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Subsystems: Improved exception handling for Java (DRAFT)

Subsystems: Improved exception handling for Java (DRAFT). Bart Jacobs, Frank Piessens. Outline. Try-Catch: Problem + Solution Locking: Problem + Solution Cancellation: Problem + Solution Cleaning Up: Problem + Solution. Outline. Try-Catch: Problem + Solution Locking: Problem + Solution

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Subsystems: Improved exception handling for Java (DRAFT)

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  1. Subsystems: Improved exception handling for Java (DRAFT) Bart Jacobs, Frank Piessens

  2. Outline • Try-Catch: Problem + Solution • Locking: Problem + Solution • Cancellation: Problem + Solution • Cleaning Up: Problem + Solution

  3. Outline • Try-Catch: Problem + Solution • Locking: Problem + Solution • Cancellation: Problem + Solution • Cleaning Up: Problem + Solution

  4. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:A Typical Try-Catch Pattern This program is broken! Why?

  5. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:An OutOfMemoryError… m.count == 11 m.widgets.length == 10

  6. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:An OutOfMemoryError… 3 4 1 2 m.count == 11 m.widgets.length == 10

  7. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:An OutOfMemoryError… 3 4 1 2 5 m.count == 12 m.widgets.length == 10

  8. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:A Typical Try-Catch Pattern

  9. class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } void removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } Proposed Solution:Re-entering the Owner Subsystem child subsystem root subsystem

  10. class Program { public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } } … } class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } void removeWidget(Widget w) { … } } Proposed Solution:Re-entering an Outer Subsystem

  11. Subsystems: Basic Operation class Program { public static void main(String[] args) { Subsystem s = Subsystem.getCurrent(); try { reenter (s) { throw new RuntimeException(); } } catch (Throwable t) { System.out.println(t + “ was caught.”); } } } Not caught

  12. Subsystems: Basic Operation class Program { public static void main(String[] args) { try { Subsystem s = Subsystem.getCurrent(); try { reenter (s) { throw new RuntimeException(); } } catch (Throwable t) { System.out.println(t + “ caught by inner.”); } } catch (Throwable t) { System.out.println(t + “ caught by outer.”); } } } Caught by outer

  13. class Subsystem { static Stack<Subsystem> stack = new Stack<Subsystem>(); static { stack.push(new Subsystem(); } Throwable exception; Subsystem parent; List<Subsystem> children = new ArrayList<Subsystem>(); static Subsystem getCurrent() { return stack.peek(); } static void enterNew() { Subsystem s = new Subsystem(); s.parent = getCurrent(); getCurrent().children.add(s); stack.push(s); } static void reenter(Subsystem s) { s.checkNotFailed(); stack.push(s); } static void exit(Throwable e) { if (e != null) getCurrent().setFailed(e); stack.pop(); getCurrent().checkNotFailed(); } void setFailed(Throwable e) { exception = e; for (Subsystemc : children)c.setFailed(e); } void checkNotFailed() { if (exception != null) throw exception; } static void finish(Throwable e) { getCurrent().parent.children.remove(getCurrent()); exit(e); } } Expansion [[try { S } catch (Throwable e) { S’ } ]] = Subsystem.enterNew(); Throwable t = null; try { S } catch (Throwable e) { t = e; } Subsystem.finish(t); if (t != null) { Throwable e = t; S’ } Expansion [[reenter (s) { S } ]] = Subsystem.reenter(s); Throwable t = null; try { S } catch (Throwable e) { t = e; } Subsystem.exit(t); if (t != null) { throw t; } Subsystems: Implementation

  14. Outline • Try-Catch: Problem + Solution • Locking: Problem + Solution • Cancellation: Problem + Solution • Cleaning Up: Problem + Solution

  15. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } synchronizedvoid removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:A Typical Locking Pattern This program is broken! Why?

