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Tag Archives: Buffer Pool

SQL Server Memory–Rise of the Clerks – Part 2


Previous posts in this series.

SQL Server Memory – Underground

SQL Server Memory – The Evolution

SQL Server Memory – Rise of the Clerks – Part 1

In the last post, I had talked about the memory architecture in SQL Server 2005. In this post, we will talk more about the clerks.

As mentioned in my previous post, memory clerks are consumers of memory. The allocate memory from the memory nodes using one of the allocators as exposed by the memory node. In general there are 4 different types of clerks in SQL. Generic Clerks, Cache Store, Object Store, and User Store.

Before we talk about these stores and the clerks, its important to understand the difference between a Cache and a pool in SQL. Cache in SQL is a mechanism to Cache different types of data, which are typically controlled by a cost for each entry. Ideally, each of these entries in the cache has a state value associated with it. Caches typically implement a lifetime control on the entry. This is controlled by some sort of LRU policies. Depending on the type of data caches, the entries could be shared between multiple users. Pool on the other hand is a mechanism for caching homogenous data(same type of data). The data in these does not have have cost or state associated with it. There is no lifetime control implemented on the pool.

Buffer Pool, as the name suggest is NOT a pool, but a cache, as we will see later.

Cache Store/User Store: These are typical caches in SQL. The Cache store provides support for hashed tables, while the user store provides caching for user created objects. Caches implement two types of control on the entries,

  1. Lifetime control: As the name suggests, it provides time management for the entries.
  2. Visibility control: As the same suggests, it provides control over, whether the entry in the cache is visible to any look ups or not.

In both the case, the SQLOS’s caching framework provides visibility control and lifetime control (not in case of User store). Life Time of the entry is managed using the reference count in the Clock Entry info, once the reference count becomes zero, the entry is removed from the cache. Visibility is maintained by the Pin Count in the clock entry info. For an entry to be visible, its pin count needs to have a value grater than zero (along with being non-dirty and no marked for single usage).

A Cache Store, can support multiple types of Hash Tables, this design is very useful for lookups. For example the Procedure Cache, has hash tables for both name and id’s.

SQL implements lifetime/visibility control using the clock algorithm. The Clock algorithm implements two clock hands.

  • External Hand: Moved by the Resource Monitor, when there is a memory pressure.
  • Internal Hand: Used to control the size of a cache, relative to other caches in SQL. In other words, this hand puts a cap on the memory usage by a single cache.

So what does the clock hands really do??  Every time the clock hands sweeps through the cache, it divides the cost of the entry by 2. If the entry is not in use and its cost is zero, the clock hands first marks the entry as invisible and then removes it from the cache.

To read more on Clock hands, refer this post by Slava.

Object Store: Object store is a simple pool in SQL. it does not implement any costing for the entries. It controls the size, using a preconfigured max cap. In addition on every memory pressure notification from the resource monitor in SQL, it removes a preconfigured number of entries.

Generic Clerks: Provides mechanisms to allocate memory from the memory node using any of the allocators as exposed by the memory node.

In SQL Server 2005, Buffer Pool allocation is done in a similar way as with SQL 2000. We start of with checking how much RAM is installed on the machine, based on which we calculate the Buf array (array to monitor the buffers in the buffer pool). Once we have calculated the Buf array, we calculate the size of the BPool and allocate it. As with SQL 2000, the max server memory setting is only used to control the amount of memory used by buffer pool and not the entire SQL server. Buffer pool commits memory on demand and depending on the requirement, it calculates its Target Memory, the amount of memory it thinks it should commit before the server goes into memory pressure.

Unlike SQL 2000, Performance Monitor SQL Memory Manager:Total Server memory counter does not indicate the total SQL memory, but only the BPool usage.

Buffer Pool is a cache for Data Pages. Its primary purpose is to cache SQL Database pages, but in addition to caching data pages, other memory consumers(clerks) can borrow memory from the Buffer Pool. This memory is always in 8KB chunk and is not always contiguous. In other words, the buffer pool can be used as an underlying memory manager for other SQL Server memory clerks, as long as they allocate memory in the 8KB size.

In SQL 2005, during Startup, the SQL OS, configures the Buffer Pool to use single page allocators, which allows the Buffer Pool to provide for all Single page Allocations in SQL. In other words, all single page allocations come from the BPool.

To sum it up: During startup, the SQL OS allocates the Buffer Pool using the VM/AWE allocators (and not the page allocators, so in theory BPool is nothing but a large chunk of reserved memory) and configures it to use the Single Page Allocators. Once this is done, any of the memory clerks which need memory in the 8KB boundary, they would allocate the memory from the BPool (stolen pages). If the clerks require memory in large buffers (not in 8KB boundary), in those case, the Multi Pages Allocator or the virtual allocators and these would be allocated outside the buffer pool, area fondly called as the Mem-To-Leave.

In the next post in this series, I will talk about the NUMA support in SQL 2005 and how it affects SQL memory.


SQL Server Memory–The Evolution

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.

  • Windows Performance Monitor Counters


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.


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.