On Saturday, 6 January 2024 15:28:53 GMT Wols Lists wrote: > As far as I'm aware, there's no mystery. On a single machine you get the > exact same thing ... it's all down to parallelism. > > Make asks itself "how many separate tasks can I do at the same time, > which won't interfere with each other". In gcc's case, the answer > appears to be two. It doesn't matter how much resource is available, > make can only make use of two cores.
Yet, if I set -distcc and -j12 -l12, I get 12 threads in parallel. That's the mystery. > In other cases, there may be a hundred separate tasks, make fires off a > hundred tasks shared amongst all the resource it can find, and sits back > and waits. And that's how the very first installation goes, with single-host distcc. Then, when it gets to gcc, it collapses to 2 threads and everything gained so far is lost many-fold. (I set USE=-fortran to avoid pointless recompilation, since nothing needs it here.) > Think of a hundred compile jobs all running at the same time, but then > the linker is invoked, and you can only have the one linker running, > after all the compile jobs have finished. I hadn't thought of that - another thing to consider. > And this is a HARD problem, I haven't seen it recently, but there used > to be plenty of threads about hard-to-debug compile failures that went > away with -j1. The obvious cause was two compile jobs being set off in > parallel, when in reality one depended on the other, and things messed up. I haven't either - seen it recently. -- Regards, Peter.
