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Comment by Const-me | Hacker News Reader

On modern windows the limit is 2^47 bytes: https://docs.microsoft.com/en-us/windows/win32/memory/memory... With 8GB of virtual space per engine, should be enough for 16k of these engines. Interesting that you got OOM only after 1k engines, that’s 16x below the limit. Was that on Windows? Actual hardware or some VM substitute?

In my use case, I was lucky as I only need an allocator per hardware thread. They never approach thousands. So far, the maximum is 256 hardware threads, like in dual-CPU EPYC 7713.

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It seems to depend a lot on current conditions and is ultimately probabilistic. I.e. it would not fail 100% at 1000, but some large enough fraction of the time that that strategy was not shippable for Wasm. IIRC it might only have been 10% of the time, but it quickly increases to 100% along a sliding scale up to the hard limit. Part of that is fragmentation as well, because Chrome uses an allocator underneath that can carve out fairly large reservations that fragment the address space.

Just wanted to cosign that this is absolutely a real-world concern. For that reason I always try to make huge reservations as early in the process lifecycle as possible [1] and then subdivide them from there. Then you can know ahead of time how much space you have to work with. This still has potential composability issues, but partitioning the address space like this before VA fragmentation becomes an issue really helps when you have a greedy VA reserver competing against typical workloads (and their malloc's mmap/VirtualAlloc requests), especially in long-running programs.

[1] Ideally before most shared libraries have been loaded. I should do a simulation to get some quantitative estimates, but my hunch is that ASLR hurts badly even if each loaded library only takes up one 4 kB page. E.g. if you have an empty address space, allocate a 4 kB page at a random address and split the space in two around it, then try to densely pack as many 8 GB ranges in there as possible, you lose one potential 8 GB range (even if the ranges only require 4 kB alignment for their base address). Whenever such splits don't land in the same 8 GB clump, you lose a potential 8 GB range per split.

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