Craig Shallahamer's Blog
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The text below is an except from the book,
Oracle Performance Firefighting, written by
Craig Shallahamer of
OraPub, Inc.
Figures and tables are not included on this page, only their reference.
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©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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<p>As other buffer headers are inserted into the midpoint or promoted, buffer headers naturally migrate from the hot region toward the cold region's LRU chain endpoint. After a buffer header is inserted, the only way to remain in the cache for a long time is to be repeatedly promoted. I'll detail this in the upcoming "Buffer Promotion" section.
</p><p>Because the window scheme used in the modified LRU algorithm is no longer used, the hidden instance parameter _small_table_threshold became deprecated. However, in Oracle Database 11g, it is being used again, but for a different purpose. Starting with this version, the _small_table_threshold parameter is the threshold for a server process to start issuing direct reads. Direct reads can increase performance because blocks read from disk are not placed into the buffer cache, but instead are processed in the server process's PGA memory. However, it is more selfish read and can actually slow performance, because no other server process will benefit from the IO activity. My tests have shown that Oracle Database 11g does not always respect changes to this parameter.
</p><p>Let's walk through an abstracted scenario. I'm leaving out a lot of detail, but this story will work fine for our purposes. Suppose you are a server process who must query a row, which resides in a specific block. Based on the SQL statement and the data dictionary, you know the file and block number. You're all about speed, so you're hoping the block already resides in the buffer cache. To check this, you need to get the buffer's buffer cache memory address, which resides in its buffer header.
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©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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