G1 Garbage Collector

person shubham sharmafolder_openJAVAlocal_offeraccess_time November 17, 2024

G1 Garbage Collector (Garbage First Collector)

The G1 (Garbage-First) Garbage Collector is a server-style, low-pause, concurrent, and parallel garbage collector introduced in Java 7 and became the default garbage collector in Java 9. G1 is designed to replace other collectors, such as the CMS (Concurrent Mark-Sweep) and the Parallel GC, by providing a balance between low latency and high throughput while minimizing long pause times during garbage collection (GC) events.

The name "Garbage-First" comes from the fact that G1 prioritizes collecting regions that are expected to have the most garbage (unused objects), aiming to maximize collection efficiency.

How Does G1 GC Work?

Unlike traditional garbage collectors, which divide memory into fixed young and old generations, G1 divides the heap into equal-sized regions and focuses on reclaiming as much garbage as possible while minimizing the impact on application performance.

1. Heap Structure:

The heap is divided into fixed-sized regions (typically 1-32 MB each). These regions are categorized into:

  • Eden (where new objects are allocated)
  • Survivor (where objects that survive garbage collections are moved to)
  • Old (where long-lived objects reside)
  • Humongous (for very large objects, typically more than half the size of a region)

Instead of managing entire young and old generations separately, G1 tracks regions that contain the most garbage and collects them first, which is why it’s called Garbage-First.

2. Marking and Collecting Phases:

G1 GC operates in phases to reclaim space. The two major processes are young generation collections and mixed collections (collecting both young and old regions).

  1. Young Generation Collection:

    • This is where objects from the Eden and Survivor regions are collected.
    • Happens more frequently because objects in the young generation tend to be short-lived.

  2. Concurrent Marking:

    • The G1 collector periodically performs a concurrent marking phase to identify which objects in the old generation are still alive.
    • Marking runs concurrently with the application threads, meaning it does not pause them.

  3. Mixed Collection:

    • After marking, G1 performs a mixed collection, collecting both young generation regions (Eden and Survivor) and old generation regions.
    • G1 chooses which old generation regions to include based on their garbage-to-live-object ratio, aiming to collect regions with the most garbage.

  4. Humongous Object Handling:

    • If an object is too large to fit in a single region, it’s placed in humongous regions. These objects are treated differently from smaller objects, but G1 will attempt to collect them as part of the old generation during mixed collections.

3. Pause Time Goal:

G1 GC is pause-time-focused, meaning it tries to limit the time an application is paused during garbage collection. You can specify a target maximum pause time goal (e.g., 200 milliseconds), and G1 will attempt to meet this goal by adjusting its behavior (like how many regions to collect at once).

Phases of G1 GC

  1. Initial Mark:

    • This is a stop-the-world (STW) phase where G1 identifies live objects in the young generation.
    • It is done at the start of the minor GC (young generation GC).

  2. Root Region Scanning:

    • Scans survivor regions to identify references to old generation regions.
    • Runs concurrently with the application.

  3. Concurrent Marking:

    • G1 marks the live objects throughout the entire heap (both young and old generations).
    • This phase runs concurrently with the application to minimize pause times.

  4. Remark:

    • Another STW phase that rechecks the objects marked as live in the concurrent marking phase.
    • Ensures no live objects were missed.

  5. Cleanup:

    • Identifies regions that are mostly garbage and prepares them for reclamation.
    • May perform some cleanup in parallel or stop-the-world depending on the workload.

  6. Copying/Compacting:

    • In this phase, live objects are copied to new regions, compacting the heap.
    • This reduces fragmentation in the heap and ensures that free space is available for new object allocation.

Key Advantages of G1 GC

  1. Predictable Pause Times:

    • G1 allows you to specify a pause-time goal (e.g., -XX:MaxGCPauseMillis=200), and the GC will try to respect that.
    • This is useful in real-time applications that need consistent, predictable response times.

  2. Concurrent Marking:

    • Most of the work in identifying live objects is done while the application is running (concurrently).
    • This minimizes the amount of time the application is paused during garbage collection.

  3. Region-Based Memory Management:

    • The heap is divided into small regions, and G1 collects those regions that have the most garbage first.
    • This improves efficiency and reduces unnecessary pauses.

  4. Compacts the Heap:

    • G1 performs heap compaction, reducing memory fragmentation, which is a common problem in long-running applications.
    • Compaction ensures that large objects can be allocated without issues.

  5. Parallelism:

    • G1 leverages multiple CPU cores to perform garbage collection tasks in parallel, improving throughput in systems with many cores.

  6. Adaptive to Workloads:

    • G1 adjusts itself dynamically based on the behavior of the application and the specified pause-time goals, making it highly adaptable to different workloads.

Tuning G1 GC

While G1 is designed to work well with minimal tuning, there are several options available for fine-tuning based on the application requirements:

  1. Set the Max Pause Time Goal:

    This sets the maximum GC pause time the application should experience. G1 tries to adapt to meet this target but might not always be able to do so if there is too much live data.

  2. Set Heap Size:

    Defines the minimum and maximum heap sizes. A larger heap allows for fewer GCs but can also increase pause times.

  3. Set Number of GC Threads:

    Defines the number of threads used for parallel phases of garbage collection.

  4. Enable/Disable Parallelism:
    For specific phases like Mark and Remark, you can configure the number of threads or whether they should be parallelized.

  5. Humongous Object Handling:
    By default, G1 treats large objects differently from smaller ones. If you expect a lot of large object allocations (e.g., large arrays or big strings), you can adjust the behavior for humongous objects.

When to Use G1 GC in Real-Life Projects?

G1 GC is designed for applications with large heap sizes and a need for low latency or consistent pause times. Some real-world use cases where G1 GC shines include:

  1. Enterprise Applications: Systems that need to process large amounts of data in memory (e.g., financial applications, ERP systems) benefit from G1 GC due to its adaptive behavior and region-based collection.

  2. Web Servers and Microservices: In applications where consistent response times are critical, G1 can help keep pauses short and predictable, especially when serving many concurrent requests.

  3. Gaming and Real-Time Applications: G1 is useful in real-time systems that need to keep pauses to a minimum to avoid disrupting user experiences (e.g., online gaming or real-time data processing).

  4. Big Data Processing: Applications processing huge data sets, such as those running on Apache Spark or Hadoop, can leverage G1’s region-based approach for better garbage collection performance over traditional collectors.

Conclusion

The G1 Garbage Collector offers an excellent balance between throughput and low-pause garbage collection for applications with large heaps or requiring predictable latency. With its adaptive region-based memory management, concurrent phases, and ability to meet pause-time goals, G1 is highly suited for modern server-side applications and real-time systems. Tuning G1 can further optimize its performance based on specific application needs, making it a powerful tool in Java’s garbage collection arsenal.

warningComments are closed.