Hi all, Cc: Simon Marlow I think I found a way to make the throughput of web servers much better. In short, put "Control.Concurrent.yield" after sending something. This enables good schedule shaping. Typical code for web servers is like this: loop = do keepAlive <- receiveRequest sendResponse when keepAlive loop With this code, a typical sequence of system calls is like this: recvfrom(13, ) -- thread A sendto(13, ) -- thread A recvfrom(13, ) = -1 EAGAIN -- thread A epoll_ctl(3, ) -- IO manager recvfrom(14, ) -- thread B sendto(14, ) -- thread B recvfrom(14, ) = -1 EAGAIN -- thread B epoll_ctl(3, ) -- IO manager Since each thread calls recvfrom() immediately after sendto(), the possibility that recvfrom() can receive a request is low. This involves the IO manager. To make the possibility higher, I put "yield" after sendResponse: loop = do keepAlive <- receiveRequest sendResponse yield when keepAlive loop Yield pushes its Haskell thread onto the end of thread queue. So, another thread can work. During the work of other threads, a request message would arrive. recvfrom(13, ) -- thread A sendto(13, ) -- thread A recvfrom(14, ) -- thread B sendto(14, ) -- thread B recvfrom(13, ) -- thread A sendto(13, ) -- thread A I tested this idea on SimpleServer and confirmed that its throughput is doubled: No yield: 58,152 req/s (https://gist.github.com/AndreasVoellmy/4184520#file-simpleserver-hs) Yield: 113,048 req/s (https://gist.github.com/kazu-yamamoto/6051118#file-simpleserver2-hs) Unfortunately, yield does not work for Warp as expected. It does not pass control to another thread and behaves as used to be. So, my question: why doesn't "yield" work in Warp? ResourceT is doing a special thing? My code change is here: https://github.com/yesodweb/wai/commit/4e9c6316ad59cd87be13000d15df8e6fd7c31... I'm using GHC head (with multicore IO manager). I'm sure that the following patch to fix the bug of yield is applied: https://github.com/ghc/ghc/commit/66839957a376dbe85608822c1820eb6c99210883 P.S. It would be appreciated if someone tries this idea on other web servers. --Kazu