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

[Kevin Rushforth]

On Mon, 2 Sep 2024 07:20:50 GMT, Johan Vos <jvos at openjdk.org> wrote:

> When building webkit, the perl script in `modules/javafx.web/src/main/native/Tools/Scripts/build-webkit` will check for a `-j` argument in `$makeArgs`. If that doesn't exist, it will default to `numberOfCPUs()`.
> This PR passes the `NUM_COMPILE_THREADS` from `build.gradle` to the `build-webkit` script. When not setting `NUM_COMPILE_THREADS` explicitly,it will default to the number of CPU's as well, hence it will have the same behavior as without this PR.
> When setting `NUM_COMPILE_THREADS` explicitly, the provided number will be used by cmake as well now.

Here are the numbers for our CI build with / without the patch:

OS / Arch|Current|This PR
---------|-------|-------
macOS / aarch64|13m:41s|56m:36s
macOS / x64|28m:30s|2h:24m:52s

To answer your question:

> I read the cryptic comment in build.gradle `// On Mac we limit it to 1 by default due to problems running gcc in parallel` What is the problem that is being referred to here? And why would that problem occur in the other native compile jobs but not in the (resource intensive) webkit compile jobs?

That was added for this bug:

[JDK-8094590](https://bugs.openjdk.org/browse/JDK-8094590): Mac: Intermittent build failures in task graphics:ccMacFontT2K

When I "fixed" that bug (workaround, really) by disabling parallel compilation on macOS, I filed the following as a follow-up, but never got to it:

[JDK-8089417](https://bugs.openjdk.org/browse/JDK-8089417): Mac: intermittent T2K gcc failure prevents parallel compilation

It's quite possible that this was a bug in the Xcode gcc compiler, and if so, would be no longer relevant. Xcode switch from gcc to clang a long time ago.

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PR Comment: https://git.openjdk.org/jfx/pull/1554#issuecomment-2326872149