Enhancing the region model of RTSJ
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This document proposes a new type of region called AGCMemory for the Real-Time Specification for Java (RTSJ) to improve portability while maintaining predictability. AGCMemory reduces the "floating garbage" problem seen in RTSJ regions by automatically recycling objects created during method invocations. It uses constant-time barriers and internal data structures to track object allocations and support partial deallocation in a way that is more intuitive than nested scopes. The authors evaluate AGCMemory and discuss implementing it in an RTSJ virtual machine and evaluating its performance trade-offs.
![Motivation: PeriodicCounter
1. import javax.realtime.*
2. public class PeriodicCounter extends RealtimeThread{
3. public PeriodicConter(){
4. super( null,//Schedulling par
5. new PeriodicParameters(null,
6. new RelativeTime(1000,0), //T
7. new RelativeTime(50,0), //C
8. new RelativeTime(100,0), //D
9. null,null /*Handlers*/);
10. null, //Memory parameters
11. new LTMemory(100,100), //Mem. param
12. null);
13. start(); //starts thread.
14. }
15. int counter=1;
16. public void run(){
17. do{
18. System.out.println(counter); // Allocates 88 bytes in
RTSJ-RI
19. counter++
20. }while(waitForNextPeriod());
21. }
22. public static void main(String s[]){
23. new HelloPeriodicCounter();
24. } 7/25](https://image.slidesharecdn.com/basanta-enhancing-falseii-130123022312-phpapp01/85/Enhancing-the-region-model-of-RTSJ-7-320.jpg)
![Solving the problem with nested scopes
1. import javax.realtime.*
2. public class PeriodicCounter extends RealtimeThread{
3. public PeriodicCounter(){
4. //UnChanged}
5.
6. int counter=1;
7. public void run(){
8. LTMemory lt=new LTMemory(150,150); //Nested scope
9. Runnable impr=new Runnable(){
10. public void run(){
11. System.out.println(counter);};
12.
13. do{ lt.enter(impr); counter++;
14. }while(waitForNextPeriod());
15. }
16. public static void main(String s[]){
17. new HelloPeriodicCounter();
18. }
19. }
9/25](https://image.slidesharecdn.com/basanta-enhancing-falseii-130123022312-phpapp01/85/Enhancing-the-region-model-of-RTSJ-9-320.jpg)
![AGCMemory
1. import javax.realtime.*
2. public class PeriodicCounter extends RealtimeThread{
3. public PeriodicConter(){
4. super( null,//Schedulling par
5. new PeriodicParameters(null,
6. new RelativeTime(1000,0), //T
7. new RelativeTime(50,0), //C
8. new RelativeTime(100,0), //D
9. null,null /*Handlers*/);
10. null, //Memory parameters
11. new AGCMemory(250,250), //Mem. param
12. null);
13. start(); //starts thread.
14. }
15. int counter=1;
16. public void run(){
17. do{
18. System.out.println(counter); counter++
19. }while(waitForNextPeriod());
20. }
21. public static void main(String s[]){
22. new HelloPeriodicCounter();
23. }
24. } 12/25](https://image.slidesharecdn.com/basanta-enhancing-falseii-130123022312-phpapp01/85/Enhancing-the-region-model-of-RTSJ-12-320.jpg)
![Runtime Barriers
Barrier Pseudo-code Functionality
pre_invocation Agc.stack.push(memPtr, memPtr) It identifies the objects
top→scape_ptr = free_mem_ptr; that will be created in the
(before executing a invocation of a method
method) top→method_ptr = free_mem_ptr;
post_invocation If(top→scape_ptr ≥ top→method_ptr ) It destroys floating
Free objects [top→method_ptr, free_prtr] objects or propagates
(after executing a their destruction
else (top_1→scape_ptr=top→scape_ptr)
method)
Agc.stack.pop()
Intra_asig. Attrib=ref It detects whether the
if (memArea(attrib)==memArea(ref)) objects of a method may
(before executing an be destroyed when the
&& (memArea(attrib) instance of AGCMemory)
assigment) methods ends
&& (ref≥attrib)
top→scape_ptr=min{top→scape_ptr, attrib}
19/25](https://image.slidesharecdn.com/basanta-enhancing-falseii-130123022312-phpapp01/85/Enhancing-the-region-model-of-RTSJ-19-320.jpg)
![Example revisited (and III)
free_mem_ptr (0)
0 250 free bytes
0
agc_stack
top
Post-invocation barrier (executed after System.out.println())
If(top→scape_ptr ≥ top→method_ptr )
