3 ms·
The article does include: > The allocation fast path of the RPython GC is a simple bump pointer, and the assembly for it. Indeed a bump allocator has the dis
by dzaima 1y ago
The article does include:
> The allocation fast path of the RPython GC is a simple bump pointer,
and the assembly for it.
Indeed a bump allocator has the disadvantage that it effectively continuously thrashes the lower cache layers, but I'd think that'd be a pretty universal workload-independent issue. (and the cache's LRU will still preserve used parts in the cache, and the thrashing is by definition gonna be pretty evenly spread, giving the user code plenty of time to do other things in between using the thing desired to stay in cache again, slightly lowering the issue)
And given that a Python impl will necessarily need to have a GC path anyway, which already will thrash cache (and Python users aren't and won't be writing free()s regardless), it's probably not a bad trade overall.
I'd assume for pushing to lists the runtime just explicitly reserves capacity ahead-of-time instead of trying to integrate into the memory manager in any way. You'd want to do that anyway even if using malloc & malloc_usable_size or similar, as you'll end up reallocating way too much without that (in a quick test I see my system malloc + malloc_usable_size + realloc used to push 100000 8-byte elts ending up reallocating 8500 times, with the entire code being just this, realloc settling on rounding up to the next 4KB; the actual pointer only changes 10-50 times, but that's still pretty bad, and would presumably get much worse if the code did anything other than just pushing to a single vec)
- forrestthewoods 1y agoThe bump allocator declaration occurs after the 2.1 cycles per allocation line. Perhaps all the information is the article. But it is not presently clearly imho. YMMV. FWIW I’m super supportive of bump allocators and arenas. I just don’t find this particular analysis compelling or sufficiently informative. YMMV.
- dzaima 1y agoYeah, it is a good bit down; that said, bump allocators are a very common (most common?) option around GC anyway; I personally scrolled straight to the disassembly, and it's pretty clear from that, and the explicit mention of bump allocator is just around there too.
- kragen 1y agoThe presentation clearly presumes a lot of implicit knowledge about generational garbage collectors that you not only don't have but firmly believed to be false.
- forrestthewoods 1y agoI have spent quite a lot of time updating code to not generate garbage because the ostensibly super fast GC was infact demonstrably not super fast! I could perhaps be convinced that modern GC are super fast. But in hindsight your blog post is intended to preach to the choir and not change the minds of non-believers. If you ever write an apologetic I will gladly read it. It needs to be a holistic analysis of memory operation cost across all threads.
- kragen 1y agoI haven't written a blog post about this; do you mean my HN comment or CF Bolz's blog post? Bolz's blog post is not written to persuade anyone but to provide objective, verifiable information from which people can form their own conclusions. I agree that a holistic analysis of memory operation cost across all threads would be not only more persuasive but more informative. But it is also much more difficult.
- forrestthewoods 1y agoSorry I thought you were OP. Saw your root reply and got wires crossed. Unfortunately I appear to lack sufficient context from which to draw any conclusions from the OP post. More context and more data needed (for me).
- kragen 1y agoNo, Bolz is way more polite and circumspect than I am. They also know a great deal more.