Dear all, At work, I'm developing a webapplication with happstack. When I let it run for some time (< 5min.) it crashes with the following message: "failed to create OS thread: Resource temporarily unavailable" This happens because the server continuously creates OS threads but never terminates them. This keeps going on until I hit the OS thread limit. I would like to know why the OS threads are created and more importantly why they aren't terminated. Unfortunately I can't publish the source code because it's proprietary software. Hopefully the following description may give an idea what's going on: The webapplication is basically a simple measurement application: The client side (HTML, Javascript, JQuery) contains a plot which shows the measurements that the server generates. The client contains a loop which first longpolls[2] the server to wait for a new measurement, the server will then block (using takeMVar) until (using putMVar) a measurement is available. The client then retrieves the measurement from the server including all other relevant data. These requests all happen asynchronously using ajax calls[1]. The server starts by forking a thread which runs the measure loop. At each iteration the measure loop reads data from a file then fits that data to a model. For fitting I use my levmar fitting library[3]. The coefficients of the fitted model are then used to calculate a measurement. I believe the OS threads are created by my levmar library. This library uses bindings-levmar[4] which is a binding to a C library. bindings-levmar uses safe FFI calls because the levmar C procedures are reentrant (they need to call back into Haskell to execute the model function). I believe the RTS creates an OS thread for each safe FFI call. When I replace my levmar fit function with a function which just returns a random number the problem disappears. The problem also doesn't appear in a standalone application I have which just uses my levmar fit function to fit all the datafiles, without the webserver. It seems that the combination of happstack and levmar causes the OS threads to not terminate. I tried space profiling the application but that did not reveal anything interesting. I even used threadscope to look at the application eventlog (see: [5]) but that also looked normal. Hopefully someone can point me where to look next. Thanks, Bas [1] http://api.jquery.com/jQuery.ajax/ [2] http://en.wikipedia.org/wiki/Push_technology#Long_polling [3] http://hackage.haskell.org/package/levmar [4] http://hackage.haskell.org/package/bindings-levmar [5] http://bifunctor.homelinux.net/~bas/crash.eventlog