The Basics What is memory management? Programs contain Objects - - PowerPoint PPT Presentation
The Basics What is memory management? Programs contain Objects Data Occupy memory Runtime system must allocate and reclaim memory for program in an efficient manner Why is this important? Why is this hard? Why is
Objects Data Occupy memory
Why is this important? Why is this hard? Why is this interesting?
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Objects dynamically allocated on HEAP malloc(), new()
Manual/Explicit
free() delete()
Automated
Garbage collection (GC)
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new allocate storage for new object delete reclaim storage
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Foo* p = new Foo(); Foo* q = p; delete p; p->DoSomething(); p = NULL; q->ProcessFoo();
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#include <stdlib.h> void f(void){ void* s; s = malloc(50); return; } int main(void){ while (1) f(); return 0; }
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Detects dead objects (garbage detection) Reclaims dead objects (garbage reclamation) Garbage collection
Relieves programmer burden Less prone to errors
Python, Java, C#, php
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public void f(){ startLaser(); Obj o = new Obj(); stopLaser(); } public static void main(…){ while (true) f(); } Time
GC, Bad for Real- Time
Software execution engine that executes your Java
Java interpreter that converts byte code into OS specific
Handles related tasks
Memory management (GC implemented in JVM) Security Multithreading
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Reclaiming objects at the right time
Discriminating live objects from garbage
Fast allocation Fast reclamation Low fragmentation
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stack heap Uninitialized data (bss) Initialized data Text / code High address Low address Command line args and environment variables Initialized to 0 by exec Read from program file by exec
Now or in the future
Approximate liveness by reachability from outside the
Unreachable objects are garbage (reclaim storage) Reachable objects are live and must not be reclaimed
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Stack references Heap references
stack heap 1 2 2 1 1 1
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stack heap 1 2 1 1 1 1
Processor registers Program stack Global variables
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Find all live objects All others are dead
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stack heap
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stack heap
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stack heap
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stack heap