201211 - Session 1 TITLE: Minding SQL Server Memory SPEAKER: Susantha Bathige (@sbathige)

1. By
Susantha Bathige
MINDING SQL SERVER MEMORY

2. AGENDA
• Introduction
• 32 bit vs 64 bit
• Terminology
• Memory support in Windows
• SQL Server Memory

3. PHYSICAL MEMORY
• RAM (system memory, main memory, primary storage)
• DIMM’s
• Measured in MB or GB
• Faster access (5-12GB/sec)
• Volatile
• Expensive
• Windows Server 2008 R2 Datacenter and Enterprise Editions can now support systems
with 2TB of RAM

4. VIRTUAL ADDRESS SPACE (VAS)
• Mechanism to share the limited physical memory among the processes
• Set of addresses that the processor uses to access anything on its bus
• Some are reserved for
• Video RAM
• HW buffers, etc.
• 32-bit w/o PAE, the address bus is 32 bits (4GB)
• 32-bit Intel server processors with PAE, the address bus was 36 bits (64GB)
• Limit address bus to 44 bits on 64-bit (16TB)

5. VAS
Source : Professional SQL Server® 2008 Internals and Troubleshooting

6. • Address space is limited to 4GB
• /3GB switch increases the user mode VAS to 3GB
• /PAE Switch
• Intel technology
• Increases address bus to 36bits
• Up to 64GB or 128GB of RAM

7. • Theoretical limit is 18,446,744,073,709,551,616 Bytes
• 16 exabytes =
16,777,216 petabytes (16 million PB)
17,179,869,184 terabytes (17 billion TB)
17,592,186,044,416 gigabytes (17 trillion GB)
• Practical limitation – 44 bits address bus (16TB)
• Enough address space for the foreseeable future
• Logically it's split into an 8TB range for user mode and 8TB for kernel mode

8. VIRTUAL MEMORY MANAGER (VMM)
• Joins physical memory and virtual address space together
Source : Professional SQL Server® 2008 Internals and Troubleshooting

9. TERMINOLOGY
• Processes
• essentially container that hosts the execution of an executable image file
• process has a virtual address space that holds the process’s private and shared data
• Threads
• a process includes one or more threads that actually execute the code in the process
• PTE
• OS mechanism of mapping VAS to physical memory page
• Page file
• a disk file that the computer uses to increase the amount of physical storage for
virtual memory

10. TERMINOLOGY
• Reserved memory/MemToLeave
• applications reservations to create a large block of VM. only in the startup process in 32-
bit SQL Server
• Current commit charge
• amount of committed virtual memory for all the active processes. RAM size is the max limit
• Page fault
• writes the data from page file into memory
• Soft page faults
• occur when a new page of memory is required (VirtualAlloc)
• Hard page faults
• occurs when SQL Server tries to access a page of its memory that has been paged out to
the page file

11. TERMINOLOGY…
• Private bytes
• bytes allocated exclusively for a specific process
• Committed memory / Working Set
• the shared and private bytes allocated to a process backed by RAM. reflects the
physical memory usage of a process
• Paged pool
• windows can write the data it stores to the paging file
• Non-paged pool
• the kernel and device drivers use non-paged pool to store data that might be
accessed when the system can’t handle page faults

12. SQL SERVER MEMORY
• Three levels (bottom to top)
• Memory nodes
• Memory clerks, cache stores
• Memory objects
• Only memory clerks can access memory
• Create new memory clerks if want to allocate more memory

13. MEMORY NODES
• DMVs
• sys.dm_os_memory_nodes
• More memory nodes if NUMA enabled
• Each memory node has its own memory clerks and caches
• Total usage is calculated using the sum of all the nodes

14. MEMORY CLERKS, CACHES, AND THE BUFFER
POOL
• Clerks
• DMV - sys.dm_os_memory_clerks
• MEMORYCLERK_SQLGENERAL
• MEMORYCLERK_SQLQUERYPLAN
• Troubleshooting much easier
• Cache
• object store, cache store, user store
• DMV - sys.dm_os_memory_cache_counters
• Buffer pool
• manage data cache
• DMV - sys.dm_os_buffer_descriptors

