📘 Overview of G1 Garbage Collector in Java
The G1 (Garbage-First) Garbage Collector is a modern, low-latency garbage collector introduced in JDK 7 and the default in JDK 9. It aims to strike a balance between throughput and pause time predictability, making it ideal for multi-core processors and large heap sizes. It optimizes memory management while allowing developers to control and minimize GC pauses.
🌟 Key Features of G1 GC
Region-based Heap Structure: Divides the heap into fixed-size regions for more flexible memory management. Concurrent and Parallel GC: Runs concurrently with the application and utilizes multiple threads for faster collection. Automatic Compaction: Performs heap compaction to reduce memory fragmentation. Pause Time Predictability: Allows users to define maximum GC pause times. Young and Old Generations: Retains generational garbage collection (young and old generations) but without rigid boundaries. 🏗️ Heap Structure in G1 GC
🗂️ Region-based Organization
The heap is divided into small regions instead of contiguous blocks. Regions are typically between 1 MB to 32 MB, depending on the heap size. 📦 Types of Regions:
Eden Regions: Where new objects are allocated. Survivor Regions: Objects promoted from Eden, surviving multiple GC cycles. Old Regions: Objects that survived long enough to be promoted. Humongous Regions: Regions for large objects that exceed half the region size. Example of Setting Region Size:
-XX:G1HeapRegionSize=16m
🔄 G1 GC Process and Phases
1. 🟢 Young Collection (Minor GC)
Triggers when the Eden space is full. Collects live objects and moves them to Survivor or Old regions. This is a stop-the-world (STW) event, but G1 minimizes the pause. 2. 🔵 Concurrent Marking (Old Collection)
Initial Mark Phase: Marks live objects from root references (STW, but brief). Concurrent Mark Phase: Identifies live objects across the heap while the application runs. Remark Phase: Ensures no live objects were missed (STW, parallelized). Cleanup Phase: Reclaims space in regions with mostly dead objects. 3. 🟡 Mixed Collection
G1 performs mixed collections, where both young and some old regions are collected together. This prevents full GC and improves collection efficiency. 4. 🔴 Full GC (Fallback)
When G1 cannot meet its goals or the heap is full, a full GC is triggered. This is a stop-the-world event and should be avoided for performance reasons. ⚙️ Tuning and Configuration Options
1. Pause Time Goals (-XX:MaxGCPauseMillis)
Allows developers to specify the maximum desired GC pause time. G1 adjusts its collection strategy to meet this pause time. Example:
-XX:MaxGCPauseMillis=200
Sets a pause time target of 200 milliseconds.
2. Heap Size Configuration (-Xms, -Xmx)
Configures the initial and maximum heap size. Example:
-Xms2g -Xmx8g
Sets the initial heap size to 2 GB and the maximum heap size to 8 GB.
3. Parallel and Concurrent Threads (-XX:ParallelGCThreads, -XX:ConcGCThreads)
Sets the number of threads for parallel and concurrent GC phases. Example:
-XX:ParallelGCThreads=4 -XX:ConcGCThreads=2
4. GC Logging for Monitoring
Enabling GC logs helps monitor and tune the performance of G1 GC. Example:
-XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:gc.log
This outputs detailed GC logs to gc.log.
📊 Comparison with Other Collectors
G1 vs CMS (Concurrent Mark-Sweep):
Pause Times: G1 offers more predictable pause times than CMS, which suffers from long pauses during concurrent marking and sweeping. Fragmentation: G1 compacts the heap during garbage collection, unlike CMS, which can experience memory fragmentation. Mixed Collection: G1 collects old and young generations together, avoiding long old generation collections like in CMS. G1 vs Parallel GC:
Throughput vs Latency: G1 optimizes for low-latency (lower pause times) while Parallel GC focuses on throughput, often leading to longer pauses. Dynamic Region Assignment: G1 uses regions, which can dynamically change roles (young, old), whereas Parallel GC has fixed-generation boundaries. 🚀 Best Practices for G1 GC
Set Realistic Pause Goals: Don’t over-optimize for very low pause times (-XX:MaxGCPauseMillis); it can negatively impact throughput. Monitor GC Logs: Use GC logging to fine-tune heap size, region sizes, and pause times. Heap Size: Ensure that the heap is large enough to prevent frequent full GCs. Use G1 with Large Heaps: G1 performs better in applications with large heap sizes (typically 4 GB or more). 🛠️ Sample G1 GC Configuration
java -Xms2g -Xmx8g -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:G1HeapRegionSize=16m -XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:gc.log -XX:ParallelGCThreads=4 -XX:ConcGCThreads=2
In this configuration:
2 GB initial heap size, 8 GB maximum. G1 GC enabled with a 200 ms pause time goal. GC logs are printed to gc.log for further analysis. 🎯 Conclusion
The G1 Garbage Collector is an advanced, low-latency GC designed for large Java applications, offering predictable pauses, automatic memory compaction, and flexible heap management. By leveraging parallel and concurrent phases, G1 reduces the impact of garbage collection on application performance, making it ideal for multi-threaded and memory-intensive environments.
With careful tuning, G1 can provide an excellent balance between throughput and latency, ensuring that Java applications remain responsive even under heavy memory usage.
