1. Multitasking without Compromise: a Virtual Machine Evolution
Grzegorz Czajkowski Laurent Daynes
Sun Microsystems Laboratories
Presented by: Subodh J. Lele
12/02/2004
CS Fall 2004

2. Introduction - Java VM Used by applications, applets Appears as an OS
Ad hoc layer
JVM
Base OS
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CS Fall 2004

3. Problems Interference between two apps Examples All are undesired!
Monopoly over heap space
Leaky object references
Sharing event and finalization queues
Internalized strings
All are undesired!
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CS Fall 2004

4. Internalized Strings Avoids duplicates String a = “abc”;
String b = a.intern();
Need to use
synchronized (a) { … }
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CS Fall 2004

5. Solutions Separate VMs Multitasking VM (MVM) Each VM as a OS process
Multitasking version of JVM
OS-style multitasking
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CS Fall 2004

6. Separate VMs Advantages Disadvantages Fault isolation
Higher Programmer productivity
Disadvantages
Inefficient resource utilization
Downgraded performance
Poor scalability
Poor application startup time
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CS Fall 2004

7. MVM Applications are protected from each other Multitasking features
TASKS
Multitasking features
Safe
Secure
Scalable
NO COMPROMISE approach
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CS Fall 2004

8. MVM Goals Just like those of an OS No interference JVM illusion
Perform and scale well
Just like those of an OS
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CS Fall 2004

9. Design decisions Examine each component Changes to JVM
Shareable
Non-Shareable… replicate
Changes to JVM
No bytecode editing
Separate task data sets
End task at any time
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CS Fall 2004

10. MVM Design Lightweight Isolation Memory Management
User-Level Native Code
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11. Lightweight Isolation
Share as much as possible
Replicate everything else
Issues
Class Initialization
Accessing appropriate copy
Efficient retrieval
Making it scalable
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CS Fall 2004

12. In MVM
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13. Class Initialization Different from object initialization
Done by <clinit>
class ClassInitialization {
static int first = 3;    static int second = 1 + first;    public static void main (String [] args)  {
X.y++;   } }
class X {
static {
System.out.println("Hello World"); }
static int y;
0   iconst_3
1   putstatic ClassInitialization/first I
4   iconst_1
5   getstatic ClassInitialization/first I
8   iadd
9   putstatic ClassInitialization/secondI
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CS Fall 2004

14. Loading and Linking Loading Linking Create main memory representation
Create TCM
Add static variables
Mark “loaded”
Linking
Verify class
Check loader constraints
Build constant pool cache
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CS Fall 2004

15. Dynamic code rewriting
Class initialization barrier
Check for entry in TCM table
Once per task
Necessitates two entries
Link resolution barrier
Unchanged from JVM
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CS Fall 2004

16. Other issues Quickened bytecodes Sharing Complied Code
Re-introduce barriers
Affects only four methods
Sharing Complied Code
Native code for method
Patching stubs
Class initialization barriers
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CS Fall 2004

17. Performance – Memory footprint
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CS Fall 2004

18. Performance – Time overheads
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CS Fall 2004

19. Memory Management Heap Memory Mechanisms Surplus memory management
Memory accountability
Per-task Garbage Collections
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CS Fall 2004

20. In HSVM New Generation Old Generation Eden Aging
For new objects
Aging
From-space
Objects garbage collected (GC)
To-space
Old Generation
Collection triggered when filled up
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CS Fall 2004

21. In MVM Old generation shared New generation common
More activities occur here
Eliminates heap-related interference
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CS Fall 2004

22. Surplus Memory Management
Postpone or avoid GC
Given to new generation
Handled by New Space Manager (NSM)
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CS Fall 2004

23. Memory Accounting
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CS Fall 2004

24. Performance – Memory Management
Overheads
Replicated new generation
Memory accounting
Extending eden and
book-keeping
Conclusion
Execution time benefits from larger eden
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CS Fall 2004

25. Handling Native Code Isolation layer Native library replicated
Redefines functions to include arguments
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26. My evaluation - pros MVM is a complete system
Appears just like a JVM for apps
Provides excellent multitasking features
Safe
Secure
Scalable
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CS Fall 2004

27. My evaluation - cons One app can crash the VM
When interaction is required, costly local RPC needed
GC forced by one task can interfere with another task’s performance
It does compromise!
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CS Fall 2004

28. Questions?
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CS Fall 2004