haskell profiling says "total time = 0.00 secs", but it's not true

Question

I'm trying to profile my program.

I compile it like that:

ghc -rtsopts -O3 -prof -auto-all Main.hs

And run:

./Main +RTS -p

And read Main.prof:

Fri Jul 15 13:06 2011 Time and Allocation Profiling Report  (Final)

       Main +RTS -p -RTS

    total time  =        0.00 secs   (0 ticks @ 20 ms)
    total alloc = 266,726,496 bytes  (excludes profiling overheads)

COST CENTRE                    MODULE               %time %alloc

trySub                         Main                   0.0   14.3
ourPalindroms                  Main                   0.0   15.0
isPalindromic                  Main                   0.0   70.7


                                                                                               individual    inherited
COST CENTRE              MODULE                                               no.    entries  %time %alloc   %time %alloc

MAIN                     MAIN                                                   1           0   0.0    0.0     0.0  100.0
 CAF                     Main                                                 240          10   0.0    0.0     0.0  100.0
  asSquareSum            Main                                                 253           0   0.0    0.0     0.0    0.0
  squares                Main                                                 252           2   0.0    0.1     0.0    0.1
  maxN                   Main                                                 248           1   0.0    0.0     0.0    0.0
  ourPalindroms          Main                                                 247           1   0.0   15.0     0.0   85.7
   isPalindromic         Main                                                 249     1000000   0.0   70.7     0.0   70.7
  main                   Main                                                 246           1   0.0    0.0     0.0   14.3
   asSquareSum           Main                                                 250        1998   0.0    0.0     0.0   14.3
    trySub               Main                                                 251        1998   0.0   14.3     0.0   14.3
 CAF                     GHC.IO.Handle.FD                                     176           2   0.0    0.0     0.0    0.0
 CAF                     GHC.IO.Encoding.Iconv                                137           2   0.0    0.0     0.0    0.0
 CAF                     GHC.Conc.Signal                                      130           1   0.0    0.0     0.0    0.0

This awesome speed of my program is actually a lie:

[.../P125]$ time ./Main +RTS -p
...output...

real    0m4.995s
user    0m4.977s
sys 0m0.010s

(Yes, I tried running both with and without time, and profiler keeps telling lies)

What can I do?

[.../P125]$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.0.3

The OS Mac OS X 10.6.8. I'm pretty sure I've installed ghc from homebrew

Answer 1

Short answer: The workaround is to compile with -threaded.

Details for over-interested souls: The reason is a GHC bug (see #5282): The profiling timer always gets triggered when the run time system wants to perform garbage collection. As at that point the execution is not in Haskell code, the cost gets attributed to the "SYSTEM" cost centre - which is then dropped from the profiling view.

The reason -threaded fixes the problem is probably that it makes the profiling timer tick in "real" time instead of the time the process was actually executing. Even though that is a less accurate form of profiling, it seems to decouple the timer enough from the program's execution to not trigger the bug.