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 I skimmed through the design and it looks reasonable. The data/analysis to give an insight into pool to cache and their size is good analysis. 4 (Maybe in the future we can look at the NIO thread-local buffer cache to see if it could use a confined arena. It differs to what you have here in that it may cache large buffers (8k or 16k is typically for networking I/O) and a larger number to deal with scatter/gathers ops with I/O with a large number of buffers.) Hanging the cache off of Thread.FieldHolder for platform threads is good. Interestingly, and if I read the code correctly, if some accidentally forgets to close a confined arena before thread exit then the memory will be released if the arena came from the cache. Using a shared pool for virtual threads make sense, I was a bit concerned with the early versions that put the cached pools into every Thread. Allocating a pool based on ncores*2 is probably okay, I assume time will tell if any contention will be observable. ------------- PR Comment: https://git.openjdk.org/jdk/pull/31365#issuecomment-4885614764
