I m using while reading

Question

I m using <http Dispatchers IO|Dispatchers IO> while reading data from files on disk It can take anywhere from 1 5 mins to complete But it s using up basically all of the available resources Is there a way to kind of cap the amount of work being done on a particular dispatcher

Tim Malseed · Answer

For example create some kind of Dispatcher similar to the IO one but it only uses 75 of the available threads

Zach Klippenstein (he/him) [MOD] · Answer

Use a semaphore to limit how much work you re doing

Tim Malseed · Answer

Could you elaborate a little

TwoClocks · Answer

I m confused Are you launching more the one disk operation at a time

TwoClocks · Answer

because one launch shouldn t ever use more than one thread at least I don t think so

Tim Malseed · Answer

The operation involves reading a file from disk then parsing it It s bottlenecked by CPU rather than disk access

TwoClocks · Answer

makes sense It still shouldn t use more than one CPU though unless it spawns other threads somehow

Tim Malseed · Answer

I m not sure what happens under the hood Running these operations in parallel is about 7x faster on my device But it means all other coroutine based operations are pretty much suspended until this work is complete

TwoClocks · Answer

yeah <http Dispatches IO|Dispatches IO> == threads it s the same if you spun up a bunch of threads that don t yeid anywhere They take up all the reources

Tim Malseed · Answer

Ok So should I be using some other Dispatcher

TwoClocks · Answer

maybe depends trying doing the file IO from a <http Dispatcher IO|Dispatcher IO> but do the CPU work from a different type of dispatcher

Tim Malseed · Answer

I don t have the luxury it s all happening in native code under a single function entry point

TwoClocks · Answer

but you probably need to look at the guts of the CPU intensive code and figure out if it s creating threads or what it s doing

TwoClocks · Answer

is the code kotlin and coroutine aware or is it java

Tim Malseed · Answer

I m calling a native function which takes a file descriptor reads in the file and parses it and returns an object back to Java This is called from Kotlin with Coroutines

TwoClocks · Answer

oh native like C or some other language nmm

Tim Malseed · Answer

Yes sorry for not being clear C++ JNI

Tim Malseed · Answer

I think there s nothing wrong with the approach I just don t want to use max threads for this task

TwoClocks · Answer

you can try a different dispatcher but I don t think it ll matter The C code is just gonna run unless you modify it you can t control it from the JVM

TwoClocks · Answer

I m not sure what you mean by max threads

Tim Malseed · Answer

I don t want to use all available resources to do this work

TwoClocks · Answer

you mean JVM threads or just all the processors are being used

TwoClocks · Answer

are you sure you arn t launching more that one of these native things at a time

Tim Malseed · Answer

Whatever is going on behind the scenes when I pass 3500 tasks to be completed with <http Dispatchers IO|Dispatchers IO> I would like it to use less than the maximum available resources

TwoClocks · Answer

oh yeah

TwoClocks · Answer

that s your poblrm

Tim Malseed · Answer

I am intentionally launching more than one native task at a time

TwoClocks · Answer

don t do that

Tim Malseed · Answer

lol

Tim Malseed · Answer

That s not helpful at all

TwoClocks · Answer

you can limit the of thread <http Dispatcher IO|Dispatcher IO> will use

Tim Malseed · Answer

I want parallelism But I don t want to use max available resources

TwoClocks · Answer

yeah you should launch N and wait for one to finish before you launch the next where N is lt the number of CPUs you have

TwoClocks · Answer

I think there is launch code to do that for you

Tim Malseed · Answer

Yeah so that s what I m asking about here I don t want to actually calculate number of available CPUs if I don t have to I m hoping there s some sort of Coroutine abstraction that does this for me

TwoClocks · Answer

TwoClocks · Answer

I remember looking at the constructor for <http Dispachers IO|Dispachers IO> It knows how many CPUs there are and you can pull that value out of it Use that 1 for the cap for the of threads and you should be fine

Tim Malseed · Answer

OK sounds like a reasonable approach

TwoClocks · Answer

here ya go `Runtime getRuntime availableProcessors `

TwoClocks · Answer

limit the number of threads to that 1 and your system won t get swamped

Tim Malseed · Answer

Yeah I m wondering whether cores 1 is the optimal number I actually have no idea I don t want to jam everything up but I also want this to complete fairly quickly relatively speaking

TwoClocks · Answer

well once you exceede the number of CPUs it s not going to go any faster in fact it ll likely slow down assuming it s all CPU bound

Tim Malseed · Answer

Does this assume no hyperthreading possibly dumb question for mobile devices

TwoClocks · Answer

in theory `availableProcessors ` should give you execution channels cores or hyperthreads or whatever

Tim Malseed · Answer

Ah OK

Tim Malseed · Answer

Thanks for your help

TwoClocks · Answer

in reality I m not sure anyone uses hyperthreads any more That was kinda a 90 s intel thing

TwoClocks · Answer

but I m not up on my ARM microcode these days