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SQL Server Memory–Rise of the Clerks – Part 1

3 Comments Posted by on June 10, 2012

Pervious Posts in the Series

SQL Server Memory – Underground

SQL Server Memory – The Evolution

SQL Server 2005 introduced many changes to the Memory Architecture. Majority of these architectural changes continued till SQL Server 2008 R2. In this post, I will try to cover a high level overview of architectural changes introduced in SQL 2005.

The New Architecture

The new memory architecture consists of 4 major components

  1. Memory Nodes
  2. Memory Clerks
  3. Memory objects
  4. Memory Broker

Slava Oak’s has a blog post explaining these components in details. Most of what follows is an attempt at simplifying what Slava mentions his blog.

Memory Node: Memory nodes are internal SQLOS objects. The number of memory nodes depends on the NUMA configuration of the server. Memory nodes can be thought of as a large pool of Memory, which different components (Clerks) can allocate memory from. Memory Nodes expose a set of Allocators which are used by all memory Clerks to allocate the desired amount of memory from the Node.

  1. Page Allocators: There are 4 primary types of page allocators exposed by the memory node. These allocators are used to allocate a set of 8KB pages. Remember that SQL Server page granularity is 8KB and any memory allocated by these page allocators would always be in multiple of 8KB.
    • Single Page Allocator: As the name suggests, this allocator is used to allocate one 8KB page at a time.
    • Multi Page Allocator: Allocates Multiple contigous 8KB Pages
    • Large Page Allocators: These are used to allocate Large Pages. This feature is only present in IA64 or X64 editions of SQL, running on servers with more than 8GB of memory. Trace Flag 834 has to be enabled in order for SQL to use Large Pages. On a X64 server the Large Page granularity is 2MB, while on IA64 its 16MB.
    • Reserved Pages: Is special purpose allocator, which allocates a set of pages for emergencies. Typically the “Fail Safe” memory for SQL.
  2. Virtual Memory Allocator: Uses Windows VirtualAlloc API’s, when SQL needs to allocate contigous memory which does not necessarily fall in the 8K page boundary.
  3. Shared Memory Allocator: Uses windows file mapping API’s and provides LPC (shared memory) capabilities.

DBCC memory status gives some information about the Memory Nodes, but they are NOT exactly memory node but CPU nodes.

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Locked Pages allocated would only show up when SQL Server has been configured to use Lock Pages in memory.

Memory Clerks: Clerks are consumers of memory in SQL. In general there are 4 kinds of clerks, generic, cache store, object store, user store. The clerks make use of the memory node allocators to allocate memory. SQL has a vast number of Clerks predefined, and each of these clerks corresponds to the type of allocation which needs to be done. SQL exposes the clerk information using the “sys.dm_os_memory_clerks” DMV. This information is also exposed through the DBCC MEMORYSTATUS. In the memory status output one can see the clerks being listed multiple times. This is because of the fact that the clerks are created per NUMA node. So if we have 4 NUMA nodes on the machine, we could see the clerks being reported 4 times.

Memory Objects: Memory objects are the memory allocations made through the clerks. There are 3 kinds of memory objects which are supported by SQL. Memory objects can be viewed using the “sys.dm_os_memory_objects” DMV.

  1. Variable Memory Object: Supports variable size memory allocations.
  2. Mark/Shrink Objects: This allocations are typically done in two stages. During the first stage the allocations grows in size (more and more memory are added as part of this allocation) and during the second phase the memory is de-allocated. As Slava points out, these type of allocations are very useful in scenarios like compilation or execution.
  3. Fixed Size Memory objects: Supports memory allocations which are fixed in size.

Memory Broker: Memory broker can be though of as the Ring Master in a Circus. The purpose of Memory Broker is to provide a centralized mechanism to distribute memory or control the allocations made by each component in SQL Server. Memory Broker monitors the demand consumption of memory by each component and then based on the information collected, it calculates and optimal value of memory for each of these components. This information is then broadcast to each of the components, which grow or shrink their usage as required. Memory broker is exposed through the DMV “sys.dm_os_ring_buffers” where the Ring_buffer_type is “RING_BUFFER_MEMORY_BROKER”.

To tie these pieces together. 

In order for the Clerks to allocated Memory, they have to create a memory object using one of the Allocators exposed by the Memory Node. These allocations (memory objects) could be of any type as mentioned above. This entire allocation and de-allocation is monitored by the memory broker, which redirects the clerks to grow/shrink their memory usage as required.

