Using _spin_count

Mohan asks: I hear that the spin_count parameter can help relieve latch contention (high latch wait values in STATSPACK reports), but I would like to know how Oracle 9i and later versions moved the spin_count parameter to a hidden _spin_count parameter. Why did Oracle 9i depreciated the spin_count parameter?  Is there any other parameter to replace spin_count in oracle 9i and beyond?
 

Answer:  The spin_count was designed to allow the DBA to control the amount of time spent "spinning" (waiting) for a serialization latch to be released and has a default value of 2,000.  Since spin_count sets the number of spins a process will undergo before trying to get a latch, if the CPU is not fully loaded, a high value may be best; for a fully-loaded CPU, a smaller value for spin_count may help.

As you noted, spin_count is still there, re-named to an undocumented parm _spin_count.  While Oracle does publish their reasons, MOSC Note:30832.1 says that spin_count is of limited value:

?If a system is not tight on CPU resource SPIN_COUNT can be left at higher values but anything above 2000 is unlikely to be of any benefit.?.

Some DBA's will try to increase spin_count in an attempt to reduce latch contention. This is a brute force approach where we are increasing the probability of obtaining the latch by increasing the number of requests for the latch. In most cases, this addresses the symptom of the problem and not the original cause.

In my experience, you rarely need to adjust your spin_count, but there are some systems where changing the now-undocumented _spin_count can help to relieve latch contention.

Also, Guy Harrison notes that spin_count can still be valuable as a tuning tool:

"Latches protect areas of Oracle shared memory from concurrent access in roughly the same way that locks protect data in tables. When a session wants a latch it will repeatedly attempt to obtain the latch until reaching the value of "_spin_count" after which it will sleep and a "latch free" wait will occur. Excessive latch sleeps can create restrictions on throughput and response time.

The two most frequently encountered forms of latch contention in modern Oracle (10g/11g) are:

- Library cache/shared pool latch contention - This is usually caused when an application issues high volumes of SQL which are non-sharable due to an absence of bind variables. The CURSOR_SHARING parameter can often be used to alleviate this form of contention.

- cache buffer chains contention - This is usually associated with very high logical read rates and "hot" blocks within the database (sometimes index blocks). After tuning SQL to reduce logical IO and eliminate repetitive reads of the same information, partitioning is often a possible solution.

If latch contention is causing serious problems, and the system has some free CPU capacity, adjusting the value of the undocumented parameter _spin_count may be effective in reducing contention. As always, modifying undocumented parameters should be approached with great caution. "
 

This script can display the value for _spin_count:

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