Hi
I've talked to John a bit, and discussed test cases etc. I've tracked
this down a little way.
Given the attached file, compiling witih SHORT_EXPORT_LIST makes the
code go _slower_. By exporting the "print_lines" function the code
doubles in speed. This runs against everything I was expecting, and
that Simon has described.
Taking a look at the .hi files for the two alternatives, there are two
differences:
1) In the faster .hi file, the body of print_lines is exported. This
is reasonable and expected.
2) In the faster .hi file, there are additional specialisations, which
seemingly have little/nothing to do with print_lines, but are omitted
if it is not exported:
"SPEC >>= [GHC.IOBase.IO]" ALWAYS forall @ el
$dMonad :: GHC.Base.Monad GHC.IOBase.IO
Sound.IterateeM.>>= @ GHC.IOBase.IO @ el $dMonad
= Sound.IterateeM.a
`cast`
(forall el1 a b.
Sound.IterateeM.IterateeGM el1 GHC.IOBase.IO a
-> (a -> Sound.IterateeM.IterateeGM el1 GHC.IOBase.IO b)
-> trans
(sym ((GHC.IOBase.:CoIO)
(Sound.IterateeM.IterateeG el1 GHC.IOBase.IO b)))
(sym ((Sound.IterateeM.:CoIterateeGM) el1 GHC.IOBase.IO b)))
@ el
"SPEC Sound.IterateeM.$f2 [GHC.IOBase.IO]" ALWAYS forall @ el
$dMonad ::
GHC.Base.Monad GHC.IOBase.IO
Sound.IterateeM.$f2 @ GHC.IOBase.IO @ el $dMonad
= Sound.IterateeM.$s$f2 @ el
"SPEC Sound.IterateeM.$f2 [GHC.IOBase.IO]" ALWAYS forall @ el
$dMonad ::
GHC.Base.Monad GHC.IOBase.IO
Sound.IterateeM.$f2 @ GHC.IOBase.IO @ el $dMonad
= Sound.IterateeM.$s$f21 @ el
"SPEC Sound.IterateeM.liftI [GHC.IOBase.IO]" ALWAYS forall @ el
@ a
$dMonad ::
GHC.Base.Monad GHC.IOBase.IO
Sound.IterateeM.liftI @ GHC.IOBase.IO @ el @ a $dMonad
= Sound.IterateeM.$sliftI @ el @ a
"SPEC return [GHC.IOBase.IO]" ALWAYS forall @ el
$dMonad :: GHC.Base.Monad
GHC.IOBase.IO
Sound.IterateeM.return @ GHC.IOBase.IO @ el $dMonad
= Sound.IterateeM.a7
`cast`
(forall el1 a.
a
-> trans
(sym ((GHC.IOBase.:CoIO)
(Sound.IterateeM.IterateeG el1 GHC.IOBase.IO a)))
(sym ((Sound.IterateeM.:CoIterateeGM) el1 GHC.IOBase.IO a)))
@ el
My guess is that these cause the slowdown - but is there any reason
that print_lines not being exported should cause them to be omitted?
All these tests were run on GHC 6.10.1 with -O2.
Thanks
Neil
On Fri, Nov 21, 2008 at 10:33 AM, Simon Peyton-Jones
| This project is based on Oleg's Iteratee code; I started using his | IterateeM.hs and Enumerator.hs files and added my own stuff to | Enumerator.hs (thanks Oleg, great work as always). When I started | cleaning up by moving my functions from Enumerator.hs to MyEnum.hs, my | minimal test case increased from 19s to 43s. | | I've found two factors that contributed. When I was cleaning up, I | also removed a bunch of unused functions from IterateeM.hs (some of | the test functions and functions specific to his running example of | HTTP encoding). When I added those functions back in, and added | INLINE pragmas to the exported functions in MyEnum.hs, I got the | performance back. | | In general I hadn't added export lists to the modules yet, so all | functions should have been exported.
I'm totally snowed under with backlog from my recent absence, so I can't look at this myself, but if anyone else wants to I'd be happy to support with advice and suggestions.
In general, having an explicit export list is good for performance. I typed an extra section in the GHC performance resource http://haskell.org/haskellwiki/Performance/GHC to explain why. In general that page is where we should document user advice for performance in GHC.
I can't explain why *adding* unused functions would change performance though!
Simon
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