?? Optimizing Java Performance with G1 Garbage Collector (G1GC) ??

Overview of G1GC

G1GC is designed to provide high throughput and low latency, making it well-suited for applications that require predictable response times. It is particularly effective for large heap sizes and is the default garbage collector starting from Java 9

Key Features of G1GC

Region-Based Memory Management:

• G1GC divides the heap into a set of equally sized regions, which can be either young or old. This segmentation allows the collector to work on smaller, manageable pieces of memory instead of the entire heap at once.

Generational Collection:

• G1GC maintains the concept of generational garbage collection. The heap is divided into:
• Young Generation: Contains newly allocated objects, similar to the Eden and Survivor Spaces in other collectors.
• Old Generation: Contains long-lived objects that have survived multiple garbage collection cycles.

G1GC Collection phase details

Initial Mark (Stop-the-World Phase)

• Objective: The goal of the initial mark phase is to quickly identify all "live" objects that are reachable from the GC roots. This is the starting point for the marking phase of garbage collection in G1GC.
• Actions:
• Marks GC Roots: All objects that can be reached directly from the GC roots (such as local variables, active threads, and static variables) are marked as "live."
• Scans Survivor Regions: It scans through regions that contain survivor spaces (where objects that survive the young generation collections are placed).
• Records Top At Mark Start (TAMS): TAMS is a critical marker used during the concurrent marking phase. It records the state of all live objects at the start of the initial mark phase, ensuring that objects created during concurrent phases aren’t lost or mismarked.

Characteristics:

• This phase stops the world—all application threads are paused during the initial mark phase to ensure that the marking of roots is consistent.
• It is usually very short, as the primary goal is to mark the reachable objects from the GC roots.

Concurrent Marking

Objective: After the initial mark, the goal of the concurrent marking phase is to identify all live objects in the heap by traversing the object graph while allowing application threads to continue running.

Action

• Traverses the Object Graph: G1GC recursively traverses the object graph, starting from the GC roots and marking all objects that are reachable.
• Updates Remembered Sets (RSets): As the application runs concurrently, objects may reference regions that have been moved (for example, in survivor spaces or older generations). The Remembered Sets (RSets) track these references so G1GC can include them in subsequent marking phases.
• Processes References While Application Runs: G1GC updates the RSets during the concurrent marking process to account for new references that were added as the application continued running. This ensures that no live objects are missed in the marking process.

Characteristics

• This phase runs concurrently with the application, minimizing pause times. However, it may take longer compared to stop-the-world phases because it processes a large portion of the heap.
• It tries to ensure that the marking process is as comprehensive as possible by taking into account references that are introduced during marking.

Remark (Stop-the-World Phase)

Objective: The remark phase ensures the finalization of object marking. It handles any updates that occurred during concurrent marking and processes references to objects that may have been missed during the concurrent phase.

Actions

• Processes Snapshot-At-The-Beginning (SATB) Buffers: These buffers track all the objects that were reachable during the concurrent marking phase but were modified during that phase. This ensures that the marking phase includes objects that were missed due to modifications (such as references created after the initial marking phase).
• Finalizes Marking: Completes the marking of all live objects in the heap.
• Handles Class Unloading: G1GC also checks for class unloading during the remark phase, ensuring that classes that are no longer in use are properly garbage collected.

Characteristics:

Like the initial mark phase, the remark phase is a stop-the-world event, meaning all application threads are paused. However, it usually takes longer than the initial mark because it processes the SATB buffers and ensures comprehensive object marking.

Cleanup

Objective: The cleanup phase frees up empty regions and prepares them for reuse.

Actions:

• Frees Empty Regions: Identifies and clears regions that are no longer in use, particularly regions in the Young Generation or the Old Generation where objects have been fully reclaimed.
• Recycles Regions for Reuse: Recycles regions to be used for new allocations, ensuring that memory is managed efficiently and that the heap doesn’t become fragmented.

Characteristics:

• This phase operates in parallel with the application and usually requires minimal pause time. However, it may be more resource-intensive if the heap is highly fragmented or if regions contain many objects to clean up.

Region Selection Algorithm

In G1GC, the heap is divided into fixed-size regions (typically between 1 MB and 32 MB in size, depending on the heap size). The Region Selection Algorithm is responsible for determining which regions should be selected for garbage collection, and it plays a key role in minimizing pause times and maximizing garbage collection efficiency.

Efficiency Calculation:

The efficiency of each region is calculated using the following formula:

javaCopyefficiency = garbage_bytes / (region_size * predicted_time_ms)

Where:

• garbage_bytes: The amount of garbage (unused memory) in the region.
• region_size: The size of the region.
• predicted_time_ms: The predicted time required to collect that region.

Factors Considered in Region Selection:

1. Live Data Percentage: Regions with a high percentage of live data (meaning objects that are still in use) are less desirable for collection because they require less reclamation.
2. Predicted GC Time: The predicted time to collect the region is calculated based on the amount of garbage and how long it would take to collect.
3. Region Age: Older regions (those that have been collected and recycled more times) may be less efficient for collection, depending on their fragmentation.
4. Reference Density (RSet Size): A region with a larger RSet (Remembered Set) indicates more inter-region references, meaning that the collector will need to perform more work in processing those references.

Collection Set Selection

The Collection Set consists of the regions selected for garbage collection during a particular cycle. The goal is to choose a set of regions that will:

• Meet Pause Time Goals: Ensure that the total time spent in garbage collection does not exceed the specified pause time goal.
• Maximize Garbage Collection: Select regions that contain the most garbage to maximize the reclamation of unused memory.
• Balance Mixed GC Load: In the mixed collection phase, G1GC tries to strike a balance between collecting young and old generations, maximizing efficiency without overloading the system.

Pause Time Calculation

G1GC aims to provide predictable pause times by breaking down the collection into smaller phases. The pause time during a collection cycle is determined by several components:

• Root Scanning Time: The time it takes to scan and mark the GC roots. This happens during the initial mark phase.
• RSet Update Time: The time it takes to update the Remembered Sets (RSet) during the concurrent marking phase, particularly as objects are moved or new references are created.
• Object Copy Time: The time it takes to copy objects from one region to another during minor and mixed collections.
• Reference Update Time: The time it takes to update references to objects that have been moved during a collection cycle.

G1GC adjusts the amount of time spent in each of these components to meet the pause time target set by the user (e.g., -XX:MaxGCPauseMillis=200).

Adaptive Sizing

G1GC uses adaptive sizing to dynamically adjust the heap's region count, collection set size, and marking thresholds to meet the application’s pause time requirements.

• Adjusts Region Count: Depending on the heap size and pause time goals, G1GC can adjust the number of regions it uses for memory management.
• Modifies Collection Set Size: If the target pause time is not met, G1GC can adjust the number of regions selected for garbage collection in the next cycle.
• Updates Marking Threshold: G1GC dynamically adjusts the marking threshold to ensure that enough live data is retained in memory, while balancing the GC workload.

Conclusion

The G1 Garbage Collector's sophisticated collection phases, region selection algorithm, and adaptive pause-time handling make it ideal for applications requiring low-latency and high throughput. By adjusting the GC process based on heap size, live data, and predicted collection times, G1GC ensures efficient memory management while meeting stringent pause time goals.

The ability to fine-tune pause times, region selection, and heap size enables developers to achieve optimal performance and manage memory more efficiently in large-scale Java applications.