For A Parallel World: Parallel building is not passé

It’s been a while since I last wrote about parallel building. This has only to do with the fact that the tinderbox hasn’t been running for a long time (I’m almost set up with the new one!), and not with the many people who complained to me that spending time in getting parallel build systems to work is a waste of time.

This argument has been helped by the presence of a --jobs option to Portage, with them insisting that the future will have Portage building packages in parallel, so that the whole process will take less time, rather than shortening the single build time. I said before that I didn’t feel like it was going to help much, and now I definitely have some first hand experience to tell you that it doesn’t help at all.

The new tinderbox is a 32-way system; it has two 16-core CPUs, and enough RAM for each of them; you can easily build with 64 process at once, but I’m actually trying to push it further by using the unbound -j option (this is not proper, I know, but still). While this works nicely, we still have too many packages that force serial-building due to broken build systems; and a few that break in these conditions that would very rarely break on systems with just four or eight cores, such as lynx .

I then tried, during the first two rebuilds of world (one to set my choices in USE flags and packages, the other to build it hardened), running with five jobs in parallel… between the issue of the huge system set (yes that’s 4.24 years old article), and the fact that it’s much more likely to have many packages depending on one, rather than one depending on many, this still does not saturate the CPUs, if you’re still building serially.

Honestly seeing such a monstrous system take as much as my laptop, which is 14 in cores and 14 in RAM, to build the basic system was a bit… appalling.

The huge trouble seem to be for packages that don’t use make, but that could, under certain circumstances, be able to perform parallel building. The main problem with that is that we still don’t have a variable that tells us exactly how many build jobs we have to start, instead relying on the MAKEOPTS variable. Some ebuilds actually try to parse it to extract the number of jobs, but that would fail with configurations such as mine. I guess I should propose that addition for the next EAPI version… then we might actually be able to make use of it in the Ruby eclasses to run tests in parallel, which would make testing so much faster.

Speaking about parallel testing, the next automake major release (1.13 — 1.12 was released but it’s not in tree yet, as far as I can tell) will execute tests in parallel by default; this was optional starting 1.11 and now it’s going to be the default (you can still opt-out of course). That’s going to be very nice, but we’ll also have to change our src_test defaults, which still uses emake -j1 which forces serialisation.

Speaking about which, even if your package does not support parallel testing, you should use parallel make, at least with automake, to call make check; the reason is that the check target should also build the tests’ utilities and units, and the build can be sped up a lot by building them in parallel, especially for test frameworks that rely on a number of small units instead of one big executable.

Thankfully, for the day there are two more packages fixed to build in parallel: Lynx (which goes down from 110 to 46 seconds to build!) and Avahi (which I fixed so that it will install in parallel fine).