Hi Eric, I am referring to the buffer pool, where data/index pages are kept.  I ran your query and have attached the result.  As you can see, only a portion of the 51GB RAM is being used, even though SQL Server says that it is using 51GB.  The PLE is at 160, so there is quite bit of RAM contention.  I have attached the output of your query for your review. 

Hi Ed, yes, I  have a query that shows the breakdown of  all  memory  by  type, and the plan cache uses 4-5GB.  Total usage by all types is not even close to 51GB, there is usually a 20-30GB gap.