Re: -CURRENT compilation time

From: Michael Schuster <>
Date: Mon, 06 Sep 2021 08:16:14 UTC

a few observations (from the POV of someone who builds current ~ once a

On Mon, Sep 6, 2021 at 10:09 AM Jeremie Le Hen <> wrote:

> Hey,
> I want to build -CURRENT again from sources. It's been a long time
> since I hadn't done that. I'm shocked by the compilation time.
> I started the whole thing on Friday night and Monday morning it's
> still in stage 4.2 (building libraries). Through occasional glancing
> at the screen over the weekend, it seems obvious to me that the
> compilation time is utterly dominated by LLVM.

Do you actually measure anything?
have you looked into what your I/O is doing? How about swap?

Compiling C++ seems
> extremely CPU heavy and this is made worse by the fact LLVM is built
> twice (once for build/cross tools, once for the actual world).
> So OK, my CPU is not the most powerful out there but it's still decent [1].

I didn't check the specs: how many cores does your box have, and how many
are you actually using (-j N)? IME, htop gives a good idea of how busy the
CPUs really are.

> So I have a couple of questions coming to my mind:
> 1. Is there any optimization I could benefit from? (I'm sure there's a
> knob to use the existing compiler instead of building a
> cross-compiler.)

look at the build man page (
for some tips on how to configure your environment.

2. More generally, isn't this compilation time not considered as a
> problem for developers? This seems to terribly slow down the iteration
> time for people working on the build system. I wouldn't be surprised
> if this drove people away from working on/improving that area.
> [1]
> Cheers,
> --
> Jeremie Le Hen
Michael Schuster
recursion, n: see 'recursion'