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Table of contents
1.
Introduction
2.
Incremental Garbage Collection
3.
Partial-Collection Basics
4.
Generational Garbage Collection
5.
The Train Algorithm
6.
Frequently Asked Questions
6.1.
What is pause time in garbage collection?
7.
Conclusion
Last Updated: Mar 27, 2024

Short-Pause Garbage Collection

Author GAZAL ARORA
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Compiler Design

Introduction

Simple trace-based garbage collectors perform stop-the-world garbage collection, which can cause long pauses in user program execution. By collecting waste one portion at a time, we can shorten the length of the pauses. 

We can divide work in two ways: time and space. The work is divided in time by interleaving garbage collection with mutation or space by collecting a portion of the garbage at a time. The first is referred to as incremental collection, whereas the second is referred to as a partial collection.

We will discuss the following Short-Pause Garbage Collection methods:

  1. Incremental Garbage Collection
  2. Partial-Collection Basics
  3. Generational Garbage Collection
  4. The Train Algorithm

Also see, Advanced Topics in Garbage Collection

Incremental Garbage Collection

An incremental garbage collector breaks up the reachability analysis into smaller units, which allows the mutator to run between them. 

Incremental collectors are conservative. A trash collector must not collect non-junk objects, but it is not required to collect all garbage in each round. The garbage that remains after collection is called floating garbage. Of course, minimizing floating garbage is preferable. 

An incremental collector, in particular, should not leave any garbage behind that was not available at the start of the collection cycle. Suppose we can be certain of such a collection guarantee. In that case, any garbage that isn't collected in one round will be collected in the next, and no memory will be released due to this garbage collection method.

In other words, incremental garbage collectors play it safe by overestimating the number of objects that can be reached. They process the program's root set atomically first, without the mutator interfering. After locating the initial set of unscanned objects, the mutator's operations are interleaved with the tracing step. During this time, any actions taken by the mutator that may affect reachability are stored in a side table as the collector can make the necessary adjustments when it resumes execution. If space runs out before tracing is finished, the collector ends the tracing process without allowing the mutator to run. In any case, after tracing is completed, space is atomically reclaimed.

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Partial-Collection Basics

The basic fact is that objects "die young" in most cases. It has been discovered that between 80% and 98% of all freshly allocated objects die after a few million instructions or before another megabyte is allocated. Before any garbage collection is performed, objects frequently become unreachable. As a result, it is very cost-effective to collect new objects regularly.

Generational garbage collection is the most well-known partial-collection algorithm. It divides objects into groups based on how long they've been assigned and collects freshly produced objects more frequently because they have a shorter lifespan. A train algorithm is an option that contains a subset of garbage at a time and is best suited to more mature objects. These two algorithms can be combined to build a partial collector differentiating between younger and older objects.

Generational Garbage Collection

Generational garbage collection is a good technique to take advantage of the fact that most objects die soon. In generational garbage collection, the heap storage is divided into a succession of divisions. The lower-numbered divisions will hold the younger things, and the higher-numbered partitions will keep the older objects. Partition 0 is where objects are produced first. When this partition becomes full, the objects that are still available are relocated to partition 1. Now that partition 0 is empty again, we can start allocating new objects to it. When partition 0 fills up again, it is garbage collected, and all of its reachable objects are copied into partition 1, where they join the previously copied items.

This pattern continues until partition 1 runs out of space, at which point garbage collection is done to partition 0 and 1.

The Train Algorithm

While the generational technique is very efficient for handling immature objects, it is less efficient for managing mature objects because mature objects are relocated every time a collection occurs, and they are unlikely to be garbage. A new approach to an incremental collection called the train algorithm was designed to improve the management of mature objects. It can be used to collect all garbage. It's probably best to use the generational technique for immature items and only "promote" them to another heap handled by the train algorithm after a few collection rounds. Another benefit of the train algorithm is that we never have to execute a complete garbage collection, which we have to do on occasion for other reasons.

Frequently Asked Questions

What is pause time in garbage collection?

Garbage collection (GC) is the method through which Java eliminates data from memory that is no longer required. When a memory region is full and the JVM wants space to proceed, a garbage collection pause, also known as a stop-the-world event, occurs. All actions are halted during a stoppage.

Conclusion

In this article, we learned about Short-Pause garbage collection. We also discussed the following Short-Pause Garbage Collection methods:

1 Incremental Garbage Collection

3 Partial-Collection Basics

4 Generational Garbage Collection

5 The Train Algorithm

Trace-based garbage collectors will run at regular intervals to find unreachable objects (garbage) and regain their space. These intervals run when the free space in storage falls below a certain threshold or is depleted. Click here to learn about the Trace-based Garbage collection.

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