Introduction to Garbage Collection
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Garbage collection is a form of automatic memory management used in computer programs to reclaim memory occupied by objects that are no longer needed. John McCarthy invented garbage collection for Lisp in 1959. Languages either use garbage collection or require manual memory management through techniques like allocating and freeing memory. Common garbage collection algorithms include reference counting, mark and sweep, and generational collection. The Java Virtual Machine uses different garbage collectors like serial, parallel, concurrent mark and sweep, and Garbage First collectors to reclaim memory in the Java heap.
![Static vs Dynamic memory allocation
Creating an array of ten integers in C (static)
int array[10];
Creating an array of integers in C (dynamic)
int * array = malloc(N * sizeof(int));
free(array);
In C++
Foo* fooPtr = new Foo()
delete fooPtr](https://image.slidesharecdn.com/garbagecollection-170109203214/85/Introduction-to-Garbage-Collection-3-320.jpg)
Introduction to Garbage Collection
- 2. In computer science, garbage collection (GC) is a form of Automatic Memory Management. The garbage collector attempts to reclaim the memory occupied by objects that are no longer in use by the program. Garbage collection was invented by John McCarthy around 1959 to abstract away manual memory management in Lisp.
- 3. Static vs Dynamic memory allocation Creating an array of ten integers in C (static) int array[10]; Creating an array of integers in C (dynamic) int * array = malloc(N * sizeof(int)); free(array); In C++ Foo* fooPtr = new Foo() delete fooPtr
- 4. Languages without GC C, C++*, D*, Objective-C*, Rust* Languages Languages wit GC Java, Go, PHP, Python, Scala*, Haskell, ….
- 5. Garbage Collection Algorithms Reference Counting Mark and Sweep Copy Collection Generational Collection
- 6. Reference Counting Keep an extra integer (“reference count”) to every heap-allocated data structure. ● With a new reference ++refCount. ● When a reference disappears --refCount. ● If refCount == 0, then reclaim the storage + Easy to implement + Real Time cleanup - Additional storage - Speed: incr, decr - Cycles not cleaned C++, Objective-C, Rust, PHP, Python….
- 7. Tracing Collectors vs Reference Counting
- 8. Mark and Sweep Mark Phase: traverses all objects, starting with roots, and marks every object found as alive. Sweep Phase: traverses all objects, reclaim the storage of unmarked objects. + Handles cycles + Easy to implement - Stops the world** - Scans the entire heap
- 9. Copy Collection + Handles cycles + Automatic Compaction - Stops the world** - Changes addresses - 2nd half is unused
- 10. Generational Collection ● Most objects die young ● Newer objects usually point to older objects + Same as Copy (unidirectional) + Less Objects to Copy + Frequency of collection + Faster - Stops the world** - Complex to implement
- 11. Java Stack vs Heap
- 12. Pre and Post Java 8 Heap
- 13. Java Garbage Collectors ● Serial Collector - -XX:+UseSerialGC ● Parallel Collector - -XX:+UseParallelGC ● Concurrent Mark & Sweep Collector - -XX:+UseConcMarkSweepGC ● Garbage First (G1) Collector - -XX:+UseG1GC
- 14. Serial GC ● Stops the world ● Uses single thread ● Designed for single CPU small Heap apps ● Do not use it! ● Young Gen - Copy Collector ● Old Gen - Mark and Sweep
- 15. Parallel GC (The Default Collector) ● Stops the world ● Designed to work with multiple CPUs ● Uses multiple threads ● Expect high latencies when GC runs (-XX:MaxGCPauseMillis=<N>) ● Young Gen - Copy Collector ● Old Gen - Mark and Sweep
- 16. Concurrent Mark and Sweep (CMS) GC ● Stops the world (relatively short pauses) ● Designed to work with multiple CPUs ● Uses multiple threads ● Good for low latency apps ● Young Gen - Copy Collector ● Old Gen - Concurrent Mark-Sweep
- 17. Garbage First (G1) Collector ● Heap is split into (typically 2048) smaller regions ● Avoids collecting the entire heap at once, instead collects incrementally ○ The regions that contain the most garbage are collected first ● Soft real-time garbage collector (Predictable | Configurable STW) ● Compaction is relatively easy ● Uses multiple threads and is good for > 6G heap sizes. Java 9 Default GC. ● Young Gen - Copy Collector ● Old Gen - Concurrent Mark-Sweep
- 18. Minor GC vs Major GC vs Full GC Minor GC cleans the Young Generation Major GC cleans the Old Generation Full GC cleans the Young and Old Generation
- 20. JVM Options to know java -XX:+PrintFlagsFinal -version | grep HeapSize uintx InitialHeapSize := 268435456 {product} uintx MaxHeapSize := 4294967296 {product} java -Xms256m -Xmx2048m -XX:MaxGCPauseMillis=200 -XX:+PrintGCDetails -XX:+PrintGCDateStamps
- 21. Java Memory Leak Hashmap keys without proper equals and hashcode Maps or lists which are growing forever
- 22. Troubleshooting UI Options: jconsole or jvisualvm. Command Line: jstat -gc -t processID 1s jmap -heap processID jmap -dump:live,format=b,file=heap.bin processID jhat heap.bin JVM Options: -XX:+PrintGCDetails-XX:+PrintGCDateStamps
- 23. Thank you!