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The buffer pool is managed by a process called the lazywriter, which uses a clock algorithm to sweep through the buffer pool. Basically, the lazywriter thread maintains a pointer into the buffer pool that sweeps sequentially through it (like the hand on a clock). As it visits each buffer, it determines whether that buffer has been referenced since the last sweep, by examining a reference count value in the buffer header. If the reference count is not 0, the buffer stays in the pool and its reference count is adjusted in preparation for the next sweep; otherwise, the buffer is made available for reuse: It is written to disk if dirty, removed from the hash lists, and put on a special list of buffers called the free list.
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NOTE: The set of buffers through which the lazywriter sweeps is sometimes called the LRU (for least recently used list). It does not, however, function as a traditional LRU because the buffers do not move within the list according to their use or lack of use; the lazywriter clock hand does all the moving. Also note that the set of buffers that the lazywriter inspects actually includes more than pages in the buffer pool. It also includes pages from compiled plans for procedures, triggers, or ad hoc queries.
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The reference count of a buffer is incremented each time the buffer s contents are accessed by any process. For data or index pages, this is a simple increment by 1. But objects that are expensive to create, such as stored procedure plans, get a higher reference count that reflects their replacement cost. When the lazywriter clock hand sweeps through and checks which pages have been referenced, it does not use a simple decrement. It divides the reference count by 4. This means that frequently referenced pages (those with a high reference count) and those with a high replacement cost are favored and their count will not reach 0 anytime soon, keeping them in the pool for further use. The lazywriter hand sweeps through the buffer pool when the number of pages on the free list falls below its minimum size. The minimum size is computed as a percentage of the overall buffer pool size but is always between 128K and 4MB. Currently, the percentage is set at 3 percent, but that could change in future releases of SQL Server 2000.
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User threads also perform the same function of searching for pages for the free list. This happens when a user process needs to read a page from disk into a buffer. Once the read has been initiated, the user thread checks to see if the free list is too small. (Note that this process consumes one page of the list for its own read.) If so, the user thread performs the same function as the lazywriter: It advances the clock hand and searches for buffers to free. Currently, it advances the clock hand through 16 buffers, regardless of how many it actually finds to free in that group of 16. The reason for having user threads share in the work of the lazywriter is that the cost can be distributed across all the CPUs in an SMP environment.
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Tables can be specially marked so that their pages are never put on the free list and are therefore kept in memory indefinitely. This process is called pinning a table. Any page (whether data, index, or text) belonging to a pinned table is never marked as free and reused unless it is unpinned. Pinning and unpinning is accomplished using the pintable option of the sp_tableoption stored procedure. Setting this option to TRUE for a table does not cause the table to be brought into cache, nor does it mark pages of the table as favored in any way; instead, it avoids the unnecessary overhead and simply does not allow any pages belonging to a pinned table to be put on the free list for possible replacement. Because mechanisms such as write-ahead logging and checkpointing are completely unaffected, such an operation in no way impairs recovery. Still, pinning too many tables can result in few or even no pages being available when a new buffer is needed. In general, you should pin tables only if you have carefully tuned your system, plenty of memory is available, and you have a good feel for which tables constitute hot spots. Pages that are very hot (accessed repeatedly) are never placed on the free list. A page in the buffer pool that has a nonzero use count, such as one that is newly read or newly created, is not added to the free list until its use count falls to 0. Prior to that point, the page is clearly hot and is not a good candidate for reuse. Very hot
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