Free objects [top→method_ptr, free_prtr]
Agc.stack.pop()
22/25](https://image.slidesharecdn.com/basanta-enhancing-falseii-130123022312-phpapp01/85/Enhancing-the-region-model-of-RTSJ-22-320.jpg)
Enhancing the region model of RTSJ
- 1. Enhancing the region model of real-time Java for large-scale systems Pablo Basanta-Val Marisol García-Valls Iria Estévez-Ayres Universidad Carlos III de Madrid Telematics Engineering Department
- 2. Outline Introduction Recycling Floating Garbage with AGCMemory Extending RTSJ Hierarchy Supporting AGCMemory Conclusions and future work 2/25
- 3. Introduction (I) Good properties of Java make it suitable for large-scale systems: Portability Automatic memory management (garbage collector) Simplicity Networking support… When we introduce real-time constraints problems with automatic memory management Garbage collector introduces unpredictable breaks in program execution deadlines may be missed 3/25
- 4. Introduction (II) Possible solutions: Real-time garbage collector techniques Regions (e.g in RTSJ Scope Memory) Neither is perfect Real-time garbage collectors maintain the Java programming style but they are not easy to tune Regions provide predictability but they modify the programming style 4/25
- 5. Introduction (and III) Our approach is a new type of region for RTSJ: AGCMemory It tries to improve the portability of regions while keeping some of the advantages of GC-techniques It maintains the Java programming style It provides the predictability of regions It reduces the number of manual changes More portable applications It enhances the reuse of code and it decreases the development time 5/25
- 6. Recycling Floating Garbage with AGCMemory Floating garbage Using nested scopes to eliminate floating garbage Using AGCMemory to eliminate floating garbage
- 7. Motivation: PeriodicCounter 1. import javax.realtime.* 2. public class PeriodicCounter extends RealtimeThread{ 3. public PeriodicConter(){ 4. super( null,//Schedulling par 5. new PeriodicParameters(null, 6. new RelativeTime(1000,0), //T 7. new RelativeTime(50,0), //C 8. new RelativeTime(100,0), //D 9. null,null /*Handlers*/); 10. null, //Memory parameters 11. new LTMemory(100,100), //Mem. param 12. null); 13. start(); //starts thread. 14. } 15. int counter=1; 16. public void run(){ 17. do{ 18. System.out.println(counter); // Allocates 88 bytes in RTSJ-RI 19. counter++ 20. }while(waitForNextPeriod()); 21. } 22. public static void main(String s[]){ 23. new HelloPeriodicCounter(); 24. } 7/25
- 8. Motivation (II) 250 free bytes 11. new LTMemory(300,300) 162 free 18. System.out.println(counter) bytes 74 free 18. System.out .println(counter) bytes -8 free 18. System.out .println(counter) bytes OutOfMemory Exception ./RTSJ-RI PeriodicCounter 1 2 3Mem-registry.c::InvokeNewInstance()- Went out of memory 8/25
- 9. Solving the problem with nested scopes 1. import javax.realtime.* 2. public class PeriodicCounter extends RealtimeThread{ 3. public PeriodicCounter(){ 4. //UnChanged} 5. 6. int counter=1; 7. public void run(){ 8. LTMemory lt=new LTMemory(150,150); //Nested scope 9. Runnable impr=new Runnable(){ 10. public void run(){ 11. System.out.println(counter);}; 12. 13. do{ lt.enter(impr); counter++; 14. }while(waitForNextPeriod()); 15. } 16. public static void main(String s[]){ 17. new HelloPeriodicCounter(); 18. } 19. } 9/25
- 10. Solving the problem with nested scopes (II) 250 bytes memoria libre new LTMemory(300,300) 220 bytes 9. lt=new LT(150,150) 150 b. impr 10. 10. Runnable impr libres 13. lt.enter(impr); 150 bytes //Using nested scope libres 62 bytes //Executes impr.run libres 150 bytes //Release nested scope libres 150 b. impr 220 bytes 13. counter++; libres 10/25
- 11. Nested Scopes Problems The programmer has to set up: Number of auxiliary scopes Maximum size of each auxiliary scope Other problems: Low portability High dependency on available classes Not especially intuitive (runnable objects) Runnable classes required Our solution: AGCMemory 11/25
