[Haskell-cafe] Compile times and separate compilation

[Benno Fünfstück]

I think there's a problem with the number of cores that they report: the
vms/containers report 16 cores, while they only have 2. Perhaps -j2 would
work better?

Wojtek Narczyński <wojtek at power.com.pl> schrieb am So., 28. Feb. 2016 18:22:

> On 22.02.2016 22:27, Johannes Waldmann wrote:
> >> I presume you are using the -j switch.
> > Does it really help? I recently found that for travis builds,
> > it is used by default (first surprise)
> > but it's faster when it's turned off (second surprise).
> >
> > https://ghc.haskell.org/trac/ghc/ticket/10818#comment:15
> >
> >
>
> I meant ghc -j, not cabal -j. It does work very well.
>
> I did the following test.
>
> wojtek at Desktop2016:~/src/tpdb-1.2.0/src$ time ghc -j -fforce-recomp
> TPDB/*.hs TPDB/XTC/*.hs TPDB/DP/*.hs TPDB/CPF/Proof/*.hs TPDB/Data/*.hs
> TPDB/Plain/*.hs TPDB/Xml/*.hs
> [24 of 24] Compiling TPDB.Mirror      ( TPDB/Mirror.hs, TPDB/Mirror.o )
> real    0m1.221s
> user    0m2.416s
> sys    0m0.604s
>
> wojtek at Desktop2016:~/src/tpdb-1.2.0/src$ time ghc -fforce-recomp
> TPDB/*.hs TPDB/XTC/*.hs TPDB/DP/*.hs TPDB/CPF/Proof/*.hs TPDB/Data/*.hs
> TPDB/Plain/*.hs TPDB/Xml/*.hs
> [24 of 24] Compiling TPDB.Mirror      ( TPDB/Mirror.hs, TPDB/Mirror.o )
> real    0m1.791s
> user    0m1.660s
> sys    0m0.060s
>
> wojtek at Desktop2016:~/src/tpdb-1.2.0$ time cabal build -j
> [24 of 24] Compiling TPDB.CPF.Proof.Util ( src/TPDB/CPF/Proof/Util.hs,
> dist/build/TPDB/CPF/Proof/Util.o )
> real    0m5.544s
> user    0m10.912s
> sys    0m2.628s
>
> Something is indeed wrong, either cabal is misusing ghc, or you are
> misusing cabal.
>
> This package is not a good test case for parallel build, it is too small.
>
> --
> Wojtek
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