12.2 一则事件enq: IV - contention

郑全

环境：exadata 双节点，oracle版本12.1.0.2
数据库服务器上多个实例出现enq: IV - contention等待事件，发现存在相关bug

In this Document

Symptoms

Cause

Solution

References

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APPLIES TO:Oracle Database - Enterprise Edition - Version 12.1.0.1 to 12.1.0.2 [Release 12.1]
Information in this document applies to any platform.

SYMPTOMS

12c RAC database seeing high "enq: IV - contention":

From awr report:

Top 10 Foreground Events by Total Wait Time
===================================
Event Waits Total Wait Time (sec) Wait Avg(ms) % DB time Wait Class
enq: IV - contention 52,914 1688.4 31.91 42.8 Other
row cache lock 44,865 896.8 19.99 22.7 Concurrency

tkprof of 10046 trace of SQL statement shows the same event in the top:

Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
enq: IV - contention 6017 0.32 287.68
row cache lock 957 0.20 7.48
library cache lock 631 0.13 15.10
library cache pin 616 0.11 14.54


CAUSE

Cluster nodes have different CPU count resulting in different number of LMD processes, on one node it has two while on the other it has three.

The issue is due to the following bug:

BUG 21293056 - PERFORMANCE DEGRADATION OF GRANT STATEMENT AFTER 12C UPGRADE

Which is closed as duplicate of:

BUG 21395269 - ASYMMETRICAL LMDS CONFIGURATION IN A CLUSTER LEADS TO POOR MESSAGE TRANSFER

SOLUTION

The fix will be included in future PSU, patch exists for certain platform/version.

The workaround is to set the following parameter to the highest value in the cluster and restart:

_ges_server_processes

To find out the highest value, run the following grep on each node:

ps -ef| grep lmd

通过文章可知，该bug是由于两节点cpu个数不同导致的，顿时觉得差异，按道理讲两台服务器cpu个数采购的时候应该是一致的，
于是查看cpu个数，发现果然不一致。