PageAccessSynchronizer

Hi, all.
We have redundant igniiton 7.8.4.

System Properties
Context State Running
Redundancy Mode Master
Redundancy State Active
Version 7.8.4 (b2016082217)
Java Version Oracle Corporation 1.8.0_102
CPU % 18,3
Memory (used/max) 314 mb / 2,0 gb
Uptime 1 hours, 1 minutes, 8 seconds
Timezone Asia/Karachi [GMT+5:00]
Locale ru_RU
Java Library Path lib;lib\core\gateway

And sometimes we see error in console:

Thread ‘qtp1983330395-42’ failed to acquire lock to page with id ‘1’, attempted for 1 minutes out of allowed 1 minute. The thread that holds the lock has name ‘qtp1983330395-562’.

Why?

ERROR 16:35:40 DefaultExceptionMapper Unexpected error occurred

org.apache.wicket.page.CouldNotLockPageException: Could not lock page 1. Attempt lasted 1 minute
more
WARN 16:35:40 PageAccessSynchronizer “qtp1983330395-3140” prio=5 tid=3140 state=TIMED_WAITING

org.apache.wicket.util.lang.Threads$ThreadDump
more
WARN 16:35:40 PageAccessSynchronizer Thread ‘qtp1983330395-2874’ failed to acquire lock to page with id ‘1’, attempted for 1 minute out of allowed 1 minute. The thread that holds the lock has name ‘qtp1983330395-3140’.
WARN 16:20:48 ClockDriftDetector Clock drift, degraded performance, or pause-the-world detected. lastTime=Tue Oct 31 16:21:04 PKT 2017 currentTime=Tue Oct 31 16:20:48 PKT 2017 deviation from expected 1000ms delta=-16795ms.
ERROR 16:17:09 DefaultExceptionMapper Unexpected error occurred

org.apache.wicket.page.CouldNotLockPageException: Could not lock page 1. Attempt lasted 1 minute
more
WARN 16:17:09 PageAccessSynchronizer “qtp1983330395-42” prio=5 tid=42 state=TIMED_WAITING

org.apache.wicket.util.lang.Threads$ThreadDump
more
WARN 16:17:09 PageAccessSynchronizer Thread ‘qtp1983330395-21’ failed to acquire lock to page with id ‘1’, attempted for 1 minute out of allowed 1 minute. The thread that holds the lock has name ‘qtp1983330395-42’.
ERROR 16:13:51 DefaultExceptionMapper Unexpected error occurred

org.apache.wicket.page.CouldNotLockPageException: Could not lock page 1. Attempt lasted 1 minute
more
WARN 16:13:51 PageAccessSynchronizer “qtp1983330395-562” prio=5 tid=562 state=TIMED_WAITING

org.apache.wicket.util.lang.Threads$ThreadDump
more
WARN 16:13:51 PageAccessSynchronizer Thread ‘qtp1983330395-42’ failed to acquire lock to page with id ‘1’, attempted for 1 minutes out of allowed 1 minute. The thread that holds the lock has name ‘qtp1983330395-562’.