  16. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } synchronizedvoid removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:An OutOfMemoryError… In thread 1 m.count == 11 m.widgets.length == 10

  17. class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } synchronizedvoid removeWidget(Widget w) { … } } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:An OutOfMemoryError… In thread 1 In thread 2 m.count == 12 m.widgets.length == 10

  18. class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Subsystems To The Rescue

  19. class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Subsystems To The Rescue In thread 1

  20. class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Subsystems To The Rescue In thread 2 In thread 1

  21. class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); new Thread() { public void run() { try { … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Subsystems To The Rescue In main thread In thread 2 In thread 1

  22. Subsystems • When an exception occurs in a subsystem s • For safety: New attempts to enter s (or a descendant) rethrow the exception, and • For liveness: Computations executing in s (or a descendant) in other threads are stopped • I.e.: In each thread that is executing in s (or a descendant), an exception is thrown (using Thread.stop()), which is caught when leaving s

  23. Outline • Try-Catch: Problem + Solution • Locking: Problem + Solution • Cancellation: Problem + Solution • Cleaning Up: Problem + Solution

  24. class Task { boolean cancel; Thread thread; synchronized void cancel() { cancel = true; if (thread != null) thread.stop(); } synchronized void setThread(Thread t) { thread = t; if (cancel) throw new ThreadDeath(); } } class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); List<Task> tasks = new ArrayList<Task>(); while (true) { String cmd = getUserCommand(); if (cmd.equals(“cancelAll”)) { for (Task t : tasks) t.cancel(); continue; } final Task task = new Task(); tasks.add(task); new Thread() { public void run() { try { task.setThread(Thread.currentThread()); … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:A Typical Cancellation Pattern This program is broken! Why?

  25. class Task { boolean cancel; Thread thread; synchronized void cancel() { cancel = true; if (thread != null) thread.stop(); } synchronized void setThread(Thread t) { thread = t; if (cancel) throw new ThreadDeath(); } } class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); List<Task> tasks = new ArrayList<Task>(); while (true) { String cmd = getUserCommand(); if (cmd.equals(“cancelAll”)) { for (Task t : tasks) t.cancel(); continue; } final Task task = new Task(); tasks.add(task); new Thread() { public void run() { try { task.setThread(Thread.currentThread()); … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:A ThreadDeath… ThreadDeath in thread 1 m.count == 11 m.widgets.length == 10

  26. class Task { boolean cancel; Thread thread; synchronized void cancel() { cancel = true; if (thread != null) thread.stop(); } synchronized void setThread(Thread t) { thread = t; if (cancel) throw new ThreadDeath(); } } class Widget { … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); List<Task> tasks = new ArrayList<Task>(); while (true) { String cmd = getUserCommand(); if (cmd.equals(“cancelAll”)) { for (Task t : tasks) t.cancel(); continue; } final Task task = new Task(); tasks.add(task); new Thread() { public void run() { try { task.setThread(Thread.currentThread()); … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Problem Statement:A ThreadDeath… ThreadDeath in thread 1 In thread 2 m.count == 12 m.widgets.length == 10

  27. class Task { boolean cancel; Subsystem subsystem; synchronized void cancel() { cancel = true; if (subsystem != null) subsystem.cancel(); } synchronized void setSubsystem(Subsystem s) { subsystem = s; if (cancel) throw new ThreadDeath(); } } class Widget { … } class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); List<Task> tasks = new ArrayList<Task>(); while (true) { String cmd = getUserCommand(); if (cmd.equals(“cancelAll”)) { for (Task t : tasks) t.cancel(); continue; } final Task task = new Task(); tasks.add(task); new Thread() { public void run() { try { task.setSubsystem(Subsystem.getCurrent()); … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Proposed Solution:Subsystem Cancellation

