JVM G1GC Tuning

By Gi Pyeong Lee
jvm g1gc tuning

JVM G1GC Overview

The Java Virtual Machine (JVM) Garbage First Garbage Collector (G1GC) is a low-pause garbage collector designed to reduce the amount of time the application is paused for garbage collection. G1GC was introduced in Java 7 and is the default garbage collector in Java 9.

G1GC works by dividing the heap into multiple regions. Each region is a fixed size, typically between 1MB and 32MB. G1GC divides the heap into Eden, Survivor, and Old regions. The Eden region is where new objects are allocated. When the Eden region is full, the objects are moved to the Survivor regions. The Survivor regions are used to hold objects that have survived one garbage collection cycle. When the Survivor regions are full, the objects are moved to the Old regions. The Old regions are used to hold objects that have survived multiple garbage collection cycles.

G1GC uses a concurrent marking algorithm to identify which objects are still in use and which objects can be reclaimed. This allows G1GC to reclaim objects without having to pause the application. G1GC also uses a concurrent compaction algorithm to move objects around in the heap to reduce fragmentation. This allows G1GC to reclaim more memory without having to pause the application.

G1GC also uses a pause time target to limit the amount of time the application is paused for garbage collection. G1GC will try to keep the pause time below the target. If the pause time exceeds the target, G1GC will try to reduce the pause time by increasing the amount of time spent in concurrent marking and compaction.

G1GC is a good choice for applications that need to minimize the amount of time spent in garbage collection pauses. G1GC can also be tuned to reduce the amount of time spent in garbage collection pauses. Tuning parameters include the size of the regions, the pause time target, and the amount of time spent in concurrent marking and compaction. Parameters

What Are G1GC Tuning Parameters?

G1GC tuning parameters are settings used to optimize the performance of the G1 garbage collector (G1GC) in the Java Virtual Machine (JVM). G1GC is a garbage collector designed to reduce the amount of time spent in garbage collection and improve application response times. It works by dividing the heap into regions, which can be collected in parallel. G1GC tuning parameters are used to configure the G1GC for the specific application and environment.

G1GC Tuning Parameters

G1GC tuning parameters can be divided into two categories: heap size parameters and garbage collection parameters.

Heap Size Parameters

Heap size parameters are used to configure the size of the heap. The most important parameters are:

  • -Xms: Sets the initial size of the heap.
  • -Xmx: Sets the maximum size of the heap.
  • -XX:MaxGCPauseMillis: Sets the maximum pause time for garbage collection.

Garbage Collection Parameters

Garbage collection parameters are used to configure the garbage collection algorithm. The most important parameters are:

  • -XX:G1HeapRegionSize: Sets the size of the heap regions.
  • -XX:G1ReservePercent: Sets the percentage of the heap to be reserved for old generation objects.
  • -XX:G1NewSizePercent: Sets the percentage of the heap to be used for new generation objects.
  • -XX:G1MaxNewSizePercent: Sets the maximum percentage of the heap to be used for new generation objects.
  • -XX:G1MixedGCCountTarget: Sets the target number of mixed garbage collections before a full garbage collection.

Example

To configure the G1GC for a web application running on a server with 8GB of RAM, the following parameters could be used:

-Xms4g -Xmx8g -XX:MaxGCPauseMillis=200 -XX:G1HeapRegionSize=4m -XX:G1ReservePercent=10 -XX:G1NewSizePercent=20 -XX:G1MaxNewSizePercent=30 -XX:G1MixedGCCountTarget=4

This configuration sets the initial and maximum heap size to 8GB, sets the maximum pause time for garbage collection to 200 milliseconds, sets the size of the heap regions to 4MB, reserves 10% of the heap for old generation objects, sets the percentage of the heap to be used for new generation objects to 20%, sets the maximum percentage of the heap to be used for new generation objects to 30%, and sets the target number of mixed garbage collections before a full garbage collection to 4.

G1GC Performance Considerations

When tuning the G1 garbage collector, there are several key performance considerations to keep in mind. These include the size of the heap, the number of garbage collections, the size of the survivor spaces, and the size of the Eden space.

Heap Size

The heap size is an important factor in G1GC performance. A larger heap size will reduce the number of garbage collections, but can also increase the pause times for those collections. It is important to find the right balance between the two.

Number of Garbage Collections

The number of garbage collections is another important factor in G1GC performance. A higher number of collections will reduce the pause times for each collection, but can also increase the overall amount of time spent in garbage collection.

Survivor Spaces

The size of the survivor spaces is also important in G1GC performance. A larger survivor space will reduce the number of objects that need to be copied during a garbage collection, but can also increase the amount of time spent in garbage collection.

Eden Space

The size of the Eden space is also important in G1GC performance. A larger Eden space will reduce the number of objects that need to be copied during a garbage collection, but can also increase the amount of time spent in garbage collection.

Conclusion

Tuning the G1 garbage collector requires careful consideration of several key performance factors. The heap size, the number of garbage collections, the size of the survivor spaces, and the size of the Eden space all need to be taken into account when tuning G1GC. By understanding the implications of these factors, it is possible to optimize G1GC performance and ensure that the garbage collector is running efficiently.