[Haskell-cafe] Compile times and separate compilation

[Wojtek Narczyński]

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