Re: -CURRENT compilation time

From: David Chisnall <theraven_at_FreeBSD.org>
Date: Wed, 8 Sep 2021 09:57:50 +0100
On 07/09/2021 18:02, Stefan Esser wrote:
> Wouldn't this break META_MODE?

I have never managed to get META_MODE to work but my understanding is 
that META_MODE is addressing a problem that doesn't really exist in any 
other build system that I've used: that dependencies are not properly 
tracked.

When I do a build of LLVM with the upstream build system with no 
changes, it takes Ninja approximately a tenth of a second to stat all of 
the relevant files and tell me that I have no work to do.  META_MODE 
apparently lets the FreeBSD build system extract these dependencies and 
do something similar, but it's not enabled by default and it's difficult 
to make work.

> I'd rather be able to continue building the world within a few minutes
> (generally much less than 10 minutes, as long as there is no major LLVM
> upgrade) than have a faster LLVM build and then a slower build of the world ...

The rest of this thread has determined that building LLVM accounts for 
half of the build time in a clean FreeBSD build.  LLVM's CMake is not a 
great example: it has been incrementally improved since CMake 2.8 and 
doesn't yet use any of the modern CMake features that allow 
encapsulating targets and providing import / export configurations.

In spite of that, it generates a ninja file that compiles 
*significantly* faster than the bmake-based system in FreeBSD.  In other 
projects that I've worked on with a similar-sized codebase to FreeBSD 
that use CMake + Ninja, I've never had the same problems with build 
speed that I have with FreeBSD.

Working on LLVM, I generally spend well under 10% of my time either 
waiting for builds or fighting the build system.  Working on FreeBSD, I 
generally spend over 90% of my time waiting for builds or fighting the 
build system.  This means that my productivity contributing to FreeBSD 
is almost zero.

For reference, changes to LLVM typically build for me in under 30 
seconds with Ninja, unless I've changed a header that everything

In particular, building FreeBSD on a 10-24 core machine has very long 
periods where a number of the cores are completely idle.

Ninja also has a few other nice features that improve performance 
relative to bmake:

  - It lets you put jobs in different pools.  In LLVM this is used to 
put link and compile jobs in different pools because linking with LLD 
uses multiple threads and a lot more memory than compilation, so a 
10-core machine may want to do 12 compile jobs in parallel but only 2 
link jobs.  This makes it much easier to completely saturate the machine.
  - Ninja provides each parallel build task with a separate pipe for 
stdout and stderr, and does not print their output unless a build step 
fails (or unless you build with -v).  With bmake, if a parallel build 
fails I have to rerun the build without -j, because the output is 
interleaved with succeeding jobs and it's difficult to see what actually 
failed.  With ninja, the output is from each failed job, with no 
interleaving.

David
Received on Wed Sep 08 2021 - 08:57:50 UTC

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