Performance Tuning with SQL Server Dynamic Management Views

System-wide memory use
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Chapter 7: OS and Hardware Interaction
There are a few ways to get a picture of the memory use in SQL Server using the
DMVs but, new to SQL Server 2008, there is an excellent DMV called sys.dm_os_sys_
memory that provides a summarization of the memory condition of the system. It shows
the current levels of memory in the system, the cache, and so on. Amongst the columns
this view returns are:
• total_physical_memory_kb – the total amount of non-virtual memory available
to the OS in kilobytes
• available_physical_memory_kb – the amount of non-virtual memory currently
available to the OS
• total_page_file_kb – the current size of the OS's virtual memory/page file
• available_page_file_kb – the amount of virtual memory currently available to
the OS
• system_memory_state_desc – calculated value based on two other columns
(system_high_memory_signal_state and system_low_memory_signal_
state) that indicates if available memory is high, low, steady, or transitioning from
one state to another.
• The final column in this list, system_memory_state_desc, requires a little more
explanation. When you start the server, this value will likely be "Available physical
memory is high," while a value of "Available physical memory is low" is an indicator of
external memory pressure, since it means that the OS has little available memory and so
may require SQL Server to reduce its memory usage. This will result in SQL Server
reducing the size of all of its memory caches, and means that plans will get flushed from
the plan cache, and so on.