Sure, except that if the server is to be used by multiple clients, you will get clashes in the PIT when say two clients both try to compile a module with the same name.
The PIT is indexed by Module, which is basically the pair (package,modulename), and the package for the main program is always the same: "main".
This will work fine if you spin up a new server for each program you want to build - maybe that's fine for your use case?
Yep, I have a new server for each CPU. So compiling one program will start up (say) 4 compilers and one server. Then shake will start throwing source files at the server, in the proper dependency order, and the server will distribute the input files among the 4 servers. Each server is single-threaded so I don't have to worry about calling GHC functions reentrantly. But --make is single-threaded as well, so why doesn't it just call compileFile repeatedly and instead bother with all that HPT stuff? Is it just for ghci?
The 'user' is low for the server because it doesn't count time spent by the subprocesses on the other end of the socket, but excluding linking it looks like I can shave about 25% off compile time. Unfortunately it winds up being just about the same as ghc --make, so it seems too low.
But that's what you expect, isn't it?
It's surprising to me that the serial --make is just about the same speed as a parallelized one. The whole point was to compile faster! Granted, each interface has to be loaded for each processor while --make only needs to do it once, but once loaded they should stay loaded and I'd expect the benefit from two processors would win out pretty quickly.
--make has a slight advantage for linking in that it knows which packages it needs to link against, whereas plain ghc will link against all the packages on the command line.
Ohh, so maybe with --make it can omit some packages and do less work. Let me try minimizing the -packages and see if that helps. As an aside, it would be handy to be able to ask ghc "given this main module, which -packages should the final program get?" but not actually compile anything. Is there a way to do that, short of writing my own with the ghc api? Would it be a reasonable ghc flag, along the lines of -M but for packages? BTW, in case anyone is interested, a darcs repo is at http://ofb.net/~elaforge/ghc-server/