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dbwr working set....

From: Aman Sharma <>
Date: 9 Oct 2007 13:58:52 -0000
Message-ID: <>

Hi there,
In database buffer cache,there is some thing called working set.This is a text quoted about this working set from a book.Please it would be realy great,if anyone can make me understand this working set and its working or point to me a link where it is described.
here is the text
"Working sets are subsets of buffers in the buffer cache that are created to improve scalability of common buffer cache operations. Each working set contains a least recently used (LRU) list and two checkpoint queues as illustrated in the slide. Each buffer in the cache is assigned to a working set at instance startup. The number of working sets depends on the number of CPUs on the system as well as the number of configured database writer processes (DBWn). Concurrent operations by processes on different CPUs can proceed without conflicting with one another by using different working sets.
A cache miss occurs when a requested block is not found in the buffer cache. When this happens, an available buffer must be found by searching the LRU list. The LRU list is a linked list of buffer headers arranged by time since last access and frequency of access. The frequency of access is measured by the number of times a buffer has been accessed. This is called touch count. A buffer that is frequently accessed and has been recently accessed is called a hot buffer or block. A buffer that has not been accessed recently is a cold buffer or block. The LRU list is arranged so that the cold buffers are at one end and hot buffers are at the other. Accesses to the LRU lists are protected by the LRU latch known as the cache buffer lru chains latch in V$LATCH. There is one latch per working set. On a cache miss, a user process picks a random working set and replaces the coldest buffer in that set with the copy read from disk. "
Thanks and best regards,

"As our circle of knowledge expands, so does the circumference of darkness surrounding it."-
Aman Sharma

Received on Tue Oct 09 2007 - 08:58:52 CDT

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