Hi! Our company's main commercial product is a Snap-based web app which we compile with GHC 7.8.4. It works on four app-servers currently load-balanced behind Haproxy. I recently implemented a new piece of functionality, which led to weird behavior which I have no idea how to debug, so I'm asking here for help and ideas! The new functionality is this: on specific url-handler, we need to query n external services concurrently with a timeout, gather and render results. Easy (in Haskell)! The implementation looks, as you might imagine, something like this (sorry for almost-real-haskell, I'm sure I forgot tons of imports and other things, but I hope everything is clear as-is, if not -- I'll be glad to update gist to make things more specific): https://gist.github.com/k-bx/0cf7035aaf1ad6306e76 Now, this works wonderful for some time, and in logs I can see both, successful fetches of external-content, and also lots of timeouts from our external providers. Life is good. But! After several days of work (sometimes a day, sometimes couple days), apps on all 4 servers go crazy. It might take some interval (like 20 minutes) before they're all crazy, so it's not super-synchronous. Now: how crazy, exactly? First of all, this endpoint timeouts. Haproxy requests for a response, and response times out, so they "hang". Secondly, logs are interesting. If you look at the code from gist once again, you can see, that some of CandidateProvider's don't actually require any networking work, so all they do is actually just logging that they're working (I added this as part of debugging actually) and return pure data. So what's weird is that they timeout also! Here's how output of our logs starts to look like after the bug happens: ``` [2015-04-22 09:56:20] provider: CandidateProvider1 [2015-04-22 09:56:20] provider: CandidateProvider2 [2015-04-22 09:56:21] Got timeout while requesting CandidateProvider1 [2015-04-22 09:56:21] Got timeout while requesting CandidateProvider2 [2015-04-22 09:56:22] provider: CandidateProvider1 [2015-04-22 09:56:22] provider: CandidateProvider2 [2015-04-22 09:56:23] Got timeout while requesting CandidateProvider1 [2015-04-22 09:56:23] Got timeout while requesting CandidateProvider2 ... and so on ``` What's also weird is that, even after timeout is logged, the string ""Got responses!" never gets logged also! So hanging happens somewhere in-between. I have to say I'm sorry that I don't have strace output now, I'll have to wait until this situation happens once again, but I'll get later to you with this info. So, how is this possible that almost-pure code gets timed-out? And why does it hang afterwards? CPU and other resource usage is quite low, number of open file-descriptors also (it seems). Thanks for all the suggestions in advance!