On Thu, 25 Jun 2026 10:22:22 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: There are up to four lazily allocated pools 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 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... > > Per Minborg has updated the pull request incrementally with two additional > commits since the last revision: > > - Add local pools > - Add 4 nested levels So, we have refactored the solution a bit: - PTs have a field (ijn the sidecar) for an array of four pools that is lazily created - VTs do not have a field at all and store that info directly in a shared Pool - An Arena always allocates a pool (called a local pool) whenever the first "small" allocation is made on the Thread. - When the Arena closes, the pool is zeroed and put into the threads array of pool (if it fits) - If the Thread's array of pool is full, the local pool is freed and then discarded - Hence, we support up to 4 nested confined arenas with pooling - Nest level 5 and upwards will still benefit from the local pool if several small allocations are made - When an Arena is created, we first check if there is an available recycled pool in the Thred's array of pools before allocating a new local pool. ------------- PR Comment: https://git.openjdk.org/jdk/pull/31365#issuecomment-4800605210