- 12. AGCMemory 1. import javax.realtime.* 2. public class PeriodicCounter extends RealtimeThread{ 3. public PeriodicConter(){ 4. super( null,//Schedulling par 5. new PeriodicParameters(null, 6. new RelativeTime(1000,0), //T 7. new RelativeTime(50,0), //C 8. new RelativeTime(100,0), //D 9. null,null /*Handlers*/); 10. null, //Memory parameters 11. new AGCMemory(250,250), //Mem. param 12. null); 13. start(); //starts thread. 14. } 15. int counter=1; 16. public void run(){ 17. do{ 18. System.out.println(counter); counter++ 19. }while(waitForNextPeriod()); 20. } 21. public static void main(String s[]){ 22. new HelloPeriodicCounter(); 23. } 24. } 12/25
- 13. Using AGCMemory 250 free bytes 11. new AGCMemory(300,300) 18. System.out.println(counter) 250 free bytes //preinvocation 162 free bytes //printing the counter 250 free bytes //memory releasing 250 free bytes 18. counter++ 13/25
- 15. AGCMemory in the RTSJ class Hierarchy ImmortalMemory Memory Area HeapMemory ImmortalPhysicalMemory Scoped Memory VTMemory LTMemory AGCMemory LTPhysicalMemory VTPhysicalMemory 15/25
- 16. LTMemory VTMemory AGCMemory comparison LTMemory VTMemory AGCMemory Allocation time Linear Not bounded Linear De-allocation after enter Yes Yes Yes method Partial de-allocation Not Yes Yes supported (when objects are (after method allocated) invocation) Predictable partial de- A priori, not Yes allocation bounded Shared Yes Yes No 16/25
- 17. Supporting AGCMemory Structures Runtime barriers Example revisited
- 18. AGCMemory Internal Structures 0xAC00 method_ptr scape_ptr top top-1 . top-2 free_mem_ptr . . 0 agc_stack 0x0C00 physical memory Chunk of physical memory: Objects are allocated in ascending memory addresses free_mem_ptr: position where the next object will be allocated AGCstack (used in memory management algorithm) scape_ptr: decides whether the objects created during the invocation of a method maybe recycled or not. method_ptr: stores information about the set of objects created during the invocation of a method 18/25
- 19. Runtime Barriers Barrier Pseudo-code Functionality pre_invocation Agc.stack.push(memPtr, memPtr) It identifies the objects top→scape_ptr = free_mem_ptr; that will be created in the (before executing a invocation of a method method) top→method_ptr = free_mem_ptr; post_invocation If(top→scape_ptr ≥ top→method_ptr ) It destroys floating Free objects [top→method_ptr, free_prtr] objects or propagates (after executing a their destruction else (top_1→scape_ptr=top→scape_ptr) method) Agc.stack.pop() Intra_asig. Attrib=ref It detects whether the if (memArea(attrib)==memArea(ref)) objects of a method may (before executing an be destroyed when the && (memArea(attrib) instance of AGCMemory) assigment) methods ends && (ref≥attrib) top→scape_ptr=min{top→scape_ptr, attrib} 19/25
- 20. Example revisited (I) free_mem_ptr (0) method_ptr 0 0 250 free bytes scape_ptr agc_stack top Pre-invocation barrier (executed before System.out.println()) Agc.stack.push(memPtr, memPtr) top→scape_ptr = free_mem_ptr; top→method_ptr = free_mem_ptr; 20/25
- 21. Example revisited (II) free_mem_ptr (88) method_ptr 0 162 free bytes 0 scape_ptr agc_stack top Object creation inside method System.out.println 21/25
- 22. Example revisited (and III) free_mem_ptr (0) 0 250 free bytes 0 agc_stack top Post-invocation barrier (executed after System.out.println()) If(top→scape_ptr ≥ top→method_ptr ) Free objects [top→method_ptr, free_prtr] Agc.stack.pop() 22/25
- 23. Conclusions Existing types of regions hard to use Floating garbage problem AGCMemory: It reduces floating garbage problem It improves the automatic memory management Easier to use than nested scopes It uses constant time barriers 23/25
- 24. Future work Implementation of AGCMemory in a RTSJ open source virtual machine We are using JRate Evaluation of the following trade-offs: Influence of the assignment AGCMemory barrier in the performance of applications Analysis of required memory for different types of applications 24/25
- 25. Enhancing the region model of real-time Java for large-scale systems Thanks 25/25