Shenandoah typically takes 4 safepoints per GC cycle. Although Shenandoah itself does not spend much time on these safepoints, it may still take quite some time for all of the mutator threads to reach the safepoint. The occasionally long time-to-safepoint increases latency in the higher percentiles. It may not be possible to eliminate all of these safepoints, but the `init-update-refs` safepoint should be the easiest to eliminate.