Several months have passed since my previous “mutex wait” post. I was so busy with work and conference presentations. Thanks to all my listeners at UKOUG2011, Hotsos2012 and Medias2012 conferences and several seminars for inspiring questions and conversations.
I. Unexpected change.
Now it is time to discuss how contemporary Oracle waits for mutexes. My previous posts described evolution of “invisible and aggressive” 10.2-11.1 mutex waits into fully accounted and less aggressive 11gR2 mutexes. Surprisingly Oracle 220.127.116.11.2 (or 18.104.22.168 PSU2) appeared in April 2011 demonstrated almost negligible CPU consumption during mutex waits. (more…)
I would like to describe how Oracle versions 22.214.171.124-126.96.36.199.1 waited for mutexes. This algorithm also appears to be used in post-188.8.131.52.2 PSUs and new 184.108.40.206 patchset as _mutex_wait_scheme=0.
My previous post demonstrated that before version 11.2:
- “Cursor: pin S” was pure wait for CPU. Long “cursor: pin S” waits indicated CPU starvation.
- Mutex contention was almost invisible to Oracle Wait Interface
- Spin time to acquire mutex was accounted as CPU time. It was service time, not waiting time.
Things changed. Mutex waits in Oracle 11.2 significantly differ from previous versions. Contemporary mutex waits are not CPU aggressive anymore, completely visible to Oracle Wait Interface and highly tunable.
Oracle KGX mutexes appeared more than 7 years ago. However, mutex waits are still obscure. Oracle Documentation provided only brief description of mutex wait events without any information about wait durations and timeouts.
Look at the following timeline:
Oracle 220.127.116.11 contains enhancements 9282521 and 9239863 named “Library cache: mutex X” for objects highly contended for. Part I and II. These enhancements introduce new interesting possibilities to tune some types of the mutex contention.
Contention for heavily accessed objects can now be divided between multiple copies of object in the library cache. According to notes 9282521.8 and 9239863.8 describing the patches, the enhancements should be used:
When there is true contention on a specific library cache object….
Let me investigate this deeper. I will use Oracle 18.104.22.168.2 for Solaris SPARC (64-bit) on 8Cores/32Threads Sun Fire T2000. I chose this platform in order to emphasize how the enhancements work. (more…)