#13296: stat() calls can block Haskell runtime -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nh2): `slyfox` on IRC answers question (2): * slyfox: yeah, ghc's RTS does not limit amount of outstanding safe FFIs. example: http://dpaste.com/2YT058D * slyfox: `ghc --make a.hs -threaded -debug && time ./a +RTS -Ds` this runs in 6 seconds and nicely prints full list of blocked threads on FFI I've also put this into a gist and confirmed it on my GHC 8.0.2: https://gist.github.com/nh2/bcf583721213d34e9f464558a91a682e Further discussion: * rwbarton: stat() in a loop in C costs about 300ns per stat call here * nh2: much easier to program than building HTTP wrappers around all software, and basic * useful features * rwbarton: so 100ns for safe call overhead is kind of significant * nh2: slyfox: I guess asking for a better API is thinking ahead a bit too much when there * isn't even an async syscall interface for it in Linux yet * rwbarton: I think that is the API slyfox means * rwbarton: getFileStatus costs another 300 ns in userspace * rwbarton: so it would be about 100 ns more on top of 600 ns * nh2: rwbarton: that is a fair point, but it is hard to get completely right: on a local FS with a fast backend (e.g. ramdisk, SSD or FS metadata in the buffer cache), stat will be very fast, but if you are on a spinning disk on a part of FS metadata that's not currently cached, a NFS, or a network-mounted VM image (e.g. Amazon EBS and the various * equivalents), it'll be 1000x slower. And there's no way to tell on which you are -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13296#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler