Profiling builds with Stack

Question

How do I tell stack to build my executable and all its dependencies with -prof?

Simply adding it to ghc-options in the .cabal file is not enough, because it only tries to build the executable with profiling enabled, which fails.

Answer 1

Profiling builds with Stack 1.0.0 and newer

To build with profiling enabled:

stack build --profile

You may need to run stack clean first, but this should be fixed in Stack 1.5.0.

To profile:

stack exec --profile -- <your program> +RTS <profiling options>

where for <profiling options> you might want -p for time profiling or -h for memory profiling. For time profiling, the profile appears in ./<your program>.prof, and for memory profiling, the profile appears in ./<your program>.hp.

See GHC profiling documentation for more profiling options.

Avoiding unnecessary rebuilding of local packages (fixed in Stack 2.X?)

Due to a long standing Stack issue, switching between profiling and non-profiling builds can cause a lot of unnecessary rebuilding of local packages and extra-deps. To work around this, you can use separate build caches for your profiling and non-profiling builds. For example, where you use stack <cmd> for non profiling you can use

stack --work-dir .stack-work-profile --profile <cmd>

for a profiling version of <cmd>. This uses a separate cache in .stack-work-profile for profiling artifacts, whereas non profiling artifacts will be preserved in the default .stack-work cache.

Profiling builds with Stack versions before 1.0.0 (i.e. from 2015)

To build with profiling enabled:

stack build --executable-profiling --library-profiling --ghc-options="-fprof-auto -rtsopts"

To profile:

stack exec -- <your program> +RTS <profiling options>

Example for Stack 1.0.0 and newer

Suppose you have a package called test with a single executable test defined by main here:

module Main where

main :: IO ()
main = do
  print $ foo 0

foo :: Int -> Int
foo x = fooSub (x+1)
  where
    fooSub x = bar (x+1)

bar :: Int -> Int
bar x = barSub (x+1)
  where
    barSub x = barSubSub (x+1)
      where
        barSubSub x = x+1

then doing stack build --profile && stack exec -- test +RTS -p will produce a file ./test.prof which includes

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

  [... many lines omitted ...]
  main                      Main                  src/Main.hs:(4,1)-(5,15)    97          0    0.0    0.0     0.0    0.0
   foo                      Main                  src/Main.hs:(8,1)-(10,24)   98          1    0.0    0.0     0.0    0.0
    foo.fooSub              Main                  src/Main.hs:10:5-24         99          1    0.0    0.0     0.0    0.0
     bar                    Main                  src/Main.hs:(13,1)-(17,46) 100          1    0.0    0.0     0.0    0.0
      bar.barSub            Main                  src/Main.hs:(15,5)-(17,46) 101          1    0.0    0.0     0.0    0.0
       bar.barSub.barSubSub Main                  src/Main.hs:17:9-46        102          1    0.0    0.0     0.0    0.0
 main                       Main                  src/Main.hs:(4,1)-(5,15)    95          0    0.0   20.5     0.0   20.5

I.e., there is profiling information for all definitions, including local definitions in where clauses.

If you only want to profile top-level definitions, you can build with the GHC option -fprof-auto-top instead: doing stack build --profile --ghc-options=-fprof-auto-top && stack exec -- test +RTS -p produces a ./test.prof which includes

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

 [... many lines omitted ...]
  main      Main                  src/Main.hs:(4,1)-(5,15)    97          0    0.0    0.0     0.0    0.0
   foo      Main                  src/Main.hs:(8,1)-(10,24)   98          1    0.0    0.0     0.0    0.0
    bar     Main                  src/Main.hs:(13,1)-(17,46)  99          1    0.0    0.0     0.0    0.0
 main       Main                  src/Main.hs:(4,1)-(5,15)    95          0    0.0   20.5     0.0   20.5

instead.

Finally, note that stack build --profile also turns on stack traces. If you change the program so that barSubSub x = error $ show x, then running stack build --profile && stack exec test produces

test: 4
CallStack (from HasCallStack):
  error, called at src/Main.hs:17:23 in main:Main
CallStack (from -prof):
  Main.bar.barSub.barSubSub (src/Main.hs:17:9-36)
  Main.bar.barSub (src/Main.hs:(15,5)-(17,36))
  Main.bar (src/Main.hs:(13,1)-(17,36))
  Main.foo.fooSub (src/Main.hs:10:5-24)
  Main.foo (src/Main.hs:(8,1)-(10,24))
  Main.main (src/Main.hs:(4,1)-(5,15))
  Main.CAF:lvl8_r4Fc (<no location info>)

Pretty cool!