The changes described in SERVER-20306 eliminated a common source of memory fragmentation, but it can still occur for other reasons. Here's an example from a node undergoing initial sync:

Over time

• allocated memory never exceeds 8 GB
• but heap size and resident memory reach nearly 14 GB
• this is due to an accumulation of pageheap_free_bytes

A common cause of this is a shifting distribution of allocated memory sizes, which leaves free pages dedicated to one size of buffer unable to be used for new memory requests because they are for a different size buffer.

Setting TCMALLOC_AGGRESSIVE_DECOMMIT can address this issue by causing tcmalloc to aggressively return the free pages to the o/s where they can then be re-used by tcmalloc to satisfy new memory requests. However can have an unacceptable negative performance impact. Is there a tweak to tcmalloc that can give us better behavior for workloads like this?