RFR: 8339335: set number of parallel jobs when building webkit

[Johan Vos]

On Tue, 3 Sep 2024 17:46:17 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> Btw, I ran another CI build with the `NUM_COMPILE_THREADS=1` workaround for Mac removed, and the build times are good again. In fact, the x64 build runs even faster than before -- possibly because WebKit build defaults to a lower number than num_processors, and it happens to be 12 on that x64 build system.
> 
> Here are the updated numbers:
> 
> OS / Arch	Current	This PR	PR + don't limit num threads on macOS
> macOS / aarch64	13m:41s	56m:36s	13m:43s
> macOS / x64	28m:30s	2h:24m:52s	23m:01s
> I should also add that the Linux and Windows build times are largely unaffected by this PR (which is good). They do run slightly faster with your patch than current, but the percentage gain is small.

Interesting.
The default NUM_COMPILE_THREADS comes from what Gradle gives us, using `${Runtime.runtime.availableProcessors()}`
The default threads that webkit uses `numberOfCPUs()` which is on mac implemented as `sysctl -n hw.ncpu`
On my macOS/aarch64, both implementations give "10", but this can indeed give different values.

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PR Comment: https://git.openjdk.org/jfx/pull/1554#issuecomment-2327173344