In version 9.2 Oracle introduced new possibilities for fine grain latch tuning. Latch can be assigned to one of eight classes. Each class can have a different spin and wait policy. In addition, exclusive and shared latches behave differently.
By default, all the latches except “process allocation latch” belong to the standard class 0. In my previous posts, I discussed how the standard class exclusive and shared latches spin and wait. Now, it is the time to explore the non-standard class latch behaviors.
My previous experiments demonstrated that, opposite to common belief, spin count for exclusive latches in Oracle 9.2-11g cannot be tuned dynamically. The _spin_count parameter is effectively static for exclusive latches. This seems to disagree with the well-known study “Using _spin_count to reduce latch contention in 11g” by Guy Harrison. The study explored how dynamic tuning of _spin_count influenced latch waits, CPU consumption and throughput. I think that there is no contradiction. Probably Guy Harrison’s experiments have been performed with the cache buffers chains latch contention. This is the shared latch.
We already know that exclusive latch in Oracle 9.2-11g uses static spin value from x$ksllclass fixed table. This spin can be adjusted by _latch_class_0 parameter. By default, the exclusive latch spins up to twenty thousand cycles.
This post will show that shared latch in Oracle 9.2-11g is governed by _spin_count value and spins upto four thousand cycles by default.
The way an Oracle process waits for a latch. This seems trivial. Oracle 11.2 documentation explicitly states about “latch free” wait event:
“… The wait time increases exponentially and does not include spinning on the latch (active waiting). The maximum wait time also depends on the number of latches that the process is holding. There is an incremental wait of up to 2 seconds…”
In this post, I would like to show that this exponential backoff was obsoleted in Oracle 9.2. Since that time, all the Oracle versions use completely new latch wait posting mechanism and FIFO queuing for all, except one latch.
Oracle no longer uses “repeatedly spin and sleep” approach. The process spins and waits only once. The pseudo code for contemporary latch acquisition should looks like:
Immediate latch get
Spin latch get
Add the process to the queue of latch waiters
Sleep until posted
As I described in my previous post, Shared and Exclusive Oracle latches differ significantly. Shared latch behaves like enqueue. It has S and X incompatible modes. Moreover X mode serializes the shared latch. The contention for shared and exclusive latches has different patterns. This leads to different methods to tune such contentions.
But we do not know which latches are shared. Oracle never published the list of shared latches. Every time looking in the AWR or Statspack report we had to guess the type of contending latch. We only know that “cache buffer chains” latches became shared since in 9.2.
Oracle executable internally determines that latch is shared using flag hidden somewhere in x$kslld (v$latchname) structure. Google search shows that KSLLD means [K]ernel [S]ervice [L]ock [L]atch [D]escriptor. Unfortunately this shared flag was not externalized to SQL. It is possible to check the flag manually using oradebug peek or DTrace. But the flag offset is version and platform dependent. We need more systematic way to determine the latch type.
We know a lot about the exclusive latches. This is Oracle realization of TTS spinlocks. Only one process at a time can hold the exclusive latch. Other processes compete for the exclusive latch by spinning. If process can not get the latch by spinning, it will wait until the latch becomes free.
But since the version 8.0 Oracle had another spinlock – shared latch. This is a realization of “Read-Write” spinlocks. Such spinlocks can be held by several “reader” processes simultaneously in SHARED mode. But if the process needs to write to the protected structure it must acquire RW spinlock in EXCLUSIVE mode. This mode prevents any concurrent access to the latch.
From version to version the number of shared latch increased. Several widely known latches like “session idle bit”, “In memory undo latch”. “Result Cache: RC Latch”, “resmgr group change latch” are shared.
Famous “cache buffers chains” latch was also became shared in Oracle 9.2. We usually react on “cache buffers chains” latch contention finding ineffective SQL plans and “hot blocks”. Recently Kyle Hailey posted an excellent graphical explanation of Oracle mechanics related to “latch: cache buffers chains” contention. But it always was a mystery to me why the sessions have to wait for SHARED latch during READ operations like searching the hash chains. Other busy shared latches like “session idle bit” do not experience such contention. This is why I would like to dive deeper into shared latch internals.