15. PLAN CACHE
• Cache all the execution plans
• DMV - sys.dm_exec_cached_plans
• Look for the procedure cache option in DBCC MEMORYSTATUS

16. PLAN CACHE IN A PROD SERVER
• What is Adhoc?
• Use of DBCC FREESYSTEMCACHE('SQL Plans')

17. QUERY MEMORY/WORKSPACE MEMORY
• Used to temporarily store results during hash and sort operations
• Allocated out of the buffer pool
• DMV – sys.dm_exec_query_memory_grants
• Perfmon counters
• Granted Workspace Memory (KB)
• Maximum Workspace Memory (KB)
• Memory Grants Pending
• Memory Grants Outstanding
• RESOURCE_SEMAPHORE wait type
• Demo

18. MIN AND MAX SERVER MEMORY
• Control only buffer pool size
• Min
• min physical memory try to keep committed
• when the SQL Server service starts, it does not acquire all the memory configured in
Min
• Max
• when the SQL Server service starts, it does not acquire all the memory configured in
Max
• a "ceiling" for the buffer pool
• Can be used “sp_configure” to set

19. AWE ENABLED OPTION
• In SQL Server 2008 R2
• SQL Server process needs to have the "Lock Pages in Memory" Advanced
User Right in Windows
• Only data pages can be stored in AWE mapped memory

20. AWE ENABLED OPTION
• Deprecated in SQL Server 2008 R2
• SQL Server 2012, this configuration option and the feature that uses this configuration
option are removed from the product (32 bit and 64 bit)
Source : http://support.microsoft.com/default.aspx?scid=kb;EN-US;2644592

21. LPIM
• AWE APIs to allocate and map physical memory to process address space
• OS never paged out AWE API allocated memory
• "Lock Pages In Memory“ right need to be granted
• Simplified in SQL Server 2012 (32 bit and 64 bit)
• Need to revisit the “max/min server memory”
• Not be part of the working set
• Overall system performance is a concern due to memory starvation

22. LPIM RIGHT AND SQL SERVER
Source : http://support.microsoft.com/kb/2659143

23. IS LPIM ENABLED?
• use sys.dm_os_memory_nodes, sys.dm_os_nodes dmvs
• Memory Manager section of DBCC MEMORYSTATUS
• Current SQL Server Error log
Using
locked pages
in the memory manager

24. WINDOWS MAX MEMORY?
• Why 2TB maximum limitation on Windows?
• Windows Server 2008 Datacenter support maximum
• Not h/w limitation
• Testing limitation
•

25. CHANGES IN SQL SERVER 2012
• memory allocation is controlled by the max server memory (MB) and min server memory
(MB) configuration options.
Source : http://support.microsoft.com/default.aspx?scid=kb;EN-US;2663912

26. CHANGES IN SQL SERVER 2012
• Changes to "memory_to_reserve“
Source : http://support.microsoft.com/default.aspx?scid=kb;EN-US;2663912

27. PERFMON COUNTERS
• Process: Page Faults / sec
• Memory/Pool Paged Bytes
• Memory/Pool Nonpaged Bytes
• MemoryFree System Page Table

28. REFERENCES
• http://support.microsoft.com/default.aspx?scid=kb;EN-US;2644592
• http://support.microsoft.com/kb/2659143
• http://blogs.msdn.com/b/slavao/archive/2005/04/29/413425.aspx
• http://sqlblog.com/blogs/sqlos_team/archive/2012/06/20/sql-server-2012-memory-manager-kb-articles.aspx
• http://msdn.microsoft.com/en-us/library/aa175282(v=sql.80).aspx
• Book: Professional SQL Server® 2008 Internals and Troubleshooting
• Whitepaper - "Advances in Memory Management in Windows"

29. Q & A