Talk Proposal: Class Metadata: A User Guide

Andrew Dinn adinn at
Fri Dec 1 15:11:47 CET 2017

Class Metadata: A User Guide

The OpenJDK JVM eats class bytecode and spits out Java heap objects and
machine code as part of normal execution. However, the JVM manages more
data than just bytecode, Java objects  JITted code. In order to manage
heap memory and to ensure correct execution, the JVM and JITted code
need to retain and, occasionally, interrogate details of the loaded
class base. Bytecode is not a very 'efficient' representation for this
purpose, nor does it include details of runtime-derived info like class
and method implementation dependencies, compile history and JITted code
status, or execution profiles.

So, the JVM throws away most bytecode after parsing, in its place
constructing and maintaining its own 'efficient' in-memory (C++) data
structures which model both the loaded class and method base and
runtime-derived state. This is what is known as Class Metadata. Class
Metadata is a significant component of the Native Memory storage
allocated and managed by the JVM, alongside JIT Compiler Metadata, GC
Metadata, Thread Metadata, etc. For small applications that don't create
a large number of Java objects Metadata can constitute a large portion
of a Java application's resident memory set.

This talk will begin by describing how to use the jcmd tool to measure
summary JVM Native Memory storage costs, including overall Class
Metadata costs. It will also show how to use jcmd to obtain a detailed
breakdown of the latter costs, splitting the overheads along two axes:
firstly, by class; and secondly, dividing per-class costs into separate
sub-costs associated with different component subsets of the class
model: classes per se, constant pools, methods and annotations.

I will go on to explain how Class Metadata is carved out of the memory
regions managed by the JVM's Native Memory allocation routines and
detail the memory layout of the various C++ types which define the
elements of the Class Metadata model, thereby clarifying some of the
more detailed statistics available in jcmd output. Real-life use cases
will be employed to explain how specific costs arise from code design
choices and to suggest how alternative choices might increase or
decrease Class Metadata costs.

Andrew Dinn
OpenJDK developer and Byteman Project Lead working at Red Hat

45 min preferred

Recording video and/or audio CC-BY-2.0 license is acceptable.


Andrew Dinn
Senior Principal Software Engineer
Red Hat UK Ltd
Registered in England and Wales under Company Registration No. 03798903
Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander

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