Coping with Immutable Data in a JVM for Embedded Real-Time Systems. Christoph Erhardt, Simon Kuhnle, Isabella Stilkerich, Wolfgang Schröder-Preikschat

Transcription

1 The final Frontier Coping with Immutable Data in a JVM for Embedded Real-Time Systems Christoph Erhardt, Simon Kuhnle, Isabella Stilkerich, Wolfgang Schröder-Preikschat

2 Introduction Embedded devices Weak CPU Limited memory SRAM expensive, scarce Flash cheaper, more ample The final Frontier (JTRES 2014) Introduction 2

3 Introduction Embedded devices Weak CPU Limited memory SRAM expensive, scarce Flash cheaper, more ample Embedded programming in Java Productivity Safety The final Frontier (JTRES 2014) Introduction 2

4 Introduction MIN SPEED MAX Embedded devices Weak CPU Limited memory SRAM expensive, scarce Flash cheaper, more ample Embedded programming in Java Productivity Safety! Performance AOT compilation Dependent on effective optimisations The final Frontier (JTRES 2014) Introduction 2

5 Introduction MIN SPEED MAX Embedded devices Weak CPU Limited memory SRAM expensive, scarce Flash cheaper, more ample Embedded programming in Java Productivity Safety! Performance AOT compilation Dependent on effective optimisations! Memory footprint RAM usage in particular The final Frontier (JTRES 2014) Introduction 2

6 The Trouble with Java s final Qualifier Aids optimisations, but cannot always be declared explicitly Effectively final erhardt@cs.fau.de The final Frontier (JTRES 2014) Introduction 3

7 The Trouble with Java s final Qualifier Aids optimisations, but cannot always be declared explicitly Effectively final No way to mark pointees as constant/immutable public class ConstArray { public static final int[] ARRAY = {10, 2; erhardt@cs.fau.de The final Frontier (JTRES 2014) Introduction 3

8 The Trouble with Java s final Qualifier Aids optimisations, but cannot always be declared explicitly Effectively final No way to mark pointees as constant/immutable public class ConstArray { public static final int[] ARRAY = {10, 2; immutable erhardt@cs.fau.de The final Frontier (JTRES 2014) Introduction 3

9 The Trouble with Java s final Qualifier Aids optimisations, but cannot always be declared explicitly Effectively final No way to mark pointees as constant/immutable public class ConstArray { public static final int[] ARRAY = {10, 2; immutable heap-allocated erhardt@cs.fau.de The final Frontier (JTRES 2014) Introduction 3

10 The Trouble with Java s final Qualifier Aids optimisations, but cannot always be declared explicitly Effectively final No way to mark pointees as constant/immutable public class ConstArray { public static final int[] ARRAY = {10, 2; immutable heap-allocated No statically allocated + initialised arrays No programmatic flash allocation erhardt@cs.fau.de The final Frontier (JTRES 2014) Introduction 3

11 Remedy: Compiler Analyses

12 Platform: KESO JVM 1.2 Motivation GC/Heap Domain A Resources Alarms ISRs Portal Domain B Portal Tasks Domain C KESO Runtime Environment OSEK / AUTOSAR OS Mem-Mapped I/O Device Drivers Microcontroller Portable, scalable to low-end devices Figure 1.1: Schematic overview of a KESO system. An OSEK or AUTOSAR RTOS runs on a microcontroller. On top of the operating system, the KESO runtime environment provides services and abstractions used Staticbyconfiguration the application, such as RPC primitives or device drivers. Multiple protection realms (domains) can contain multiple tasks each, have their own resources, heap, and garbage collector and communicate Ahead-of-time safely using portals. compilation to C code erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 5 Due to the reduction of structure sizes in modern computing chips, dealing with

13 Effectively final Fields public final class Constants { public static int MAX_FRAMES = 1000; public class Main { private static void parsecmdline(final String[] v) { //... if (v[i].equals("max_frames")) Constants.MAX_FRAMES = Integer.parseInt(v[i + 1]); //... erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 6

14 Effectively final Fields public final class Constants { public static int MAX_FRAMES = 1000; public class Main { private static void parsecmdline(final String[] v) { //... if (v[i].equals("max_frames")) Constants.MAX_FRAMES = Integer.parseInt(v[i + 1]); //... unreachable erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 6

15 Effectively final Fields public final class Constants { public static int MAX_FRAMES = 1000; effectively final public class Main { private static void parsecmdline(final String[] v) { //... if (v[i].equals("max_frames")) Constants.MAX_FRAMES = Integer.parseInt(v[i + 1]); //... unreachable erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 6

16 Effectively final Fields public final class Constants { public static int MAX_FRAMES = 1000; effectively final public class Main { private static void parsecmdline(final String[] v) { //... if (v[i].equals("max_frames")) Constants.MAX_FRAMES = Integer.parseInt(v[i + 1]); //... unreachable Let s focus on static fields for now. erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 6

17 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { MAX_FRAMES = 1000; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

18 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { MAX_FRAMES = 1000; Criteria 1. Field is written exactly once, in the class constructor erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

19 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { if (...) { MAX_FRAMES = 1000; Criteria 1. Field is written exactly once, in the class constructor erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

20 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { System.out.println("MAX_FRAMES = " + MAX_FRAMES); MAX_FRAMES = 1000; Criteria 1. Field is written exactly once, in the class constructor 2. No read prior to initialisation Within class constructor Within method called from class constructor erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

21 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { MAX_FRAMES = 1000; Criteria 1. Field is written exactly once, in the class constructor 2. No read prior to initialisation Within class constructor Within method called from class constructor erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

