RFR: 8374153: Add a MAX_COMPILE_THREADS gradle property to limit number of threads

[Ambarish Rapte]

On Fri, 19 Dec 2025 18:29:43 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> This PR adds a new `MAX_COMPILE_THREADS` gradle property to to limit the number of build-time parallel compilation threads to the smaller of this property and the number of avialable processors.
> 
> Our gradle build spawns up to `NUM_COMPILE_THREADS` compilation tasks in parallel when building native libraries. By default, `NUM_COMPILE_THREADS` is set to the value returned by `Runtime.getRuntime().availableProcessors()`. That value can be overridden on the command line by setting the `NUM_COMPILE_THREADS` gradle property. We need a way to clamp `NUM_COMPILE_THREADS` at some maximum value without the caller having to compute it.
> 
> The change in `build.gradle` is simple:
> 
> * Define a new `MAX_COMPILE_THREADS` gradle property, which will default to 0 (meaning unlimited) for compatibility with current behavior.
> 
> * Set the default value of `NUM_COMPILE_THREADS` as follows:
> 
> 
>     NUM_COMPILE_THREADS = MAX_COMPILE_THREADS > 0
>                         ? Math.min(MAX_COMPILE_THREADS, availableProcessors)
>                         : availableProcessors(
> 
> 
> This will allow a test scripts that runs on a number of different platforms to clamp to a maximum value without having to use jshell or similar to implement `Math.min(MAX_COMPILE_THREADS, Runtime.getRuntime().availableProcessors())` in the script.
> 
> We have a practical need for this in our headful test systems, several of which report a higher number of processors than they can effectively support. Without clamping, the build will either run slower (due to too much swapping) or fail altogether when building WebKit.

lgtm...

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Marked as reviewed by arapte (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/2014#pullrequestreview-3602965882