OpenJDK Draft Proposal Targets JVM Code Cache Fragmentation to Improve Execution Time
A new draft JDK enhancement proposal, circulating in the OpenJDK community, aims to significantly improve Java application execution time by introducing an optional “hot” code heap within Java’s segmented code cache. This proposal is designed to address the performance impact of code fragmentation and optimize the handling of frequently executed methods.
Proposal Overview
The proposed enhancement seeks to create a dedicated “hot” code heap for methods that are frequently executed, separating them from non-profiled methods within the code cache. By marking certain methods as “hot,” this approach aims to compactly store these methods in a specific area of the cache, thereby reducing fragmentation and improving overall performance. The goal is to mitigate the adverse effects of scattered compiled code and enhance the efficiency of Java applications.
Impact on Code Fragmentation and Performance
One of the main objectives of the hot code heap proposal is to minimize the fragmentation of highly-optimized code. Fragmentation in the JVM code cache can lead to performance degradation, especially when a large amount of code is JIT-compiled and the hot code is dispersed across the cache. The proposal addresses this issue by consolidating hot code, which should lead to more efficient execution and reduced negative impacts associated with code scattering.
Historical Context and Rationale
The proposal builds on previous concepts introduced in Java Development Kit (JDK) 9, which was released in September 2017. The motivation behind this new proposal stems from the realization that sparse hot code can slow down applications. In scenarios where there is extensive JIT compilation and a significant amount of hot code, the performance can suffer due to the inefficiencies caused by a vast and fragmented code cache.
Technical Considerations
The proposal acknowledges that executing large amounts of scattered code imposes penalties on the CPU, a problem that cannot always be resolved by other means, such as large pages. The degree of performance impact is influenced by the amount and distribution of hot code and the specific processor architecture. By co-locating hot code based on profile information, the proposal aims to boost performance and streamline the code cache management.
Future Prospects
While the hot code heap proposal presents a promising approach to enhancing Java performance, it has not yet been assigned to a specific JDK release. The current JDK 22 was released recently, and the upcoming JDK 23, scheduled for September, could potentially include this proposal. However, it is possible that the implementation may be delayed to a future release, depending on the progress and feedback received during the proposal’s review process.
Conclusion
The hot code heap proposal represents a significant step forward in optimizing Java application performance by addressing code fragmentation and improving the management of frequently executed methods. By consolidating hot code and enhancing the code cache, this proposal has the potential to deliver tangible benefits in execution speed and overall efficiency for Java applications