June 17, 2010
Warning: this article is certain to bring tears of pain to your eyes while you attempt to determine what is right and what is wrong.
While reading the June 2010 printing of the “Oracle Performance Firefighting” book I found a couple of interesting lines about the buffer cache and the effects, or lack there-of, of the _SMALL_TABLE_THRESHOLD parameter. This quote is from page 208 (this is a long quote, required to not lose the context of the discussion topic - please excuse any typos):
“The single most radical departure from the modified LRU algorithm [introduced in Oracle Database 6, per the book] is known as midpoint insertion [introduced in Oracle Database 8.1.5, per the book]. Each LRU chain is divided into a hot and cold region. When a buffer is read from disk and a free buffer has been found, the buffer and buffer header replace the previous buffer and buffer header contents, and then the buffer header is moved to the LRU chain midpoint. Single-block read, multiblock read, fast-full scan, or full-table scan—it makes no difference. The buffer header is not inserted at the MRU end of the LRU chain, but rather at the midpoint…
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.”
Before deciding what about the above is correct or incorrect, take a look at Metalink Doc ID 787373.1. A couple of words from that document to encourage you to take a look at the original material:
“Applies to Version: 184.108.40.206 to 220.127.116.11.0: When loading objects and data into the buffer cache… best way to load for full scans. This functionality is called smallness logic and is driven by a hidden parameter called: _small_table_threshold. … Oracle will load the object via the buffer cache as this is more efficient than doing a direct read operation.”
Before deciding what about the above Metalink document is correct or incorrect, take a look at this AskTom article. A couple of words from that article:
“no, you are not correct on your guess about the [_SMALL_TABLE_THRESHOLD] parameter, it controls the caching of the blocks (whether they are cached like a single block IO or cached as we cache full scan blocks) – not the method of IO.”
Before deciding what about the above AskTom article is still correct, take a look at this blog article. A couple of words from that article:
“When direct path reads starts to happen? It is known that somewhat reliable figure is your _small_table_threshold multiplied by 5 (mentioned by Tanel Poder on oracle-l recently).”
One final blog article, from the comments section:
“A table is deemed ‘small’ by Oracle if it’s 2% or less of the buffer cache in size. A small table is ‘cached’ when read via a FTS and is not immediately loaded into the least recently used end of the LRU list as becomes a ‘large’ table read via a FTS…”
“When a block is read from a ‘large’ table via a FTS, the blocks are basically loaded into the LRU end of the LRU list, even though they’ve only just been loaded into the buffer cache. As such, the chances of them remaining in the buffer cache is minimal and will likely be immediately overwritten once processed.
However, when a block is read via an index, the blocks are loaded somewhere near the ‘middle’ of the LRU list, not at the LRU end of the list…”
What, if anything, is wrong with the above quote from the book? Bonus question, what, if anything, is wrong with the Metalink document? Maybe there is a problem with the other quotes?
The point of blog articles like this one is not to insult authors who have spent thousands of hours carefully constructing an accurate and helpful book, but instead to suggest that readers investigate when something stated does not exactly match what one believes to be true. It could be that the author “took a long walk down a short pier”, or that the author is revealing accurate information which simply cannot be found through other resources (and may in the process be directly contradicting information sources you have used in the past). If you do not investigate in such cases, you may lose an important opportunity to learn something that could prove to be extremely valuable.