Getting a list or count of all running threads

Question

How can I do one of the following?

• get a list of ThreadIDs all running forked threads (preferably with labels) from application code?
• get a simple (maybe approximate; e.g. from the last major GC), count of running threads
• get either of the above from gdb or similar

Things that don't work for me:

• requiring a profiling build
• maintaining some data structure to try to track this information myself

Things I thought might be promising:

• write a custom eventlog hook to try to get this info https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/runtime_control.html#hooks-to-change-rts-behaviour
• create a hacky RTS binding somehow

Calcifuge

May I ask why a tracking data structure is out of bounds? In principle, this seems like the most straightforward approach: replace calls to fork primitives with ones that update a global set or counter. – Sabellian

@dfeuer: That’s what I’d do with code I controlled, but I don’t believe you can easily instrument code that you don’t control with your replacement forks – Autoroute

Maybe the ThreadScope source code will be informative? – Sabellian

I think it's a great question (clear, well-asked, clear effort to solve it yourself first, not a dupe of a thing we've seen a million times)... I just don't think there's a satisfying answer... – Northeastwards

@Sabellian I'd like to be able to audit whether threads are getting created but not destroyed properly, or whether they are leaking from some other place (e.g. a bug in async or something). I suppose I could do the former with confidence if I just store ThreadIds in a set and have a thread that polls them periodically and only removes them when the status shows "terminated" or whatever – Calcifuge

Answer 1

GHC 9.6 introduced listThreads :: IO [ThreadId]

From GHC User’s Guide:

GHC now provides a set of operations for introspecting on the threads of a program, GHC.Conc.listThreads, as well as operations for querying a thread’s label (GHC.Conc.threadLabel) and status (GHC.Conc.threadStatus).

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