8271722: [TESTBUG] gc/g1/TestMixedGCLiveThreshold.java can fail if G1 Full GC uses >1 workers

Reviewed-by: ayang, tschatzl

Author: reinrich

test/hotspot/jtreg/gc/g1/TestMixedGCLiveThreshold.java

@@ -99,6 +99,8 @@ private static OutputAnalyzer testWithMixedGCLiveThresholdPercent(int percent) t
                                        "-XX:+UnlockDiagnosticVMOptions",
                                        "-XX:+UnlockExperimentalVMOptions",
                                        "-XX:+WhiteBoxAPI",
+                                       // Parallel full gc can distribute live objects into different regions.
+                                       "-XX:ParallelGCThreads=1",
                                        "-Xlog:gc+remset+tracking=trace",
                                        "-Xms10M",
                                        "-Xmx10M"});