Say I'm writing a package and there's a bit of code that measurably benefits from multithreading with Threads.@threads
. Should I use multithreading?
My concern is that doing so may clash with multithreading that the package user has set up in their own code, leading to overall slower execution.
It's a good question... one option might be to add an optional keyword argument multithreaded=true
or false
to whatever relevant function... optionally plus or minus some heuristics to pick the fastest option by default (e.g. https://github.com/JuliaSIMD/VectorizedStatistics.jl/blob/4839b730f88f1d39091624f9433c8a9009e9a83e/src/vcov.jl#L41-L48)
Not sure I'm 100% satisfied with that approach, but it's an option and I haven't thought of a better one yet in that case
Threads.@threads
uses the composable task-level parallelism now, so in theory it composes well with other multithreaded user code. I think Threads.@threads
still depends on Threads.nthreads()
to determine the number of tasks to use, and this could be a bad heuristic depending on your problem size. (But if this is a problem you can do manual decomposition of your problem into tasks with @spawn
.)
Overall I think it's quite reasonable to try just using @threads
unconditionally. It would be best to do some tests with some plausible outer threaded loop — such as might be written by a user — around your library to reassure yourself.
still depends on Threads.nthreads() to determine the number of tasks to use
Actually, this is no longer true in 1.9-DEV — it uses dynamic scheduling instead which is great.
Or... scratch that, it has some dynamic scheduling aspects, but the number of tasks is still the number of threads.
Anyway, I'd say try it and see!
Chris Foster said:
Threads.@threads
uses the composable task-level parallelism now, so in theory it composes well with other multithreaded user code. I thinkThreads.@threads
still depends onThreads.nthreads()
to determine the number of tasks to use, and this could be a bad heuristic depending on your problem size. (But if this is a problem you can do manual decomposition of your problem into tasks with@spawn
.)Overall I think it's quite reasonable to try just using
@threads
unconditionally. It would be best to do some tests with some plausible outer threaded loop — such as might be written by a user — around your library to reassure yourself.
Well.. if 'now' means on the bleeding edge non-released versions then yeah
I'd recommend strongly against shipping packages that use Threads.@threads
without a way to disable it
if 'now' means on the bleeding edge non-released versions then yeah
1.6 is the LTS and it uses the task system?
Oh... waait a minute. No, it kind of uses it, but I was mistaken in exactly how. Looking closer, it actually subverts the scheduler and statically assigns tasks to threads.
In that case yeah, I totally agree.
Thanks everyone for the pointers! I think I'll be conservative and disable threads by default, but use a keyword argument to allow users to enable it.
Chris Foster said:
if 'now' means on the bleeding edge non-released versions then yeah
1.6 is the LTS and it uses the task system?
Oh... waait a minute. No, it kind of uses it, but I was mistaken in exactly how. Looking closer, it actually subverts the scheduler and statically assigns tasks to threads.
In that case yeah, I totally agree.
Yeah, there's a lot of confusion about this and it's bad. It wasn't until a couple months ago that the PR was merged to make it dynamic rather than static. Jeff was on the fence about it. My argument in favour of the dynamic scheduler was that many people right now are under the mistaken assumption that it is dynamic, so that's what we should provide by default.
This is why I constantly tell people "don't use Base threads, just use ThreadsX.jl unless you know exactly what you're doing and why you need something different. "
I kind of hate that it took this long since 1.3 to get proper dynamic and nested threading...
the initial multithreading blogpost was almost 3 years ago, and while the @spawn
tasks were easily nest & composable, @threads
just wasn't
https://julialang.org/blog/2019/07/multithreading/#how_to_use_it
Last updated: Nov 06 2024 at 04:40 UTC