How does Oracle process spin for a latch? How many times does it check the latch before going to sleep? Anyone knows. This is the _spin_count=2000. Two thousand cycles by default. Oracle established this value long ago in version 6 at the beginning of 90s. However, let me count.
My previous investigation showed that latch wait was cardinally changed in Oracle 9.2. At that time, the exponential backoff disappeared. The latches have been using completely new wait posting since 2002. We may expect that latch spin have been changed too. Controversial results of _spin_count tuning in Oracle 9.2 confirm this also. In this series of posts, I will explore how the contemporary Oracle latches spin. The first post is about exclusive latches that form the majority of Oracle latches. For example, 460 out of 551 latches are exclusive in Oracle 11.2.0.2.
I will demonstrate that exclusive latches spin 10 times more than we expected. The _spin_count occurred to be effectively static for exclusive latches, and there is a big difference between not setting _spin_count and setting it to 2000.
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