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.
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.
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.
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>
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!