Transcript Real-Time Java Memory Model - People

Memory Management for Real-Time Java
Hierarchical Real-Time
Java Computation
Hierarchical
Scoped Memories
Objects
For Aircraft
Management
Computation
Aircraft
Management
Computation
Objects For Sensor
Data Processing
Sensor Data
Processing
Avionics
Control
Time
Scoped Memory
For Aircraft
Management
Computation
Objects For
Avionics Control
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Scoped Memory
For Sensor
Data Processing
Scoped Memory
For Avionics
Control
Impact
• Java has safe memory model
• No memory corruption, no memory leaks
• Implemented with garbage collection
• Unsuitable for Real-Time Java – potentially unbounded pauses
when garbage collector scans memory
• Scoped Memory Eliminates Pauses, Improves Performance,
Provides Safe Memory for Real-Time Systems
• Program Analysis Ensures Scoped Memory Used Correctly
• Guarantees NO run-time exceptions caused by incorrect use
• Guarantees NO memory corruption
• Removes dynamic checks, eliminates check overhead
Basic Idea
• Each computation has its own scoped memory
• Scoped memory holds objects for its computation
• When computation terminates, memory reclaimed
• Scoped Memory Hierarchy Matches Computation Hierarchy
• References legal within same scope or from inside
scope to outside scope
• References illegal between scopes at same level or
from outside scope to inside scope
• Violation? POSSIBLE MEMORY CORRUPTION!
• Current Real-Time Java: dynamically checks for
violations, throws exception if violation occurs
• Our solution:
• Statically analyze program, understand referencing
• Check that violations CAN NEVER OCCUR!
• No dynamic checks, no exceptions, no corruption
Accomplishments and Status
• Real-Time Java implementation in MIT FLEX compiler underway
• Initial Scoped Memory Implementation Exists
• Separate memory region for each scoped memory
• Provides allocation time (QOS) guarantees
• Code Freeze Late April
• Initial Analysis Implemented
• Uses pointer and escape analysis
• All or nothing answer
• Tested on several simple benchmark programs
• More sophisticated analyses under development
Martin Rinard, Daniel Jackson, MIT Laboratory for Computer Science