  28. class Task { boolean cancel; Subsystem subsystem; synchronized void cancel() { cancel = true; if (subsystem != null) subsystem.cancel(); } synchronized void setSubsystem(Subsystem s) { subsystem = s; if (cancel) throw new ThreadDeath(); } } class Widget { … } class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; synchronized Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } … } class Program { public static void main(String[] args) { final WidgetManager m = new WidgetManager(); List<Task> tasks = new ArrayList<Task>(); while (true) { String cmd = getUserCommand(); if (cmd.equals(“cancelAll”)) { for (Task t : tasks) t.cancel(); continue; } final Task task = new Task(); tasks.add(task); new Thread() { public void run() { try { task.setSubsystem(Subsystem.getCurrent()); … … m.allocWidget() … … … m.removeWidget(…) … … } catch (Throwable t) { showErrorMessage(t); } } }.start(); } } … } Proposed Solution:Subsystem Cancellation (Child subsystem is cancelled) ThreadDeath on entry to child

  29. Outline • Try-Catch: Problem + Solution • Locking: Problem + Solution • Cancellation: Problem + Solution • Cleaning Up: Problem + Solution

  30. class Widget implements Closeable { WidgetManager m; Widget(WidgetManager m) { this.m = m; } public void close() { m.removeWidget(this); } … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { static Widget allocGreenWidget(WidgetManager m) { Widget w = m.allocWidget(); w.setColor(Color.green); return w; } public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … Widget w = allocGreenWidget(m); try { … } finally { w.close(); } … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:A Typical Cleanup Pattern This program is broken! Why?

  31. class Widget implements Closeable { WidgetManager m; Widget(WidgetManager m) { this.m = m; } public void close() { m.removeWidget(this); } … } class WidgetManager { int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } void removeWidget(Widget w) { … } } class Program { static Widget allocGreenWidget(WidgetManager m) { Widget w = m.allocWidget(); w.setColor(Color.green); return w; } public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … Widget w = allocGreenWidget(m); try { … } finally { w.close(); } … } catch (Throwable t) { showErrorMessage(t); } } } … } Problem Statement:A StackOverflowError…

  32. class Widget implements Closeable { Subsystem client = Subsystem.getCaller(); WidgetManager m; Widget(WidgetManager m) { client.registerCleanup(this); this.m = m; } public void close() { m.removeWidget(this); } … } class WidgetManager { Subsystem s = Subsystem.getCurrent(); int count; Widget[] widgets = new Widget[10]; Widget allocWidget() { reenter (s) { Widget w = new Widget(); count++; if (count == widgets.length + 1) { Widget[] ws = new Widget[count * 2]; System.arraycopy(widgets, 0, ws, 0, widgets.length); widgets = ws; } widgets[count – 1] = w; return w; } } void removeWidget(Widget w) { reenter (s) { w.client.unregisterCleanup(w); … } } } class Program { static Widget allocGreenWidget(WidgetManager m) { Widget w = m.allocWidget(); w.setColor(Color.green); return w; } public static void main(String[] args) { WidgetManager m = new WidgetManager(); while (true) { String cmd = getUserCommand(); try { … Widget w = allocGreenWidget(m); try { … } finally { w.close(); } … } catch (Throwable t) { showErrorMessage(t); } } } … } class Subsystem { static void finish(Throwable t) { Subsystem s = getCurrent(); … while (!s.cleanupStack.isEmpty()) s.cleanupStack.peek().close(); } } Proposed Solution:Subsystem Cleanup Routines

  33. Related Work(Under Construction) • Re-entering: Similar to performing a remote procedure call in systems where each subsystem is a separate process/thread • Cancellation: • Rudys, Wallach. Termination in language-based systems. ACM TISS 5(2), 2002. • Wick, Flatt. Memory accounting without partitions. ISMM 2004. • Flatt, Findler. Kill-safe synchronization abstractions. PLDI 2004. • Flatt, Findler, Krishnamurthi, Felleisen. Programming languages as operating systems. ICFP 1999. • Subsystem cleanup stacks: • Similar to compensation stacks in Weimer. Finding and preventing run-time error handling mistakes. OOPSLA 2004.

  34. Conclusion • It’s hard in Java to catch unchecked exceptions safely • Many (most?) existing try-catch blocks for unchecked exceptions are probably unsafe • Many synchronized blocks are probably unsafe • There is no easy and safe way to cancel a computation • Subsystems make it easy to fix these problems for the example programs • Future work: Assess the severity of the problem and the effectiveness of subsystems in large programs

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