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In the next post, I will talk about the memory clerks in more details.

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SQL Server Memory–The Evolution

5 Comments Posted by on May 9, 2012

Previous Posts in this series

SQL Server Memory– Underground

In my Previous post, I had talked briefly about the windows Memory Management and how SQL calculates its Buffer Pool and MTL. In this post, I would be talking mainly about the workings of the SQL Buffer Pool and the MTL, and how to figure out the memory usage by SQL and its components. This post would be specific to SQL 2000 unless specified otherwise.

SQL Buffer Pool

SQL Memory Management is Dynamic in nature, meaning SQL can allocate or de-allocate memory in the Buffer Pool as per requirement. In order for the SQL Server to effectively manage its memory allocations SQL Server 7, introduced a layer of Memory Manager that manage various types of memory requests in the SQL Buffer pool.

The important ones are

  1. General – This memory manager handles general allocation (not handled by other managers). Includes things like locks etc..
  2. Query Plan – As the name implies, this is mainly used for query plans.
  3. Optimization – Handles memory requests during optimization.
  4. Utility – Manages Memory allocations for different utilities, like tracing, log shipping etc.
  5. Connection – Manages memory allocations for Connections. This includes memory for the input and output buffers.

The idea behind using these managers is very simple. Consider an Analogy. In a company with 1000 employees, If there is only one manager, management becomes really difficult. One the second hand if we have 10 managers managing 100 employees each, it would be easier. With multiple managers it becomes easier to manage and report the usage.

The amount of memory being consumed (out of the 2 GB user mode VAS), can be easily tracked using either of the following options.

  • DBCC MEMORYSTATUS
  • Windows Performance Monitor Counters

DBCC MEMORYSTAUS

The KB article describes the output of the DBCC MEMORYSTATUS command for SQL 2000 in great detail. The following section of the DBCC MEMORYSTATUS output can be used to track the usage per Memory Manager.

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The OS reserved/OS committed/OS in Use sections are for all allocations which have been made in the SQL VAS  by the memory managers (more than 8 KB allocations) and also for all external allocations coming from external components loaded in the SQL Space. In short, this can be used to track all the allocations made in the Mem-To-Leave area.

Apart from the output for the Memory Managers, another important section in the output is the first section of the output, which can be used to see how are the Database Pages allocated in the BPool.

Windows Performance Monitor Counters

SQL Server exposes two Performance Monitor Objects for monitoring SQL Server Memory. These performance Monitor counters are also applicable to next releases of SQL Server (SQL 2005/2008/2008 R2) with slight differences, which I would be pointing out in the next posts.

  • Memory Manager: Provides information about SQL Server memory as whole.
  • Buffer Manager: Provides information about the SQL Buffer Pool

Memory Manager

The important counters under this object are

  1. Connection Memory: Indicates the amount of memory being consumed under the Connection Memory Manager.
  2. Granted Workspace Memory: Memory allocated/reserved for operations during Query Execution. All memory for sorting, hashing come from these.
  3. Memory Grants Pending: Number of memory requests which have not been completed. A non-zero value for a sustained period would indicate a memory pressure in SQL Server.
  4. Optimizer Memory: Memory being consumed under the Optimizer Memory Manager.
  5. Total Server Memory: Total memory consumed my SQL. This included both allocation in BPool and Allocations in Mem-To-Leave. This value should be equal to the sum of the “OS in Use” under dynamic memory manager, and “Committed” under Buffer Count sections of the Memory Status values. Remember these values in Memory Status output are in number of 8 KB pages.

Buffer Manager

  1. Checkpoint Pages/Sec: Number of pages being flushed by the checkpoint thread.
  2. Database Pages: Number of BPool buffers being used for Database pages.
  3. Free Pages: Number of pages free in the buffer pool. This values should always be greater than 0.
  4. Lazy Writes/Sec: Number of lazy write operations per sec.
  5. Stolen Pages/Sec: Committed pages which are not being as a database page are termed as stolen.
  6. Total Pages: Number of pages currently being used in the Buffer Pool.

Memory discussions for SQL Server is not complete without the mention of two very important operations, Checkpoint and Lazy Writer.

Checkpoint: Checkpoint is the process wherein Dirty Buffer (database pages which have been modified by a DML operation) back to the disk.

Lazy Writer: The sole purpose of Lazy Writer is to maintain some free buffers in the SQL Server Buffer Pool. Lazy writer runs periodically and check which buffers can be flushed and returned to the free pool.

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