Fundamentals of Tuning OpenJDK's GC

Charlie Hunt (
Java & JVM
Location: E147 Level: Intermediate
Average rating: ****.
(4.17, 6 ratings)
Slides:   1-PDF 

Regardless of the garbage collector one chooses when using OpenJDK’s JVM, if one has a foundation of the fundamentals with tuning GC, he or she will be successful in tuning GC for his or her application.

The session will begin by presenting necessary background information on the HotSpot JVM’s memory manager including, in general, how it works.

Then, the presentation will shift to talking about the relationship between an application’s object allocation rate, and how that impacts both minor GC frequency and object promotion rate which dictates the frequency of full GCs. In addition, the presentation will also describe the relationship between the application’s object allocation rate and the duration of time it takes to perform a minor GC. In addition, the factors that contribute to the duration of a full GC will also be presented.

Once these relationships are well understood, a person tuning the JVM’s GC can make intelligent decisions about what tune since he or she understands the impact of a change to the JVM’s memory manager, including heap sizing and even a switch to a different garbage collector.

Charlie Hunt

Charlie Hunt is the Architect of Performance Engineering at and the lead author of the popular book, Java Performance. At, Charlie is responsible for improving the performance of’s infrastructure, mobile and UI along with the evolution of internal performance tools. Previously, Charlie was the JVM Performance Architect at Oracle where he was responsible for improving the performance of the Java HotSpot VM and Java SE class libraries. He wrote his first Java application in 1998 and joined Sun Microsystems, Inc. in 1999 as a Senior Java Architect. He has been working on improving the performance of Java and Java applications ever since. He is a regular speaker on the subject of Java performance at many world-wide conferences including JavaOne, QCon, Velocity and Dreamforce conferences.


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