Warning: VKTM detected a time drift.

 1 Warning: VKTM detected a time drift.
 2 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
 3 Tue Dec 03 16:21:35 2019
 4 Warning: VKTM detected a time drift.
 5 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
 6 Tue Dec 03 16:38:23 2019
 7 Adjusting the default value of parameter parallel_max_servers
 8 from 640 to 320 due to the value of parameter processes (350)
 9 Starting ORACLE instance (normal)
10 Tue Dec 03 21:09:32 2019
11 Warning: VKTM detected a time drift.
12 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
13 Tue Dec 03 22:00:00 2019
14 Setting Resource Manager plan SCHEDULER[0x32DA]:DEFAULT_MAINTENANCE_PLAN via scheduler window
15 Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter
16 Tue Dec 03 22:00:00 2019
17 Starting background process VKRM
18 Tue Dec 03 22:00:00 2019
19 VKRM started with pid=78, OS id=28922 
20 Tue Dec 03 22:00:02 2019
21 Begin automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
22 Tue Dec 03 22:20:55 2019
23 End automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
24 Tue Dec 03 22:46:17 2019
25 Warning: VKTM detected a time drift.
26 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
27 Wed Dec 04 00:00:29 2019
28 Thread 1 advanced to log sequence 5843 (LGWR switch)

在Oracle Database 11g中，VKTM是一个新增的后台进程。这个进程的含义是：

VKTM (virtual keeper of time) is responsible for providing a wall-clock time (updated every second) and
reference-time counter (updated every 20 ms and available only when running at elevated priority).

也就是说，这个进程用于提供一个数据库的时钟，每秒更新；或者作为参考时间计数器，这种方式每20毫秒更新一次，仅在高优先级时可用。

在系统时间出现异常或变化时，VKTM进程还会检测这些变化，提醒用户，尤其是在RAC环境中，时间的偏移和变化极有可能导致系统故障。以下是告警日志中的警告信息：

Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.

通过VKTM进程，数据库可以降低和操作系统的交互，Kamus的解释是：

在11g之前所有的Oracle数据库后台或者前台进程如果需要获得当前时间信息，就需要调用操作系统的gettimeofday()函数或者说是相类似的函数。
而VKTM进程就是专门用来获得时间信息然后将信息存放在SGA中供其它进程使用，这样其它进程当需要时间信息的时候，只要到SGA的某个内存位置去获得就好，
而不用频繁调用gettimeofday()函数。毫无疑问，这样效率会更高。

在RAC测试中，Oracle 11.1.0.6版本LMSx进程获取时间信息时，可以从VKTM进程中获益大概70%的速度提升，而11.1.0.7将会更高。

同时，因为gettimeofday()函数也引发了很多bug，所以无论是RAC还是NORAC库，都将从VKTM进程中获益。

参考
https://www.eygle.com/archives/2012/06/oracle_11g_vktm.html