[ML] Reduce test parallelism on low-core machines to avoid flaky timing tests

Using -j numCpus on 4-core macOS CI (Orka VM) caused
CKMostCorrelatedTest/testScale to fail because full CPU saturation
made wall-clock based complexity assertions unreliable (exponent
2.21 vs threshold 2.1). Cap at numCpus-1 for <=4 cores to leave
headroom for timing-sensitive benchmarks.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: edsavage

build.gradle

@@ -116,9 +116,11 @@ project.ext.numCpus = Runtime.runtime.availableProcessors()
 // Each suite spawns ctest --parallel <all_cpus> internally, so running too
 // many suites simultaneously causes resource contention. These values were
 // determined empirically (see PR #2900).
-// On low-core machines (<=4, e.g. macOS CI Orka VMs), use all cores since
-// CTest internal parallelism is modest enough that contention is minimal.
-project.ext.testParallel = isWindows ? 2 : (numCpus <= 4 ? numCpus : Math.max(2, (int) Math.ceil(numCpus / 3.0)))
+// On low-core machines (<=4, e.g. macOS CI Orka VMs with 4 cores),
+// cap at numCpus-1 to leave headroom for timing-sensitive tests
+// (e.g. CKMostCorrelatedTest/testScale). Full saturation causes
+// wall-clock based complexity checks to fail under contention.
+project.ext.testParallel = isWindows ? 2 : (numCpus <= 4 ? Math.max(2, numCpus - 1) : Math.max(2, (int) Math.ceil(numCpus / 3.0)))
 project.ext.makeEnvironment = [ 'CPP_CROSS_COMPILE': cppCrossCompile,
                                 'VERSION_QUALIFIER': versionQualifier,
                                 'SNAPSHOT': (isSnapshot ? 'yes' : 'no'),

dev-tools/docker/docker_entrypoint.sh

@@ -68,11 +68,11 @@ if [ "x$1" = "x--test" ] ; then
     echo passed > build/test_status.txt
     # Each test suite spawns ctest --parallel <nproc> internally, so limit
     # the number of suites running concurrently to avoid resource contention.
-    # On low-core machines (<=4), use all cores since CTest internal
-    # parallelism is modest enough that contention is minimal.
+    # On low-core machines (<=4), cap at nproc-1 to leave headroom for
+    # timing-sensitive tests (e.g. CKMostCorrelatedTest/testScale).
     NCPUS=$(nproc)
     if [ "$NCPUS" -le 4 ]; then
-        TEST_PARALLEL=$NCPUS
+        TEST_PARALLEL=$(( NCPUS > 2 ? NCPUS - 1 : 2 ))
     else
         TEST_PARALLEL=$(( (NCPUS + 2) / 3 ))
     fi