There is some evidence that PLAB resizing is not working as expected.
In particular, fairly well-behaved applications exhibit large volatility
in PLAB sizing when auto-sizing of PLAB's is enabled (the default).
This not only can causes wasted space in the survivor spaces, but can
also lead to larger footprint (and more cache misses), more objects
being prematurely tenured and overall cause worse GC and application
performance as a result.

See comments section for more specific detail.