We have a web application using happstack. It always used ~1% CPU, which was fine. Recently, we started caching more things in memory, and CPU usage during idle time rose to 15-30%. We eventually determined this was due to the idle time GC (we compile with -threaded). Using +RTS -I2 removed the problem.

This is similar to #3408 (closed), except that that says it is fixed in 6.12, and that the interval is set to 5s. The docs [1] still mention 0.3s, though, and since changing it to 2s fixes our problem, it seems it's not 5s by default.

It is difficult to attach a simple test case, but I can test things if needed.

[1] http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/runtime-control.html\#rts-options-gc