If you run othervm tests with a concurrency > 1, then the reported test times get inflated. You can see this by running:
time makec ../build/fastdebug test TEST=test/hotspot/jtreg/vmTestbase/vm/gc/compact JTREG="VERBOSE=all"

and looking at the elapsed time for the main action:
$ grep -r "othervm specified" -A1 jtreg_open_test_hotspot_jtreg_vmTestbase_vm_gc_compact/jtreg.log | grep elapsed
elapsed time (seconds): 121.194
elapsed time (seconds): 240.631
elapsed time (seconds): 359.092
elapsed time (seconds): 478.718
elapsed time (seconds): 597.433
elapsed time (seconds): 716.628
elapsed time (seconds): 836.638
elapsed time (seconds): 955.156
elapsed time (seconds): 1074.476
elapsed time (seconds): 1193.712
elapsed time (seconds): 1314.116
elapsed time (seconds): 1433.632
elapsed time (seconds): 1553.563
elapsed time (seconds): 1672.996
elapsed time (seconds): 1792.846
elapsed time (seconds): 1913.112
elapsed time (seconds): 1926.819
elapsed time (seconds): 1926.809
elapsed time (seconds): 1926.921
elapsed time (seconds): 1927.093
elapsed time (seconds): 1927.192
elapsed time (seconds): 1927.07
elapsed time (seconds): 1926.896
elapsed time (seconds): 1927.31
elapsed time (seconds): 1927.244
elapsed time (seconds): 1927.33
elapsed time (seconds): 1927.221
elapsed time (seconds): 1927.42
elapsed time (seconds): 1927.158
elapsed time (seconds): 1927.315
elapsed time (seconds): 1927.316
elapsed time (seconds): 1927.311
elapsed time (seconds): 1927.532
elapsed time (seconds): 1927.379

Each test is supposed to be run for 120 seconds, but the reported time ramps up in ~120s increments until it reaches the number that matches the "jtreg concurrency" number, which in my run is 16.

I think this happens because we start "concurrent" tests. Each test starts by taking a timestamp, then looks if we need "exclusiveAccess" and if we do, then a lock is taken before the othervm test process is forked. This serializes the tests and inflates the times.

From the jtreg code:
-----------
startAction(true); <<< Takes timestamps
if (script.isCheck()) {
    status = passed(CHECK_PASS);
} else {
    Lock lock = script.getLockIfRequired(); <<< serializes on lock
    if (lock != null) lock.lock();
    try {
        switch (!othervmOverrideReasons.isEmpty() ? ExecMode.OTHERVM : script.getExecMode()) {
            case AGENTVM:
                showMode(ExecMode.AGENTVM);
                status = runAgentJVM(); <<< starts test
----------

I've tested a workaround for this, and it seems to be solving the problem. However, since I'm not well-versed in this code, I don't know if this have any unforeseen consequences.

The workaround:
diff --git a/src/share/classes/com/sun/javatest/regtest/exec/MainAction.java b/src/share/classes/com/sun/javatest/regtest/exec/MainAction.java
index dc1a6d5..bea9e2a 100644
--- a/src/share/classes/com/sun/javatest/regtest/exec/MainAction.java
+++ b/src/share/classes/com/sun/javatest/regtest/exec/MainAction.java
@@ -333,13 +333,16 @@ public class MainAction extends Action
         if (nativeCode && script.getNativeDir() == null)
             return error(MAIN_NO_NATIVES);
- startAction(true);
-
         if (script.isCheck()) {
+ startAction(true);
             status = passed(CHECK_PASS);
         } else {
             Lock lock = script.getLockIfRequired();
             if (lock != null) lock.lock();
+
+ // Start action after the lock is taken to ensure correct "elapsed time".
+ startAction(true);
+
             try {
                 switch (!othervmOverrideReasons.isEmpty() ? ExecMode.OTHERVM : script.getExecMode()) {
                     case AGENTVM: