Java garbage collection, an overview.

An brief introduction about Java Garbage Collection, concurrent mark and sweep, escape analysis and garbage first (G1).

Transcript

1. Garbage Collection in Java “Heap storage for objects is reclaimed by an automatic storage management system. Objects are never explicitly de-allocated” –Java Virtual Machine Specification [1]
2. Java Variable Flavours • Stack – Local variables holding primitives. – Local variable of reference type will point at heap memory. • Heap – Objects. – Primitive fields of an object. 10
3. Mark and Sweep Approach • Simplest. Garbage collector algorithm. • Recover and reuse. Heap memory no longer in use. • Algorithm. – Stop-the-world. Non-deterministic pauses stop all the running threads. – Start. From live objects walks through the tree of references. – Mark. As live any object on the route. – Sweep. Everything left is garbage and can be collected.
4. Generational Garbage Collector • Mark-and-sweep. Improved. • Weak generational hypothesis [2]. – Most objects become unreachable quickly. – Few references from older to young objects exist. • Areas of memory. – Eden. Most new objects (very large objects directly to old generation). – Survivor. Survived, one holds object, the other empty. – Tenured. Promoted longer-lived objects. – PermGen. Not strictly in heap, internal structures (i.e. class definitions). Eden Survivor From To Tenured PermGen Young Generation Old Generation
5. Escape Analysis • Recent change. Java 6u23. • Local variables. Only used inside the method. – No passed into other methods. – No returned. • No heap. Object created on the method stack frame – Reduce objects of young collections. – Memory used freed when method returns. 10
6. Concurrent Mark-Sweep in Action • Two short pauses. Per GC cycle, initial mark and remark. • Initial Mark. Identifies set of objects immediately reachable outside old generation. • Concurrent marking phase. Marks all live objects transitively reachable from this set. • Object graph can change. Not all live objects are guaranteed to be marked. • Pre-cleaning. Revisiting objects modified concurrently with the marking phase. • Second Pause (Remark). Revisits objects modified during concurrent marking phase. • Concurrent sweep phase. Deallocates garbage objects without relocating the live ones. Initial Mark Remark Marking/Pre-cleaning Sweeping Running application thread Running GC thread
7. CMS Pros/Cons • Advantages. – Two pauses. No one single pause. – Concurrency. App and GC run in parallel. • Disadvantages. – Extra overhead. – Free lists. Free space not contiguous. – Large Java heap req. Marking cycle lasts more than stop-the-world, space reclaimed at the end. – App runs concurrently. Old generation potentially increases during marking phase. – Floating garbage. Not guaranteed all garbage objects. – Fragmentation issue. Lack of compaction, possible not efficient use of free space [3]. Start sweeping End sweeping
8. Garbage-First GC (G1) in Action • Features. Parallel, concurrent, incrementally compacting low-pause. • Heap layout. Split into regions. • Region. Equal-sized chunks. • Pause goal. How long app can pause for GC while running (20 ms every 5 min). • Pause. Objects evacuated from one or more regions to a single region. • Statistics. Average a region takes to collect. • G1?. Knows mostly empty regions. – Collects. In this regions first. – Concentrates. Collect on areas likely to be full of garbage. GC Marking GC GC Running application thread Running GC thread
9. G1 Pros/Cons • Advantages. – High performance. – Pause time goals. Prevent interruption proportional to heap or live-data size. – Compact and free up memory. Continuously work to reduce fragmentation. – Concurrent global marking phase. Determine liveness of the objects. – Pause prediction model. • Meet user-defined pause time target with high probability. • Selects number of regions to collect based on the time target. • Disadvantages. – Target. Multi-processor machines with large memories.
10. Comparison • G1. – Compacting collector. – Avoid free list, rely on regions. – More predictable pauses. – User can specify pause targets. • Switch to G1. – More than 50% heap occupied with live data. – Allocation and promotion rate varies significantly. – Undesired long collection and compaction pauses (0.5s to 1s) • CMS. – No compaction. – No control. • ParallelOld. – Whole-heap compaction, long pauses. – No control.
11. References [1] Lindholm, Tim, and Frank Yellin. Java Virtual Machine Specification, Second Edition. Addison-Wesley, Reading, MA, 1999. [2] Charlie Hunt, Binu John. Java Performance, First Edition. Pearson, 2011. [3] Jones, Richard, and Rafael Lins. Garbage Collection. John Wiley & Sons, Ltd.,West Sussex, PO19 IUD, England, 1996. [4] Memory Management in the Java HotSpot Virtual Machine. Sun Mircrosystem, April 2006. [5] B.J. Evans, M. Verburg. Well-Grounded Java Developer. Manning, 2012.
12. Questions?