Hello,

By default SQL Server is designed to use memory in large quantities, in order to cache data pages, plans and other DB objects in the Memory (Buffer pool).Under noramal conditions SQL Serevr will consume as much memory as it needs from the available RAM. When OS sets the memory low resource notification flag it is under these circumstances SQL Server will trim its working set (memory usage).

In order to avoid such a behaviour set the MAX_SERVER_MEMORY sp_configure option.

Refrence: http://www.red-gate.com/community/books/accidental-dba (Chap 4)