Java’s garbage collection (GC) process, which reclaims unused memory by deleting unreferenced objects, is a cornerstone of the language’s memory management. However, GC can introduce latency, particularly when interacting with Java Native Interface (JNI) critical regions. To address this, a new JDK Enhancement Proposal (JEP) is under consideration in the OpenJDK community. This proposal aims to improve latency by introducing region pinning to the G1 (Garbage-First) garbage collector, allowing GC to proceed without delays caused by JNI operations.
The Goals of Region Pinning
The proposed enhancement focuses on eliminating stalls caused by JNI critical regions. Key objectives include ensuring that Java threads do not stall during these critical regions, avoiding additional delays in initiating GC cycles, and maintaining minimal impact on GC pause times when JNI critical regions are not active. By meeting these goals, the proposal seeks to enhance Java’s performance in scenarios requiring frequent interaction between managed Java code and unmanaged languages such as C or C++.
Understanding JNI and Its Impact on GC
JNI enables Java to interoperate with unmanaged programming languages by providing functions to access Java objects directly. These functions, used in pairs, allow developers to get a pointer to a Java object, use it, and then release the pointer. During this process, the JVM must ensure that objects in use are not moved or reclaimed by GC. To achieve this, the JVM can either pin critical objects in place or suspend GC operations entirely. Currently, G1—the default GC in Java—opts for the latter approach, disabling garbage collection during each critical region. While effective, this method increases latency and affects application responsiveness.
How the Proposal Aims to Solve the Problem
The new proposal suggests extending G1 to pin arbitrary regions of memory during both minor and major GC operations. By doing so, critical objects can remain stationary without halting the entire garbage collection process. This approach would allow Java threads to operate seamlessly during JNI critical regions, eliminating the need for GC pauses. If implemented, this enhancement could significantly reduce latency in applications that rely heavily on JNI, such as those requiring tight integration with native libraries or high-performance computing tasks.
This JEP underscores Java’s ongoing commitment to improving performance and adaptability, ensuring the language remains competitive in environments demanding both speed and interoperability.