High CPU usage during idle time due to GC
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  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.