Build time regressions

[John Lato]

Hi Edward,

This is possibly unrelated, but the setup seems almost identical to a very
similar problem we had in some code, i.e. very long compile times (6+
minutes for 1 module) and excessive memory usage when compiling generic
serialization instances for some data structures.

In our case, I also thought that INLINE functions were the cause of the
problem, but it turns out they were not.  We had a nested data structure,
e.g.

> data Foo { fooBar :: !Bar, ... }

with Bar very large (~150 records).

even when we explicitly NOINLINE'd the function that serialized Bar, GHC
still created a very large helper function of the form:

> serialize_foo :: Int# -> Int#  -> ...

where the arguments were the unboxed fields of the Bar structure, along
with the other fields within Foo.  It appears that even though the
serialization function was NOINLINE'd, it simply created a Builder, and
while combining the Builder's ghc saw the full structure.  Our serializer
uses blaze, but perhaps Binary's builder is similar enough the same thing
could happen.

Anyway, in our case the fix was to simply remove the bang pattern from the
'fooBar' record field.  Then the serialize_foo function takes a Bar as an
argument and serializes that.  I'm not entirely sure why compilation takes
so much longer otherwise.  I've tried dumping the output of each simplifier
phase and it clearly gets stuck at a certain point, but I didn't really
debug in much detail so I don't recall the details.

If you think this is related, I can investigate more thoroughly.

Cheers,
John L.

On Wed, Oct 1, 2014 at 4:54 AM, Edward Z. Yang <ezyang at mit.edu> wrote:

> Hello Joachim,
>
> This was halfway known, but it sounds like we haven't solved
> it completely.
>
> The beginning of the sordid tale was when Cabal HEAD switched
> to using derived binary instances:
> https://ghc.haskell.org/trac/ghc/ticket/9583
>
> SPJ fixed the infinite loop bug in the simplifier, but apparently
> the deriving binary generates a lot of code, meaning a lot of
> memory. https://ghc.haskell.org/trac/ghc/ticket/9630
> hvr's fix was specifically to solve this problem.
>
> But it sounds like it didn't eliminate the regression entirely?
> If there's an unrelated regression, we should suss it out.  It would
> be helpful if someone could revert just the deriving changes,
> and see if this reverts the compilation time.
>
> Edward
>
> Excerpts from Joachim Breitner's message of 2014-09-30 13:36:27 -0700:
> > Hi,
> >
> > the attached graph shows a noticable increase in build time caused by
> >
> > Update Cabal submodule & ghc-pkg to use new module re-export types
> > author    Edward Z. Yang <ezyang at cs.stanford.edu>
> >
> https://git.haskell.org/ghc.git/commit/4b648be19c75e6c6a8e6f9f93fa12c7a4176f0ae
> >
> > and only halfway mitigated by
> >
> > Update `binary` submodule in an attempt to address #9630
> > author    Herbert Valerio Riedel <hvr at gnu.org>
> >
> https://git.haskell.org/ghc.git/commit/3ecca02516af5de803e4ff667c8c969c5bffb35f
> >
> >
> > I am not sure if the improvement is related to the regression, but in
> > any case: Edward, was such an increase expected by you? If not, can you
> > explain it? Can it be avoided?
> >
> > Or maybe Cabal just became much larger... +38% in allocations when
> > running haddock on it seems to confirm this.
> >
> > Greetings,
> > Joachim
> >
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