LruBlockCache size should not exceed acceptableSize too many

Our regionserver has a configuation as bellow：
-Xmn4g -Xms32g -Xmx32g -XX:SurvriorRatio=2 -XX:+UseConcMarkSweepGC
also we only use blockcache,and set hfile.block.cache.size = 0.3 in hbase_site.xml,so under this configuration, the lru block cache size will be(32g-1g)*0.3=9.3g. but in some scenarios，some of the rs will occur continuous FullGC for hours and most importantly, after FullGC most of the object in old will not be GCed. so we dump the heap and analyse with MAT and we observed a obvious memory leak in LruBlockCache, which occpy about 16g memory, then we set set class LruBlockCache log level to TRACE and observed this in log:

2016-07-22 12:17:58,158 INFO [LruBlockCacheStatsExecutor] hfile.LruBlockCache: totalSize=15.29 GB, freeSize=-5.99 GB, max=9.30 GB, blockCount=628182, accesses=101799469125, hits=93517800259, hitRatio=91.86%, , cachingAccesses=99462650031, cachingHits=93468334621, cachingHitsRatio=93.97%, evictions=238199, evicted=4776350518, evictedPerRun=20051.93359375

we can see blockcache size has exceeded acceptableSize too many, which will cause the FullGC more seriously.
Afterfter some investigations, I found in this function:

  public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf, boolean inMemory,
      final boolean cacheDataInL1) {

No matter the blockcache size has been used, just put the block into it. but if the evict thread is not fast enough, blockcache size will increament significantly.
So here I think we should have a check, for example, if the blockcache size > 1.2 * acceptableSize(), just return and dont put into it until the blockcache size if under watrmark. if this is reasonable, I can make a small patch for this.