On Fri, 7 Nov 2025 12:54:27 GMT, Erik Österlund <[email protected]> wrote:
>> This is the implementation of JEP 516: Ahead-of-Time Object Caching with Any >> GC. >> >> The current mechanism for the AOT cache to cache heap objects is by using >> mmap to place bytes from a file directly in the GC managed heap. This >> mechanism poses compatibility challenges that all GCs have to have bit by >> bit identical object and reference formats, as the layout decisions are >> offline. This has so far meant that AOT cache optimizations requiring heap >> objects are not available when using ZGC. This work ensures that all GCs, >> including ZGC, are able to use the more advanced AOT cache functionality >> going forward. >> >> This JEP introduces a new mechanism for archiving a primordial heap, without >> such compatibility problems. It embraces online layouts and allocates >> objects one by one, linking them using the Access API, like normal objects. >> This way, archived objects quack like any other object to the GC, and the GC >> implementations are decoupled from the archiving mechanism. >> >> The key to doing this GC agnostic object loading is to represent object >> references between objects as object indices (e.g. 1, 2, 3) instead of raw >> pointers that we hope all GCs will recognise the same. These object indices >> become the key way of identifying objects. One table maps object indices to >> archived objects, and another table maps object indices to heap objects that >> have been allocated at runtime. This allows online linking of the >> materialized heap objects. >> >> The main interface to the cached heap is roots. Different components can >> register object roots at dump time. Each root gets assigned a root index. At >> runtime, requests can be made to get a reference to an object at a root >> index. The new implementation uses lazy materialization and concurrency. >> When a thread asks for a root object, it must ensure that the given root >> object and its transitively reachable objects are reachable. A new >> background thread called the AOTThread, tries to perform the bulk of the >> work, so that the startup impact of processing the objects one by one is not >> impacting the bootstrapping thread. >> >> Since the background thread performs the bulk of the work, the archived is >> laid out to ensure it can run as fast as possible. >> Objects are laid out inf DFS pre order over the roots in the archive, such >> that the object indices and the DFS traversal orders are the same. This way, >> the DFS traversal that the background thread is performing is the same order >> as linearly materializing the objects one by one in the or... > > Erik Österlund has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 34 commits: > > - Fix test group anomaly > - Merge branch 'master' into 8326035_JEP_object_streaming_v6 > - Merge branch 'master' into 8326035_JEP_object_streaming_v6 > - Merge branch 'master' into 8326035_JEP_object_streaming_v6 > - Comment update > - Merge branch 'master' into 8326035_JEP_object_streaming_v6 > - Merge branch 'master' into 8326035_JEP_object_streaming_v6 > - remove include > - Interned string value word accounting > - Dont load all objects when JVMTI CFLH is on > - ... and 24 more: https://git.openjdk.org/jdk/compare/167c952b...754006b5 Marked as reviewed by aboldtch (Reviewer). ------------- PR Review: https://git.openjdk.org/jdk/pull/27732#pullrequestreview-3433727151
