Long G1 young pause collection due to external root scanning

Issue

• Many long G1 young pause collections, one after another, due to external root scanning. For example:

12345.678: [GC pause (G1 Evacuation Pause) (young), 8.9540383 secs]
   [Parallel Time: 8902.8 ms, GC Workers: 18]
      [GC Worker Start (ms): Min: 42422201.9, Avg: 42422202.9, Max: 42422204.0, Diff: 2.1]
      [Ext Root Scanning (ms): Min: 8881.9, Avg: 8887.2, Max: 8899.2, Diff: 17.4, Sum: 159969.9]
      [Update RS (ms): Min: 0.0, Avg: 3.1, Max: 5.5, Diff: 5.5, Sum: 56.5]
         [Processed Buffers: Min: 0, Avg: 8.4, Max: 37, Diff: 37, Sum: 152]
      [Scan RS (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 1.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 2.3, Avg: 9.0, Max: 11.5, Diff: 9.3, Sum: 161.1]
      [Termination (ms): Min: 0.0, Avg: 1.4, Max: 1.8, Diff: 1.8, Sum: 24.7]
         [Termination Attempts: Min: 1, Avg: 6.2, Max: 13, Diff: 12, Sum: 112]
      [GC Worker Other (ms): Min: 0.1, Avg: 0.3, Max: 0.4, Diff: 0.3, Sum: 4.9]
      [GC Worker Total (ms): Min: 8900.0, Avg: 8901.0, Max: 8902.1, Diff: 2.1, Sum: 160218.1]
      [GC Worker End (ms): Min: 42431103.7, Avg: 42431103.9, Max: 42431104.1, Diff: 0.3]
   [Code Root Fixup: 0.1 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 1.3 ms]
   [Other: 49.9 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 1.5 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 4.8 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.7 ms]
   [Eden: 748.0M(748.0M)->0.0B(744.0M) Survivors: 61440.0K->65536.0K Heap: 5553.3M(16192.0M)->4807.5M(16192.0M)]
 [Times: user=10.50 sys=0.09, real=8.96 secs]