Optimization

Compiling your Program for Profiling

The GHC compiler has mature support for compiling with profiling annotations.

Using the -prof and -fprof-auto flags when compiling will add support to your binary for profiling flags for use at runtime.

Suppose we have this program:

main = print (fib 30)
fib n = if n < 2 then 1 else fib (n-1) + fib (n-2)

Compiled it like so:

ghc -prof -fprof-auto -rtsopts Main.hs

Then ran it with runtime system options for profiling:

./Main +RTS -p

We will see a main.prof file created post execution (once the program has exited), and this will give us all sorts of profiling information such as cost centers which gives us a breakdown of the cost associated with running the various parts of the code:

    Wed Oct 12 16:14 2011 Time and Allocation Profiling Report  (Final)

           Main +RTS -p -RTS

        total time  =        0.68 secs   (34 ticks @ 20 ms)
        total alloc = 204,677,844 bytes  (excludes profiling overheads)

COST CENTRE MODULE  %time %alloc

fib         Main    100.0  100.0


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

MAIN        MAIN                    102           0    0.0    0.0   100.0  100.0
 CAF        GHC.IO.Handle.FD        128           0    0.0    0.0     0.0    0.0
 CAF        GHC.IO.Encoding.Iconv   120           0    0.0    0.0     0.0    0.0
 CAF        GHC.Conc.Signal         110           0    0.0    0.0     0.0    0.0
 CAF        Main                    108           0    0.0    0.0   100.0  100.0
  main      Main                    204           1    0.0    0.0   100.0  100.0
   fib      Main                    205     2692537  100.0  100.0   100.0  100.0

Cost Centers

Cost centers are annotations on a Haskell program which can be added automatically by the GHC compiler — using -fprot-auto — or by a programmer using {-# SCC "name" #-} <expression>, where “name” is any name you wish and <expression> is any valid Haskell expression:

-- Main.hs
main :: IO ()
main = do let l = [1..9999999]
          print $ {-# SCC "print_list" #-} (length l)

Compiling with -fprof and running with +RTS -p e.g. ghc -prof -rtsopts Main.hs && ./Main.hs +RTS -p would produce Main.prof once the program’s exited.