Performance issues uncovered with weighttp

[ezyang]

A few years ago I did some benchmarking of happstack using weighttp, and ran into some curious issues, which I reported here: http://redmine.lighttpd.net/issues/2554#change-8686

The maintainer of weighttp did some very nice analysis on happstack, and identified some places where happstack can do better. Would be worth looking at closely and trying to address!

[stepcut]

Nice. Some of the issues he identified are buried deep in the GHC RTS. But it looks like there is some low hanging fruit there as well. For example this line:

https://github.com/Happstack/happstack-server/blob/master/src/Happstack/Server/Internal/Handler.hs#L283

Probably results in some of the excessive system calls. We should probably concat the bytestrings and make a single system call.