Choosing the Right Garbage Collector in Java: G1 vs. CMS vs. Parallel GC

Introduction

In Java, Garbage Collection (GC) is an essential process for automatic memory management. Java provides several types of garbage collectors, each designed to balance performance, latency, and memory management needs. Choosing the right garbage collector is crucial to ensure that Java applications perform efficiently without unnecessary delays or resource consumption. This decision is especially important when optimizing for throughput, low-latency, or large-scale applications.

Three popular garbage collectors—G1 (Garbage-First), CMS (Concurrent Mark-Sweep), and Parallel GC—offer different strategies for managing memory. Each has its strengths and weaknesses, and the right choice depends on the nature of your application and its performance requirements.

In this article, we will compare G1, CMS, and Parallel GC to help you understand which one fits your application’s needs. Additionally, we will provide practical insights into how to fine-tune the garbage collection process for optimal performance.

Overview of Garbage Collectors in Java

Garbage collectors in Java are responsible for cleaning up memory by identifying and reclaiming unused objects that are no longer in use. The main objective is to free up memory while minimizing the impact on application performance. The JVM (Java Virtual Machine) provides several garbage collection algorithms that differ in how they manage memory, the frequency of collection, and the amount of time the application spends in garbage collection.

The most common garbage collectors are:

G1 GC (Garbage-First Garbage Collector)
CMS GC (Concurrent Mark-Sweep Garbage Collector)
Parallel GC (Throughput Collector)

Let’s take a closer look at each garbage collector and its specific features, advantages, and use cases.

1. Garbage-First (G1) Garbage Collector

Introduced in Java 7, G1 GC was designed to provide a balance between low-latency and high throughput. It is a server-style garbage collector that performs well with large heap sizes and applications that require predictable pause times. G1 divides the heap into multiple regions (instead of generations), which helps optimize both garbage collection time and heap management.

Key Features of G1 GC:

Predictability: G1 GC aims to minimize pause times by dividing the heap into multiple regions, which allows the garbage collector to perform incremental collections in parallel.
Concurrent Marking: G1 performs most of its garbage collection in parallel with application threads. This helps in reducing pause times.
Parallelism: G1 GC utilizes multiple threads for different tasks like marking, sweeping, and compacting.
Region-based Heap: The heap is divided into regions rather than generations, which allows more flexibility in memory management.

When to Use G1 GC:

Low-Latency Requirements: G1 GC is ideal for applications that need predictable and low-latency performance.
Large Heap Sizes: It works well with large heaps and multi-core systems, as it scales efficiently with heap size.
Applications with Mixed Workloads: G1 is suitable for applications that have both large numbers of short-lived objects and long-lived objects.

Example JVM Option for G1 GC:

-XX:+UseG1GC

Pros of G1 GC:

Reduces the frequency and duration of full garbage collection pauses.
Allows you to set pause-time goals, which helps meet real-time requirements.
Efficient with large heap sizes, as it dynamically adapts to memory usage.

Cons of G1 GC:

Slightly more complex to tune than other garbage collectors.
May not be as efficient in handling small heaps or low-throughput workloads.

2. Concurrent Mark-Sweep (CMS) Garbage Collector

The CMS Garbage Collector was designed for applications requiring low-latency performance. Unlike G1, which divides the heap into regions, CMS focuses on minimizing pause times by performing garbage collection concurrently with the application threads. CMS performs the marking and sweeping phases concurrently, which allows application threads to continue running while garbage collection is taking place.

Key Features of CMS GC:

Concurrent Collection: CMS works concurrently with application threads for most of the garbage collection process.
Low-Latency: It reduces stop-the-world pause times, making it ideal for applications with real-time requirements.
Generational Approach: CMS works on a generational heap, where young objects are collected more frequently than older objects.
Short Pauses: CMS is designed to minimize the duration of stop-the-world pauses, which is particularly beneficial for low-latency systems.

When to Use CMS GC:

Real-Time Applications: CMS is ideal for applications that require very low-latency and cannot afford long pauses.
Multi-Core Systems: It works well on multi-core machines where concurrent processing can be used to reduce pause times.
Applications with Mixed Memory Needs: CMS is effective for applications with mixed memory workloads where both short-lived and long-lived objects are common.

Example JVM Option for CMS GC:

-XX:+UseConcMarkSweepGC

Pros of CMS GC:

Low-latency and short garbage collection pauses.
Reduces the impact on application performance by performing most of the collection concurrently.
Suitable for applications requiring predictable performance.

Cons of CMS GC:

Can lead to fragmentation in the Old Generation, requiring occasional Full GC cycles.
May experience long pauses if the Old Generation becomes too full.
Tuning CMS can be challenging, as improper configuration may lead to issues like frequent Full GCs.

3. Parallel Garbage Collector (Throughput Collector)

The Parallel GC (also known as the Throughput Collector) is designed to maximize throughput by utilizing multiple threads for garbage collection tasks. Unlike CMS and G1, Parallel GC prioritizes throughput and works best for batch-processing systems where long pause times are acceptable. Parallel GC is the default garbage collector in many JVM configurations.

Key Features of Parallel GC:

High Throughput: Parallel GC maximizes throughput by using multiple threads to perform garbage collection tasks in parallel.
Multi-Threading: It uses multiple threads for both minor and major garbage collection, which helps improve efficiency in multi-core systems.
Generational Collection: Like CMS, Parallel GC uses generational garbage collection to efficiently manage short-lived and long-lived objects.

When to Use Parallel GC:

High-Throughput Applications: Parallel GC is ideal for applications where throughput is more important than latency.
Batch Processing: It is best suited for applications that can tolerate longer GC pauses but need to process large volumes of data.
Multi-Core Systems: Parallel GC performs well in environments with multiple cores, where concurrent garbage collection can reduce the overall time spent in GC.

Example JVM Option for Parallel GC:

-XX:+UseParallelGC

Pros of Parallel GC:

Maximizes throughput by using multiple threads for garbage collection.
Suitable for applications with large heaps and workloads that can tolerate long pauses.
Easy to configure and generally works well out of the box.

Cons of Parallel GC:

Not suitable for low-latency applications due to long stop-the-world pauses.
Tuning may be required for applications with specific pause-time requirements.
Can cause unpredictable pause times when the heap becomes too large.