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by Rendello
5 days ago
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In Rust at least, most things have a with_capacity(n) constructor to ensure there's space for n elements (or n bytes, in the case of strings). I suppose there's no getting around the fact that if your collection has no known bounds, you'll have to do bounds checking + potential reallocation in the hot (push) path or risk having your program SIGSEGV. https://doc.rust-lang.org/std/?search=with_capacity |
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The problem with overcommit, is that there is no reliable way to know in advance how much memory your program can actually use. That system with 16GB of RAM will allow you to "allocate" 128GB of memory. Just try it, call `malloc()` or the `with_capacity()` constructor, it will return a valid pointer or container. Try to write in that giant "buffer" at the beginning, at the end, somewhere in the middle at random… it will still work. Everything works exactly as if you really had a giant 128GB buffer, that you can write to and read back from…
…Until you hit somewhere below 5M different pages, where instead of getting a new page after the page fault, you get killed by the out of memory killer. With SIGTERM if you have the relevant privilege, so you have at least a chance of exiting cleanly, but user programs just get SIGKILL. No appeal, no way to check.
Well there is a way to check, kinda: allocate a fixed amount of memory up front, hit all the pages, and if your program didn't crash, it really has the amount of memory you just gave it. Then perform all your allocations within this real buffer you really have (this means a custom allocator). Any allocation success will be real, and bounds checks will actually work. It just doesn't play well with programs whose actual memory needs are highly unpredictable: most of the time you'll use much more memory than you need, and sometimes you won't have enough, forcing you to raise the threshold.