On Wed, 3 Jun 2026 08:50:01 GMT, Per Minborg <[email protected]> wrote:
> ## Summary > > This PR proposes to introduce a pooled confined arena as an optimization for > `Arena.ofConfined()`, where small native allocations can be served from a > reusable per-thread/per-slot memory pool instead of calling the regular > native allocator for every short-lived arena. The arena remains confined to > its owner thread and is still closed normally, but its backing storage can be > reset and reused when the arena closes. The feature requires no API changes. > > ### Outline > > Platform threads: one lazily allocated pool per Thread, encoded in > `Thread.confinedMemoryPool`. > Virtual threads: fixed shared native pool with CAS-protected slots, because > per-virtual-thread native pools would not scale. > > Pooled memory is zeroed out upon _closing_ an Arena to minimize data > visibility between reuse. This means the data is visible only within a TWR > block, and never outside it. > > By default, a confined arena has access to 64 bytes of pooled data. The pool > size is configurable via a system property and can be 8, 16, 32, or 64 bytes. > Pooling can also be turned off completely by setting the pool power-of-two > size to zero. Nested confined arenas are not supported > > ## Static Analysis > > An extensive static corpus analysis of third-party libraries and the JDK > itself has been conducted with respect to `Area.ofConfined()` usage, > revealing that confined arenas were used _only_ in TWR blocks and _never_ in > an unstructured way. The static analysis further revealed that in most cases, > only a small amount of native memory was ever allocated, usually less than 32 > bytes, and in many cases, 8 bytes or less. This usage pattern lends itself > well to pooling. > > ## Dynamic Analysis > > A dynamic statistical analysis of actual runs was also made, where various > properties of confined arenas were recorded and summarized during a complete > tier1 test run. While a tier1 run is not necessarily representative of a > typical application workload, it provided some interesting results: > > The run produced 93 per-process histogram blocks and 788,773,092 closed > confined arenas. The result is dominated by arenas with no native allocation > at all: 375,934,768 arenas (47.661%) are in the zero-byte bucket. Counting > arenas up to 63 bytes covers 99.997% of all arena closures. > > The largest count bucket is 8-15 bytes per arena with 400,951,293 arenas > (50.832% of all arenas). The largest byte bucket is 8-15 bytes per arena with > 3,207,623,039 B (3,059.03 MiB) (46.794% of all bytes). Buckets below 64 KiB > preserve very close t... src/java.base/share/classes/java/lang/Thread.java line 401: > 399: */ > 400: @Stable > 401: private AutoCloseable confinedArenaAllocator; I'm on the fence about this—adding a new field into Thread (which also means for all Virtual Threads that get allocated) for a very specific use-case—what other options exist, and do VirtualThreads need their own caches or can they rely on the caches of their carriers? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/31365#discussion_r3348947158
