Debugging the runtime system

If you link with -debug you'll get the "debugging RTS". This has some extra consistency checking (which makes it run a little slower), and has lots of logging infrastructure to tell what is going on.

The extra features are controlled with the -D runtime system flag. So you might say

sh$ ghc -o MyProg Main.hs
sh$ ./MyProg +RTS -Ds

to see logging information from the scheduler.

Use +RTS -? to see a list of all the -D flags. The current list (GHC 6.12) is:

  -Ds  DEBUG: scheduler
  -Di  DEBUG: interpreter
  -Dw  DEBUG: weak
  -DG  DEBUG: gccafs
  -Dg  DEBUG: gc
  -Db  DEBUG: block
  -DS  DEBUG: sanity
  -Dt  DEBUG: stable
  -Dp  DEBUG: prof
  -Dr  DEBUG: gran
  -DP  DEBUG: par
  -Dl  DEBUG: linker
  -Dm  DEBUG: stm
  -Dz  DEBUG: stack squezing
  -Dc  DEBUG: program coverage

Of these, -DS (sanity checking) is special. It switches on a rather expensive heap-consistency check which runs after every garbage collection. Your program will run a lot slower, but it helps when tracking down garbage-collection errors.