Proposal to Implement Region Pinning in Default GC to Minimize GC Latency for Java Threads
Java garbage collection, a crucial process for managing memory by removing unused objects, is set for a potential enhancement through a new proposal being discussed in the OpenJDK community. This proposal, known as a JDK Enhancement Proposal (JEP), aims to reduce latency issues associated with garbage collection (GC) by introducing region pinning in the G1 (Garbage-First) collector. This approach would address some of the current limitations, particularly concerning Java Native Interface (JNI) critical regions.
The core idea of the proposal is to enable the G1 garbage collector to avoid disabling garbage collection during JNI critical regions by implementing region pinning. This would ensure that Java threads do not experience delays or stalls related to GC operations while in these critical regions. The proposal outlines several key goals:
- No Thread Stalling Due to JNI Critical Regions: By pinning memory regions, Java threads would not need to wait for garbage collection operations to complete when they are in a critical region. This would help maintain application responsiveness and performance.
- Reduced Latency for GC Operations: The proposal aims to eliminate the additional latency that arises from disabling GC during JNI critical regions. This would result in more predictable and efficient garbage collection times.
- Minimized Regressions in GC Pause Times: The introduction of region pinning would aim to ensure that GC pause times do not regress when JNI critical regions are not active. This means that the performance improvements should be consistent across different scenarios.
- No Impact on Performance When No JNI Regions Are Active: It is important that the proposal does not introduce any performance regressions in garbage collection times when JNI critical regions are not in use. The focus is on maintaining or improving current performance levels.
The motivation behind this proposal stems from the need to improve interoperability with unmanaged programming languages, such as C and C++, which use JNI to interact with Java objects. JNI functions require the use of direct pointers to Java objects, which must be managed carefully to avoid disrupting the GC process. Currently, G1 handles this by disabling GC during critical regions, which can lead to increased latency.
By implementing region pinning, the proposal suggests that the G1 collector could maintain the placement of critical objects during GC operations, thus avoiding the need to halt GC altogether. This approach is expected to enhance performance by allowing garbage collection to proceed concurrently with JNI critical operations.
While the proposal presumes that JNI critical regions will remain relatively short and infrequent, there is a consideration for potential heap exhaustion issues when multiple regions are pinned simultaneously. However, the experience with the Shenandoah garbage collector, which also uses region pinning without encountering such issues, suggests that G1 could similarly handle these scenarios without significant problems.
Overall, this proposal represents a promising development in improving Java’s garbage collection efficiency and latency, potentially leading to more responsive and high-performance applications.