Limit the number of parallelism

[uncttao]

Hi, how can I limit the Cilk runtime scheduler to a maximum N number of threads (instead of all available threads)? I need this for benchmark and other purposes. Thanks!

[neboat]

You can use the CILK_NWORKERS environment variable to set the number of Cilk workers the program uses at runtime. For example, to run the classic Cilk fib(40) on 1 worker:

$ CILK_NWORKERS=1 ./fib 40

Equivalently:

$ export CILK_NWORKERS=1
$ ./fib 40

Let me know if that works for you.

[uncttao]

This works. Thank you!

[vivkumar]

Can we change the worker count dynamically during the program execution?

[neboat]

Currently, no. We don't offer an API function to change the number of workers during program execution. Such a function turns out to be dangerous outside of a few specific contexts, and as a result, we have been reluctant to add that functionality without understanding how people want to use it.

What would you want to do with that functionality, @vivkumar?

[vivkumar]

We wanted to use OpenCilk for evaluating https://ftp.cs.wisc.edu/sohi/papers/2014/PLDI_2014_Adaptive.pdf