1. (Not too) Real-Time JVM (Progress Report)
Dec. 28, 2001
Jong-Koo,Lim
RealTimeSystems Lab

2. Contents Goal Related Works Java Virtual Machine Real-Time Java
System Architecture
Inside Java Virtual Machine
Problems with typical JVM
Real-Time Java
Seven Enhanced Areas

3. Goal Design and Implementation of
Real-Time Extensions in Java Virtual Machine

4. Related Works Java Virtual Machine Real-Time Java
Current JVM : Kaffe, Hotspot, CVM, KVM
Platform : Real-Time Linux
Real-Time Java
Java is not appropriate for real-time software
It is possible to support real-time software if java has any special implementation
Seven Enhanced Areas

5. Java Virtual Machine System Architecture Inside Java Virtual Machine
Problems with typical JVMs

6. System Architecture Class Libraries Java Threads Native Threads
Java Virtual Machine
Native Methods
(Real-Time) Operating System
Processor and Other Hardware

7. Inside a Java Virtual Machine
Find and load referenced class
in the CLASSPATH
Java Application
ClassFile {
u4 magic;
u2 minor_version;
u2 major_version;
u2 constant_pool_count
cp_info constant_pool[ ];
u2 access_flags;
u2 this_class;
u2 super_class;
u2 interfaces_count;
u2 fields_count;
u2 methods_count;
u2 attributes_count; }
Class Libraries
Class Loader
Execution Engine performs the final translation
- Bytecode to native opcode
Difference engine types
- Bytecode interpreter
- Just-in-time compiler
- Hybrid
Bytecode Verifier
Native
Methods
Execution Engine
Run-time
Data
Area
Garbage
Collector
Method
Area
heap
Java Stacks PC
Registers
Native Method Stacks
Run-time Data Areas
Simplest Garbage Collector
- Incremental Garbage Collector
(Real-Time) Operating System 7
RealTimeSystems Lab

8. Garbage Collection Two basic sequence Two basic approach
Detect garbage objects
Reclaim the heap space
Two basic approach
Reference-counting garbage collector
a reference count is maintained for each object
Tracing garbage collector
trace out the graph of object references, starting with the root nodes.
class A {
….. ; }
A a1 = new A();
A a2 = new A();
A a3 = a1; // a1’s rc = 2
A a4 = a1; // a1’s rc = 3
a2 = null; /* a2’s rc =0, so, a2 is garbage */

9. Inside a Java Class Files
- Class File Header
CAFEBABE magic = ca fe ba be
minor version = 3
d major version = 45
Constant Pool
constants
00000a 0A006600AB
1. Methodref class #102 , name-and-type #171
00000f AC
2. Fieldref class #101, name-and-type #172
(the following is omitted)
Constant Pool Tag
Class index
Constant Pool Entry

10. Oolong (Assembly Language for JVM)
DumpClass : => Display and Analyze ‘class file’ in a bytecode unit
Gnoloo : *.class => *.j
Oolong : *.j => *.class

11. Problems with typical JVM
Too Big
A full-featured JVM is huge(~500k of ROM)
In addition there are class libraries, heap, etc
Too Slow
Bytecode interpretation
Run-time checks
Too Unpredictable
Default garbage collectors are not very good in RTS

12. Real-Time Java (next work)
Seven Enhanced Areas
Thread Scheduling and Dispatching
Memory Management (Garbage Collector)
Synchronization and Resource Sharing
Asynchronous Event Handling
Asynchronous Transfer of Control
Asynchronous Thread Termination
Physical Memory Access