22 Finding Effectively final Fields public final class Constants { public static int MAX_FRAMES; static { MAX_FRAMES = 1000; Criteria 1. Field is written exactly once, in the class constructor 2. No read prior to initialisation Within class constructor Within method called from class constructor Effect on optimisations Constant folding, check elision Potentially many indirect effects! erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 7

23 Finding Constant Arrays public class ConstArray { public static final int[] ARRAY = {10, 2; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 8

24 Finding Constant Arrays public class ConstArray { public static final int[] ARRAY; static { int[] a = new int[2]; a[0] = 10; a[1] = 2; ARRAY = a; Criteria 1. Array created with constant size, in class constructor erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 8

25 Finding Constant Arrays public class ConstArray { public static final int[] ARRAY; static { int[] a = new int[2]; a[0] = 10; a[1] = 2; ARRAY = a; Criteria 1. Array created with constant size, in class constructor 2. All writes are to constant indices, with constant values, not more than once per index erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 8

26 Finding Constant Arrays public class ConstArray { public static final int[] ARRAY; static { int[] a = new int[2]; a[0] = 10; a[1] = 2; ARRAY = a; Criteria 1. Array created with constant size, in class constructor 2. All writes are to constant indices, with constant values, not more than once per index 3. No reads prior to initialisation Consider aliasing! erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 8

27 Finding Constant Arrays public class ConstArray { public static final int[] ARRAY; static { int[] a = new int[2]; a[0] = 10; a[1] = 2; ARRAY = a; Criteria 1. Array created with constant size, in class constructor 2. All writes are to constant indices, with constant values, not more than once per index 3. No reads prior to initialisation Consider aliasing! Multi-dimensional arrays: bottom-up approach erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 8

28 Static Allocation of Constant Arrays Java source: public class ConstArray { public static final int[] ARRAY = {10, 2; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 9

29 Static Allocation of Constant Arrays Java source: public class ConstArray { public static final int[] ARRAY = {10, 2; Emitted C code: typedef struct { uint16_t classid; uint32_t length; const int32_t data[2]; int_array2_t; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 9

30 Static Allocation of Constant Arrays Java source: public class ConstArray { public static final int[] ARRAY = {10, 2; Emitted C code: typedef struct { uint16_t classid; uint32_t length; const int32_t data[2]; int_array2_t; const int_array2_t ca = { /*.classid = */ INT_ARRAY_ID, /*.length = */ 2, /*.data = */ {10, 2, ; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 9

31 Static Allocation of Constant Arrays Java source: public class ConstArray { public static final int[] ARRAY = {10, 2; Emitted C code: typedef struct { uint16_t classid; uint32_t length; const int32_t data[2]; int_array2_t; const int_array2_t ca = { /*.classid = */ INT_ARRAY_ID, /*.length = */ 2, /*.data = */ {10, 2, ; void ConstArray clinit_(void) { ConstArray_ARRAY = &ca; erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 9

32 Making Use of Flash Memory Constant arrays Declare as const in emitted C code Emit section attribute Adapt linker script erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 10

33 Making Use of Flash Memory Constant arrays Declare as const in emitted C code Emit section attribute Adapt linker script Other candidates for flash allocation Strings from constant pools Runtime-system data structures Type-information store Dispatch table erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 10

34 Flash-Memory Pitfalls Mark-and-sweep GC! Shouldn t try to flip colour bit in flash-allocated object header Don t scan flash-allocated objects erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 11

35 Flash-Memory Pitfalls Mark-and-sweep GC! Shouldn t try to flip colour bit in flash-allocated object header Don t scan flash-allocated objects AVR! Separate access instructions for RAM and flash (ld vs. lpm) For each use: determine correct instruction through alias analysis Prevent aliasing between RAM and flash objects erhardt@cs.fau.de The final Frontier (JTRES 2014) Remedy: Compiler Analyses 11

36 Evaluation

37 Collision Detector CD j 1.2 Real-time air-traffic Variant simulator text data and collision detector CiAO OS Table 1 Baseline final analysis constant TriCore 150 MHz, 2 MiB flash, 1 MiB SRAM strings text data Baseline + final analysis + constant strings erhardt@cs.fau.de The final Frontier (JTRES 2014) Evaluation 13

38 Collision Detector Baseline With effectively-final analysis Relative execution time Iteration FoldedFigure primitive 4.1: Relative constants execution time of the CD x on-the-go with runtime final analysis. Execution time improved by 5 to 12%. Singleton objects 30 % fewer null-checks erhardt@cs.fau.de The final Frontier (JTRES 2014) Evaluation 14

39 SPiCboardTest Test application Evaluation board for teaching JOSEK OS Variant Flash RAM Baseline AVR ATmega32 + 1arrays MHz, KiB + flash allocation flash, 2 KiB SRAM Flash RAM Baseline + constant arrays + flash allocation erhardt@cs.fau.de The final Frontier (JTRES 2014) Evaluation 15

40 Conclusion Summary Effectively-final analysis Can greatly support optimisations Static initialisation + flash allocation of arrays Improves startup times Conserves precious RAM erhardt@cs.fau.de The final Frontier (JTRES 2014) Conclusion 16

41 Conclusion Summary Effectively-final analysis Can greatly support optimisations Static initialisation + flash allocation of arrays Improves startup times Conserves precious RAM Outlook Permit programmer intervention through annotations Cross-check against code to detect contradictions Exploit knowledge about target platform (e.g. memory map) erhardt@cs.fau.de The final Frontier (JTRES 2014) Conclusion 16