The G1 verification can be very slow when running some tests. Compared to Parallel Scavenge, G1 runs DoubleArrayLow, FloatArrayHigh and FloatArrayLow in 21m14s compared to 1m8.7s for PS.

After adding some logging, the main issue seems to be:

     // If it returns false, verify_for_object() will output the
     // appropriate messasge.
     if (do_bot_verify && !_offsets.verify_for_object(p, obj_size)) {
       *failures = true;
       return;
     }

in HeapRegion::verify.

Impact = Medium, since the test times out due to 30 min limit in nightly testing
Likelihood = Medium, since it happens intermittently in nightly testing
Workaround = High, since verification has to be turned off

ILW = MMH = P3