Greetings Currently GHC supports two kinds of threads - pinned to a specific capability (bound threads) and those it can migrate between any capabilities (unbound threads). For purposes of achieving lower latency in Haskell applications it would be nice to have something in between - threads GHC can migrate but within a certain subset of capabilities only. I'm developing a program that contains several kinds of threads - those that do little work and sensitive to latency and those that can spend more CPU time and less latency sensitive. I looked into several cases of increased latency in those sensitive threads (using GHC eventlog) and in all cases sensitive threads were waiting for non-sensitive threads to finish working. I was able to reduce worst case latency by factor of 10 by pinning all the threads in the program to specific capability but manually distributing threads (60+ of them) between capabilities (several different machines with different numbers of cores available) seems very fragile. World stopping GC is still a problem but at least in my case is much less frequently so. It would be nice to be able to allow GHC runtime to migrate a thread between a subset of capabilities using interface similar to this one: -- creates a thread that is allowed to migrate between capabilities according to following rule: ghc is allowed to run this thread on Nth capability if Nth `mod` size_of_word bit in mask is set. forkOn' :: Int -> IO () -> IO ThreadId forkOn' mask act = undefined This should allow to define up to 64 (32) distinct groups and allow user to break down their threads into bigger number of potentially intersecting groups by specifying things like capability 0 does latency sensitive things, caps 1..5 - less sensitive things, caps 6-7 bulk things. Anything obvious I'm missing? Any recommendations to how to implement this?