Subject: Strange behavior of "blockCacheSize" metric with LruBlockCache

When enabling the direct memory allocation [HBASE-4027] I'm observing
strange values for the blockCacheSize metric.

With usual cache, values are slowly growing around 1 GB
When I'm enabling direct allocation (~4 GB off-heap), then it looks like
this: 1 GB, 4 GB, 1, 4, 1, 4...

When looking at the code, the double cache is just adding values of both
caches. I don't think my cache is evicting/filling that quick, and looking
at the debug logs nothing is happening (very low cache activity).

Can anyone explain this metric under such circumstances?
Perhaps it should be CC to dev mailing list?
Adrien Mogenet