[Qbs] Qbs (way) faster than cmake (or benchmark issue)

[Christian Gagneraud]

On Tue, 16 Jul 2019 at 18:29, Christian Gagneraud <chgans at gmail.com> wrote:
>
> Hi there,
>
> I finally found time to start my qbs vs cmake build perf.
>
> What i found is that qbs beats cmake 'big time' on my build machine
> and i was actually surprised, so surprised that i'm trying to find
> where is the mistake.

Here is a graph showing qtcreator build time vs number of parallel
jobs, using cmake and qbs.

Builds were done in a docker container (same for both), stock Ubuntu
19.04/amd64, all dependencies (inc. qbs tool and dev libs) installed
using apt.

Number of compilations were measured by generating a Clang compilation
database and counting the number of entries:
cmake: 2881 (2869)
qbs: 4362 (2848)

The first number is the number of compiler calls, the second number if
the same but ignoring the pattern 'moc_'.

Using 2 jobs:
qbs: 26:30.78
cmake: 30:11.44
=>Cmake is 15% slower than qbs

Using 56 jobs:
qbs: 2:35.95
cmake: 4:33.79
=> CMake is 75% slower than bqs

At the 20 jobs mark, cmake start to stagnate, whereas qbs still make
use of parallelism, at the 30 jobs mark, cmake completely stopped
reducing overall build time, whereas qbs start to stagnate. At the 40
jobs mark, both systems are stale.

Chris
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