MongoDB Java Driver every time execution update method will from to get a PoolOutputBuffer in SimplePool, returned to the pool after use, but when set WriteConcern for SAFE mode, the drive itself will perform DBTCPConnector._checkWriteError () method to check the safety of the method from the pool made ​​a PoolOutputBuffer, after not returned to the pool, this will consume a PoolOutputBuffer, and this PoolOutputBuffer default at least occupy more than 50KByte memory space, and a tremendous waste of resources.When high concurrent to monitoring jvm memory usage knew terror Eden area is growing very fast.