JVM garbage collection using CMS

To ensure reliable server performance with Java, the PingDirectory server depends on Java’s Concurrent Mark and Sweep process (CMS) for background garbage collection.

Out of several garbage collection options, CMS is the ideal choice for consistent system availability. For the most part, the CMS collector runs as one or more background threads within the Java virtual machine (JVM), freeing up space in JVM Heap from an area called the old generation. One of the criteria used by CMS to determine when to start background garbage collection is a parameter called CMSInitiatingOccupancyFraction. This percentage value, which applies to the Old Generation, is recommended for the JVM to initiate CMS when data occupancy in Old Generation reaches the threshold.

Ideally, the database cache takes less than 70% of the space available in the Old Generation, and the CMSInitiatingOccupancyFraction value of 80 leaves plenty of headroom to prevent the JVM from running out of space in Old Generation because of an inability for CMS to keep up. Because CMS takes processing resources away from the PingDirectory server, do not set theCMSInitiatingOccupancyFraction at or below the expected database cache size, which would result in the constant running of CMS in the background. For a description of determining Old Generation size, see the section on Memory Footprint and Database Cache.

When the CMS collection process cannot keep pace with memory demands in the Old Generation, the JVM resorts to pausing all application processing to allow a full garbage collection. This event, called a "stop-the-world pause", does not break existing TCP connections or alter the execution of the PingDirectory server requests. The goal in tuning CMS is to prevent the occurrence of these pauses. When one does occur, the server generates an alert after the pause and records the pause time in the error log.

Because determining an ideal CMSInitiatingOccupancyFraction can be difficult, we warn if the PingDirectory server detects a garbage collection pause by generating a recommended value for the occupancy threshold based on the current amount of memory being consumed by the backend caches. Unfortunately, an administrator cannot determine the ideal occupancy threshold value in advance. To warn of any impending garbage collection pauses, the server calculates a recommended value for the CMSInitiatingOccupancyFraction property and exposes it in the JVM Memory Usage monitor entry in the following attribute:

Also, when you start the server, an administrative alert indicates the current state of the CMSInitiatingOccupancyFraction and its recommended value.

The PingDirectory server only makes a recommendation if all of the backends are preloaded and the CMSInitiatingOccupancyFraction JVM property is explicitly set, which is done automatically. For example, if you installed the PingDirectory server and specified that the database be preloaded (or "primed") at startup, then the server can make a good recommendation when a pause occurs. If the backend database cache is not full and has not been preloaded, then the recommended value might be an inaccurately low value.