To ensure reliable server performance with Java, the Directory 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 Directory Server, do not set the CMSInitiatingOccupancyFraction 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 Directory Server requests. The goal in tuning CMS is to prevent the occurrence of these pauses. When one does occur, the Directory 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 Directory 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 Directory 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.
$ bin/start-server [20/April/2012:10:35:25 -0500] category=CORE severity=NOTICE msgID=458886 msg="PingDirectory Server 126.96.36.199 (build 20120418135933Z, R6226) starting up" ... (more output) ... [20/April/2012:10:35:53 -0500] category=UBID_EXTENSIONS severity=NOTICE msgID=1880555580 msg="Memory-intensive Directory Server components are configured to consume 71750382 bytes of memory: ['userRoot local DB backend' currently consumes 26991632 bytes and can grow to a maximum of 64323584 bytes, 'changelog cn=changelog backend' currently consumes 232204 bytes and can grow to a maximum of 2426798 bytes, 'Replication Changelog Database' currently consumes 376661 bytes and can grow to a maximum of 5000000 bytes]. The configured value of CMSInitiatingOccupancyFraction is 36 which is less than the minimum recommended value (43) for the server's current configuration. Having this value too low can cause the Concurrent Mark and Sweep garbage collector to run too often, which can cause a degradation of throughput and response time. Consider increasing the CMSInitiatingOccupancyFraction value to at least the minimum value, preferably setting it to the recommended value of 43 by editing the config/java.properties file, running dsjavaproperties, and restarting the Directory Server. If the server later detects that this setting actually leads to a performance degradation, a separate warning message will be logged. If this server has not yet been fully loaded with data, then you can disregard this message" [20/April/2012:10:35:53 -0500] category=CORE severity=NOTICE msgID=458887 msg="The Directory Server has started successfully"
The Directory 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 Directory Server and specified that the database be preloaded (or "primed") at startup, then the Directory Server can make a good recommendation for the Directory Server 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.
The generated value for the Directory Server property could change over time with each Directory Server build, Java release, or changes in data set. If the current value is close to the recommended value, then you do not need to change the property unless the server experiences a JVM pause.
If the Directory Server experiences a JVM garbage collection pause, you can retrieve the recommended value from the server, reset the Directory Server property, run dsjavaproperties, and restart the server.