🚀 Optimizing JVM with G1 Garbage Collector (G1GC)

1. Set Initial and Maximum Heap Size

Setting appropriate initial (-Xms) and maximum (-Xmx) heap sizes ensures that the JVM starts with an optimal heap and can grow as necessary, but without triggering excessive garbage collection cycles.

Why it’s important:

• -Xms (Initial heap size): Defines the heap size when the JVM starts. If set too small, the JVM will trigger frequent Minor GC events as it runs out of space.
• -Xmx (Maximum heap size): Specifies the upper limit of the heap. Setting a high value allows the heap to grow as needed, but allocating too much memory might lead to inefficient garbage collection.

Best Practice: Set both values to the same size if you know the application will require a stable, predictable heap size. This avoids the overhead of resizing the heap during runtime.

• -Xms4g: Start with 4 GB of heap.
• -Xmx4g: Maximum heap size is 4 GB.

Adjust Pause Time Goals

The -XX:MaxGCPauseMillis flag sets the desired pause time goal for G1GC. This parameter helps limit the length of stop-the-world pauses (when application threads are paused for garbage collection).

Why it’s important:

• Throughput vs. Latency: Lower values can help reduce application pause times, but might result in more frequent GC cycles. Higher values allow more GC work to be done, but the pause times may increase.
• The JVM will attempt to meet the pause-time goal, but depending on the heap size and workload, it may be difficult to achieve without adjusting other parameters.

Sets the maximum pause time goal to 200 milliseconds. G1GC will attempt to keep all stop-the-world pauses under 200ms.

Control Parallelism

These flags control the number of threads used for parallel and concurrent garbage collection. G1GC uses different threads for young generation (minor GC) and old generation (major GC) collections.

Why it’s important:

• Increasing the number of threads improves GC performance on multi-core systems.
• ParallelGCThreads sets the number of threads used for minor GC, and ConcGCThreads sets the number of threads for concurrent marking and cleanup tasks during major GC.
• Properly configuring these threads based on available CPUs ensures that garbage collection is parallelized and the application stays responsive.

• ParallelGCThreads=8: Sets the number of parallel threads used during minor GCs to 8.
• ConcGCThreads=4: Sets the number of concurrent GC threads for handling Old Generation to 4.

Adjust the Size of G1 Regions

G1GC divides the heap into smaller regions. The size of these regions can impact the frequency of garbage collection cycles and pause times.

Why it’s important:

• Smaller regions increase the granularity of GC cycles, making them more frequent but potentially faster.
• Larger regions reduce the frequency of GC cycles but may increase GC pause times.

G1HeapRegionSize=8m: Sets the size of each G1 region to 8 MB. A smaller region size typically leads to more frequent but shorter GC pauses, while a larger region size reduces the frequency of GC but may increase the pause time.

Set the Initiating Heap Occupancy Percentage

The InitiatingHeapOccupancyPercent parameter controls when G1GC begins its concurrent marking cycle. The marking cycle helps identify live objects, and triggering it too late may lead to Full GCs.

Why it’s important:

• A lower value triggers concurrent marking when the heap is less full, preventing Full GCs and improving overall throughput.
• A higher value means the marking phase starts later when more of the heap is used, potentially increasing GC pause times.

This triggers the concurrent marking cycle when 45% of the heap is occupied, helping prevent Full GCs and optimizing throughput for memory-heavy applications.

Optimize Mixed GC Behavior

This configuration optimizes Mixed GCs, where both young and old generations are collected together. Mixed GCs help reclaim memory from the Old Generation.

Why it’s important:

• Mixed GCs are more efficient but may increase pause times if not tuned correctly.
• The G1MixedGCCountTarget controls how many mixed GCs will be performed before a Full GC is triggered, and G1MixedGCLiveThresholdPercent determines which regions are eligible for mixed collection.

• G1MixedGCCountTarget=8: This ensures that 8 Mixed GCs are performed before G1GC triggers a Full GC.
• G1MixedGCLiveThresholdPercent=85: Only regions with less than 85% live data will be included in mixed collections, reducing the overhead.

Set Heap Waste Target

Heap waste refers to memory that remains unused after a garbage collection cycle. This parameter controls how much heap waste is acceptable before G1GC triggers additional collections.

Why it’s important:

• Allowing heap waste to accumulate can lead to inefficient memory utilization and trigger unnecessary garbage collection.
• Lower values lead to more frequent GC cycles to reclaim space, while higher values delay collection and may increase heap fragmentation.

This sets the maximum allowed heap waste to 5%. If heap waste exceeds this threshold, G1GC will initiate a cleanup phase to reclaim unused memory.

Configure String Deduplication

String deduplication reduces memory usage by eliminating duplicate strings in the heap. This is particularly useful for applications with many repeated string values (e.g., web servers, data processing systems).

Why it’s important:

• String deduplication helps save memory by storing only one copy of each unique string in the heap, significantly reducing memory consumption.

Enables string deduplication, which can significantly reduce heap usage for applications with many repeated strings, improving throughput and memory efficiency.

Monitor and Adjust Based on GC Logs

Enable GC logging to gather detailed insights into the behavior of garbage collection, such as pause times, heap usage, and collection types

Why it’s important:

• GC logs provide real-time feedback on the garbage collection process, allowing you to make data-driven decisions for future tuning.
• By analyzing GC logs, you can track performance metrics like pause times, frequency of full GC events, and memory usage patterns.

This logs GC details to /var/logs/gc.log and rotates logs after they exceed 10MB. This provides a continuous history of GC behavior that you can analyze later.

Example JVM Configuration for High Throughput:

Combining all of the above parameters, a sample JVM configuration for high throughput with low latency could look like this:

-Xms4g -Xmx4g

-XX:+UseG1GC

-XX:MaxGCPauseMillis=200

-XX:ParallelGCThreads=8 -XX:ConcGCThreads=4

-XX:G1HeapRegionSize=8m

-XX:InitiatingHeapOccupancyPercent=45

-XX:G1MixedGCCountTarget=8 -XX:G1MixedGCLiveThresholdPercent=85

-XX:G1HeapWastePercent=5

-XX:+UseStringDeduplication

-Xlog:gc*:file=/var/logs/gc.log:time,uptime,filecount=5,filesize=10M