On Fri, 14 Aug 2026 11:06:06 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 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: There are up to four lazily allocated pools per Thread, >> encoded in `Thread.FieldHolder.confinedMemoryPool`. >> Virtual threads: Works in the same way but uses its _carrier thread's _ >> cache instead. >> >> 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. >> >> A confined arena has access to four pools, each of size 64 bytes. The pool >> sizes are 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. As there can be up to four pools per thread, >> nested confined arenas are supported (i.e., up to four nested arenas). >> >> ## 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 ar... > > Per Minborg has updated the pull request incrementally with one additional > commit since the last revision: > > Address performance regression src/java.base/share/classes/java/lang/Thread.java line 301: > 299: * Access is confined to this platform thread, directly or as a > carrier. > 300: */ > 301: long[] confinedMemoryPools; If we only care about platform threads, maybe a carrier thread local might also be ok (and leave Thread alone) ? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/31365#discussion_r3805150933
