Memory Management

This is excerpt from memory management chapter of book Troubleshooting SQL Server, A guide for accidental DBA (Some notes I put together from the chapter for self help: This chapter in the book is a wonderful read.)

No memory leak- SQL Server keeps all the memory allocated to it, whether it uses it or not. It does not return memory if the memory is not in use. So basically it looks like there is a memory leak. PS: SQL server will not release this memory unless the OS sets memory low resource notification flag, which indicates SQL server to release its memory allocations. While the memory high feature lets SQL Server know that it can grow to use additional memory. (A dedicated thread was introduced in SQL 2005 to monitor these memory notifications)

SQL      64 bit                                          SQL  32bit

Windows 2008                       OS limit 32 GB                                    limit  4GB --(enabling AWE helps SQL use additional memory unto 64gb)

SQL 2000 enterprise or SQL 2005 standard and above only has AWE enabling feature. SQL 2000 standard edition and below don't have this feature. (PS*: Remember, If the RAM or physical memory is less than 32GB then accordingly the SQL will have an upper limit which is less that the physical memory available. The above examples are based on RAM being around 64gb)

SQL Server 32 bit:

• Memory allocation: 2GB to User Mode VAS and 2GB to Kernel Mode VAS though Windows assigns it a complete of 4GB to operate. Out of 2GB User mode: most of it is buffer pool memory, and some is non-buffer pool memory.


• Buffer pool memory: SQL server calls VirtualAlloc function in WinAPI to allocate memory to itself. (VirtaulAlloc returns 32bit pointer which limits amount of usermode VAS to 2GB) Buffer pool memory allocations are for data pages and execution plans. This memory is pageable by Windows.


• Non-buffer pool memory allocations: Thread stack allocations, heap allocations, exteded stored procs, SQLCLR, linked servers, backup buffers. SQL Server calculates this MemToLeave which is: MaxWorkerThreads * 0.5MB + default revervation (256MB) where MaxWorkerThreads = (ProcessorCount-4)+256  (generally comes to about 0.4GB) Generally buffer pool will not require more memory than this, but incase this memory becomes Fragmented and there is not enough contingeous memory available, the amount of VAS reserved can be increased using -g 256> startup parameter. In 64-bit SQL, VAS is 8TB, greater than allowable physical RAM on a windows server, so this issue does not exist.
  So for 2GB user mode VAS: 1.6Gb is buffer pool and 0.4 GB is non-buffer pool memory.


• To allow 32-bit SQL to use more buffer pool memory 2 options exist:
  1.VAS tuning (available for Windows server with 4GB memory-RAM) -use with extreme caution, since here you are taking away about 1GB of Kernel VAS and trying to allocate to User VAS. In most cases avoid!
  2. enabling AWE -Address Windowing Extensions (for greater than 4GB RAM).  AWE is used to extend Buffer pool User memory. It requires additional configuration of the OS to use Physical Address Extensions (PAE). When PAE is enabled, the 32 bit memory management pointer is exapanded to 36-bit allowing OS to address 64GB of RAM or RAM's upper limit. For applications to make use of this additional memory they must use AWE. Thus instead of calling VirtuallAlloc, now SQL Server calls AllocateUserPhysicalPages function in Windows to allocate memory. This is non-pageable  by Windows.
  To use AWE, PAE is enabled in Windows 2008 using BCDEdit /set from command prompt. Next, 'awe enabled' sp_configure option is set in SQL server and SQL Server service account must have Locak Pages in Memory user right (assigned using Windows Group Policy Tiil, gpedit.msc). Restart SQL server required after enabling this. 'max server memory' sp_configure option used to set max memory allowed for AWE eg: 4gb to (MAX RAM not recommended), any memory that we think is required by SQL server.Since this is non-pageable memory, Windows OS cannot get this locked memory back if it needs it, so be careful while assigning AWE memory limit.

SQL Server 64 bit:

• In 64-bit SQL, VAS is 8TB for Kernel mode and 8TB for user mode, greater than allowable physical RAM on a windows server. Hence, AWE enabled bit has no use in 64-bit SQL. But Lock Pages in Memory option still exists.


• Also, since there is so much memory, The MemtoLeave for non-buffer pool and -g startup parameter also has no significance in 64-bit. But you have to manually monitor to ensure that Memory/Available Mbytes (non-buffer pool memory) remains abover 300MB as we gradually increase value of max server memory.


• Configuring memory through Min Max server memory bits(via SSMS or sp_configure): Mim server memory is minimum size to which SQL server can shrink buffer pool when WIN memory is under pressure. Max server memory is how much maximum memory a buffer pool can use). Min Server memory should always be set much lower than max memory (If min and max are set to same(not recommended!), we are intentionally locking pages in memory which will limit how much space SQL can free up for windows when windows hits low memory)(More on how to lock pages in memory:http://msdn.microsoft.com/en-us/library/ms190730(v=sql.105).aspx)

Diagnosing Memory pressure: low memory allocated to SQL means more pages to get from disk, means more physical IO and poor performance. Results in continuous buffer pool flushing called buffer pool churn.

SQL Server: Buffer manager\Page Life Expectancy: (time in seconds that a page exists in cache): This counter must be monitored over long periods of time in order to properly identify normal trends and away from normal value.

Free List Stalls/sec > 0 frequently, indicates memory pressure. If free pages counter is near to or is 0 and PLE drops at the same time means memory pressure.