Craig Shallahamer's Blog
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You were brought to this page based on an internet search
and as a free service to Oracle DBAs.
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.
To order the book in either print or PDF form, click
here.
©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>Figure 6-19. By default, a buffer header's touch count can be incremented only once every 3 seconds. This helps ensure a buffer deemed popular is active more than just a few seconds.
p><p>When a buffer is inserted into the buffer cache, its touch count is set to zero. However, if the buffer is repeatedly touched within a short period of time, there is a good chance the touch will not count. In Figure 6-19, the time period labeled A will result in the touch count being incremented only once, as is the case for all the periods diagrammed. Without the 3-second rule, the buffer header's touch count would be increased by 14; with the 3-second rule, the touch count will be increased by only 5. This ensures that for a buffer to be deemed popular, it can't just be popular for a few seconds. It must continue to be popular to generate a large touch count.
p><p>Oracle also allows a touch count to be missed. There is no latch involved (which is a fantastic way to eliminate latch contention), and Oracle does not pin the buffer header. Without serialization control, two server processes can increment and update a buffer header's touch count to the same value!
p>
©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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