level environment), while later in Julia the ENV is retrieved using getenvw (which access the process level environment).

Contributor guide

Tech stack
None
Domain
backendapi
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Knowledge of Julia's C APIFamiliarity with Windows environment variablesC programming experienceExperience with multithreading
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the Windows environment variable handling in Julia's embedding initialization. The issue mentions that `getenv` reads machine level environment while `getenvw` reads process level. Look at the Julia source code, particularly the thread creation path and the `jl init with image threading` function, to see where `JULIA NUM THREADS` is read. The fix likely involves using the Windows `GetEnvironmentVariable` or ` wgetenv` consistently. Refer to the discussion with Jeff and Keno for further details.
`JULIA_NUM_THREADS` ignored when embedding Julia on a Windows system · JuliaLang/julia#55468 | Good First Issue