Installation manual

Performance and Scalability Recommended JVM Tuning Set

Release Notes for Release 9.1 Service Pack 3 (9.1.30151.0) Page 13

Concurrent Mark Sweep Collector

With the certification of Java 6, Blackboard recommends using the Concurrent Mark Sweep

Collector (-XX:+UseConcMarkSweepGC) instead of the Parallel Collector

(-XX:+UseParallelGC).

Local Allocation Buffer (TLAB)

The –XX:+UseTLAB parameter forces the JVM to use thread-local object allocation. A Thread

Local Allocation Buffer (TLAB) is a region of Eden that is used for object allocation by a single

thread, which is faster than performing an atomic operation on a top pointer shared across

threads. Using a TLAB improves concurrency by reducing contention on the shared heap lock.

Administrators can specify TLAB initial size and disable the resizing through the -

XX:TLABSize=<size in kb> -XX:-ResizeTLAB parameter. The default setting of 256KB

provides adequate performance. Blackboard’s tests do not show increased performance when

changing the TLAB size from the default value 256KB to 128KB or to 512KB. To discover which

settings have a positive, negative, or neutral effect on performance, trace all TLAB operations,

using the -XX:+PrintTLAB parameter.

Survivor Spaces

The -XX:SurvivorRatio parameter controls the size of the two survivor spaces. If survivor

spaces are too small, the JVM will copy collection overflows directly into the old generation. If

survivor spaces are too large, they will be empty. At each garbage collection, the JVM determines

the number of times that an object can be copied before it is tenured, which is called the tenure

threshold (-XX:MaxTenuringThreshold). After experimenting with setting -

XX:SurvivorRatio to 4, 8, 16, and 4096 with -XX:MaxTenuringThreshold set to 0,

Blackboard’s tests have found that customers who use our heap setting recommendations can

improve Eden space memory management by changing the -XX:SurvivorRatio parameter to

4 or lower. Configurations that use JVM heap sizes larger than 8GB require different settings.

New Space Region

Use the -XX:NewSize and -XX:MaxNewSize parameters instead of the previously

recommended -XX:NewRatio parameter to determine the memory needs in the New Space

region of the JVM. During performance tests, the -XX:NewRatio parameter did not size and

resize the new generation space of the JVM correctly. Size the -XX:NewSize and -

XX:MaxNewSize parameters to roughly 1/4 or 1/3 the size of the JVM.