[Python-Dev] The untuned tunable parameter ARENA_SIZE

When CPython's small block allocator was first merged in late February 2001, it allocated memory in gigantic chunks it called "arenas". These arenas were a massive 256 KILOBYTES apiece. This tunable parameter has not been touched in the intervening 16 years. Yet CPython's memory consumption continues to grow. By the time a current "trunk" build of CPython reaches the REPL prompt it's already allocated 16 arenas. I propose we make the arena size larger. By how much? I asked Victor to run some benchmarks with arenas of 1mb, 2mb, and 4mb. The results with 1mb and 2mb were mixed, but his benchmarks with a 4mb arena size showed measurable (>5%) speedups on ten benchmarks and no slowdowns. What would be the result of making the arena size 4mb? * CPython could no longer run on a computer where at startup it could not allocate at least one 4mb continguous block of memory. * CPython programs would die slightly sooner in out-of-memory conditions. * CPython programs would use more memory. How much? Hard to say. It depends on their allocation strategy. I suspect most of the time it would be < 3mb additional memory. But for pathological allocation strategies the difference could be significant. (e.g: lots of allocs, followed by lots of frees, but the occasional object lives forever, which means that the arena it's in can never be freed. If 1 out of ever 16 256k arenas is kept alive this way, and the objects are spaced out precisely such that now it's 1 for every 4mb arena, max memory use would be the same but later stable memory use would hypothetically be 16x current) * Many programs would be slightly faster now and then, simply because we call malloc() 1/16 as often. What say you? Vote for your favorite color of bikeshed now! //arry/ -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20170601/57602cee/attachment.html>