--make should have a -j flag for parallel building

[igloo]

It should be possible to give --make a -j flag, similar to make's, to tell it to use multiple proceses to build modules. This would allow executables, libraries and cabal packages to be built faster for people with multiple CPUs.

[simonmar]

This seems like a good place to hang my patch to implement ghc --make -jN, which was used for the experiments in the 2005 Haskell Workshop paper on SMP GHC, but almost certainly isn't ready for prime time.

[bos]

Would love to have this.

[simonmar]

Not for 6.8, probably.

[simonmar]

We're not planning this for 6.10. It's more likely that Cabal will get parallel make support first, in which case there's less need for us to tackle this.

[orenbenkiki]

Is this a dead ticket? Because I'd love to see it implemented. I'm working on a 32-core machine and compiling large Haskell packages (dozens of modules). A -j flag would make a real difference for me. Granted this isn't the most common case, but I'd expect a significant improvement even for smaller packages on a dual-core machine (and these days, which machine isn't at least that?). Faster builds => happier developers, and all that :-)

[simonmar]

Attached a patch I had lying around to make FastString thread-safe. IIRC it had a small compile-time performance impact.

[morabbin]

Cabal install now has -j flag, so closing this as wontfix.

[orenbenkiki]

Cabal install -j flag solves a different problem. It builds and installs different packages in parallel. This ticket is about GHC being able to build different modules in parallel in a single package, regardless of installing the results.

[nh2]

Can somebody give an idea about the difficulty of this?

From an outsider's view, GHC has a very clear idea about module dependencies and in which order it has to build them so building independent modules in parallel shouldn't be too hard, should it?