JEP 515 introduces ahead-of-time (AOT) method profiling to improve startup and warmup performance of Java applications. It leverages a new tool, jaotc, which uses a profile generated during previous runs to compile frequently used methods to native code. This AOT compiled code is then stored in a shared archive, improving startup times by reducing the amount of JIT compilation needed during initial execution and speeding up the time it takes for the application to reach peak performance. The profile data guides the AOT compiler, ensuring that only the most critical methods are compiled, thus minimizing storage overhead. This approach complements the existing tiered compilation system and doesn't replace it.
This JEP proposes preparing the Java platform for a future where final truly means final, eliminating the current capability of dynamically modifying final fields via reflection or other privileged code. The goal is to improve performance, security, and maintainability by enabling further runtime optimizations based on the immutability guarantees of final. This JEP focuses on identifying and mitigating compatibility risks posed by this change, such as existing frameworks and libraries that rely on altering final fields. It outlines an incremental approach involving a new JVM command-line option to enforce final field immutability, allowing developers to test and adapt their code before the restriction becomes the default and eventually permanent. This preparatory work will pave the way for a subsequent JEP to actually finalize the behavior of final.
HN commenters largely discuss the implications of making final mean truly final in Java. Some express concern about the performance impact, particularly for JIT compilers and escape analysis. Others question the practicality and benefit, given the existing workarounds like sealed classes and the potential disruption to existing codebases. A few commenters welcome the change, seeing it as a positive step toward stricter immutability and potentially simplifying some aspects of the language. There's also discussion around the nuances of the proposal, such as its impact on method overriding and the interaction with reflection. Several users highlight the complexity of implementing this change in the JVM and the potential for unforeseen consequences.
JEP 483 introduces a new class loading and linking mechanism called "ahead-of-time" (AOT) loading, aimed at improving startup performance. Unlike the existing dynamic class loading, AOT processes class data during build time, generating a dedicated archive. This archive contains pre-linked classes, readily available at startup, reducing the runtime overhead associated with verification and resolution. While AOT can significantly decrease startup time, particularly for applications with large class hierarchies, it comes with trade-offs. AOT-generated archives increase disk space consumption and require dedicated build-time tooling. Additionally, AOT doesn't replace dynamic class loading entirely; it complements it, handling a predefined set of classes while dynamic loading manages the rest. JEP 483 intends to improve startup, not overall performance, and introduces a new tool called jaotc to facilitate AOT compilation.
HN commenters generally express interest in JEP 483's potential performance benefits, particularly faster startup times. Some highlight the complexity of the proposed changes and the potential for subtle bugs. A few commenters question the necessity of AOT given existing JIT compiler advancements, while others point out that AOT can offer advantages beyond raw startup speed, such as reduced memory footprint and improved warmup times. One commenter notes the limited scope of the initial JEP, applying only to platform classes, and wonders about future expansion to application classes. Another expresses concern about the potential security implications of pre-compiled code. Several users discuss the interplay between AOT compilation and existing JIT compilation, specifically how the two might be used together effectively.
Summary of Comments ( 10 )
https://news.ycombinator.com/item?id=43954178
HN commenters generally express enthusiasm for JEP 515 (Ahead-of-Time Method Profiling), viewing it as a significant performance improvement for Java. Several note that tiered compilation already exists, but this JEP improves it by making profiling data available at application startup, leading to faster warmup times and potentially better peak performance. Some discuss the practical benefits, particularly for short-lived applications and serverless functions where rapid startup is crucial. Others highlight the technical details, like the ability to customize the profiling data and the use of
jaotcfor static compilation. A few commenters raise questions about compatibility and the potential overhead of storing and loading the profile data. There's also discussion around similar features in other languages and virtual machines, emphasizing the wider trend of improving runtime performance through profile-guided optimization.The Hacker News post titled "JEP 515: Ahead-of-Time Method Profiling" has generated several comments discussing the implications and potential benefits of this Java Enhancement Proposal.
Several commenters express enthusiasm for the performance improvements promised by AOT profiling. One user points out that tiered compilation already exists in Java, but it requires the application to "warm up" before reaching peak performance. JEP 515 offers a way to bypass this warm-up period by providing profile data ahead of time, leading to faster startup times and improved performance from the outset. They suggest this is particularly useful for short-lived applications, CLI tools, and serverless functions. Another commenter concurs, highlighting the benefits for serverless environments where startup time is critical for cost and responsiveness.
One commenter questions the practicality of gathering and managing profile information, particularly for applications deployed in diverse environments. They express concern about the potential overhead and complexity of generating and applying these profiles. Another commenter responds to this concern, suggesting that profiles could be generated during testing or staging environments, which are often more controlled and predictable than production. This would allow developers to capture representative performance data and then apply it to production deployments. They further suggest that the tooling and automation around profile management will likely improve over time.
There's also a discussion around the balance between AOT and JIT compilation. One commenter mentions that AOT compilation can sometimes lead to suboptimal performance in the long run compared to JIT, as JIT can adapt to runtime conditions and optimize accordingly. However, another commenter counters this by pointing out that JEP 515 uses profile-guided AOT compilation, which should provide more informed compilation decisions compared to traditional AOT, leading to better overall performance.
Finally, some commenters discuss the security implications of AOT profiling. One user raises concerns about the potential for malicious actors to manipulate profile data to influence compilation and potentially exploit vulnerabilities. While no concrete solutions are offered, the concern highlights the importance of secure profile management and verification.
Overall, the comments reflect a generally positive outlook on JEP 515, with many commenters excited about the potential performance benefits. However, some concerns regarding practicality, complexity, and security are also acknowledged, suggesting areas for further development and refinement.