[MNG-8725] Option for "-T" argument: Use max cores minus 1

**[Matthias Bünger](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=JIRAUSER302100)** opened **[MNG-8725](https://issues.apache.org/jira/browse/MNG-8725?redirect=false)** and commented

You can enable parallel builds in Maven using the `-T` argument and pass the number of threads or threads per core (see: https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3 ).

The Maven Daemon uses the maxinum number of cores minus 1 per default for max effiencey without blocking the system.

Sadly there is no option for the Maven core to behave like this. So each developer has either to have the same number of cores available (which is often not the case) or pass his/her individual number of threads on each call.

It would be great to have a built-in argument to use maximum -1 cores like the Daemon does.

Note: I write cores all the time, because threads in this context are hardware threads (and no virtual threads possible with Java 21 and I don't want to mix this even that Maven is not yet on Java 21)

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**Affects:** 3.9.9

Start by tracing Maven core's handling of the -T parallel-build argument and compare it with the Maven Daemon's max-cores-minus-one default. Define the built-in option's behavior for hardware threads and verify that parallel builds use the requested default without requiring a per-call thread count.

Written by the indexing model from the issue text.