Multiprocessor improvement in gcc?

[cfaun5]

Would I experience a significant performance increase when running gcc on a multi-processor system? Or is gcc single-threaded and would I just be wasting money? Would I need to do something w/ distcc to get a performance increase?

Any help would be greatly appreciated,
cfaun5

[bwkaz]

gcc is single-threaded, yes. In general, compilation is a single threaded problem -- you can't very easily compile independent chunks of a program at the same time.

However, you can run more than one instance of gcc at a time (on different source files), to take advantage of multiple processors. That's what make's -j flag is for -- it tells make to spawn off a bunch of processes at a time, instead of waiting for each one to finish before running the next.

So for instance, if the program's Makefile says that to build the executable, it needs 100 different object files, make -j will know that it can (probably) build each of those object files independently of each other. So it'll issue some number of gcc commands in parallel, to compile some of the object files, and when each one finishes, it'll start another, until it runs into a dependency that forces it to go single-process again. (For example, the final link requires all the object files, so make will have to go back to one process for the final link.)

[cfaun5]

Thank you; this helps greatly.

[cfaun5]

Sorry for my ignorance, but I tried the -j flag in gcc, and it said it was an unrecognized option (version 3.3.5-20050130). I looked at the manuals for newer versions and there was not any -j's there either. I couldn't find what you described.

Did I misinterpret something?

Thanks once again,
cfaun5

[jacobb]

The '-j' flag is for make, not gcc.

[cfaun5]

Whoops. Sorry about that. Thank you!