Comment by sestep
6 months ago
Haha, it seems you may have thought the person you were responding to is the post author :) but actually that would be me.
Agreed that the odd spikes don't matter, that's why I didn't bother discussing them; I was more interested in the data after the array got large enough that random access was actually slower. It looked like all those weird spikes were for arrays small enough to fit in cache anyways.
I agree that it could have been helpful if I'd drawn lines at L1/L2/L3/RAM limits, but I didn't do that because I don't think it's entirely clear where those lines should have been drawn. Specifically because there are two arrays. Should the line show just where the floating-point array is small enough to fit in cache, or where both arrays together are?
Not sure I quite follow what you're saying about mmap on Linux vs Mac; only one of the three sets of experiments used mmap, and the third was explicitly to try to tease out that effect. Especially for the first experiment, I agree that there should be no difference for arrays small enough to fit in RAM, since the whole file gets read into memory first.
> Agreed that the odd spikes don't matter, that's why I didn't bother discussing them
That was sarcasm =P Those spikes are very curious and the choice of presentation makes them seem like noise but there is something there that should be investigated further imho. In the graph it looks like noise. I mean it’s just 1ns. But a 2x throughput difference isn’t noise! Thats huge! Very curious.
> Not sure I quite follow what you're saying about mmap on Linux vs Mac
Your 4th conclusion is “On Linux, random order starts getting even slower for arrays over a gigabyte, becoming more than 50x slower than first-to-last order; in contrast, random order on the MacBook seems to just level out as long as everything fits in RAM.”. That doesn’t make sense. There shouldn’t be any OS difference here.
Gotcha, sorry for not picking up on the sarcasm. Yeah, I mean, I didn't really bother running the experiments many times for the smaller array sizes, so it could potentially be interesting to see if those artifacts persist when poked.
Could you clarify why there shouldn't be an OS difference? I was under the impression that it's the OS that handles how swap space is implemented (which was used by the first set of experiments), as well as how memory-mapped files are implemented (which was used by the second set of experiments). Am I mistaken about that?
> Could you clarify why there shouldn't be an OS difference?
Ignoring mmap. But why on a 16gb system why would performance degrade at 1Gb? You shouldn’t be hitting the swap. So any differences should be hardware architecture related. M1 unified vs Ryzen. And I wouldn’t expect 1Gb to be a magic threshold.
I would definitely expect a threshold beyond 16gb. And I’d expect the swap to come into play at maybe 12gb. I wouldn’t expect a huge difference between 500mb and 8gb. Ok there’s probability difference of hitting the L3 cache. But most of those random accesses will be hitting system RAM so it should be the same.
Could be wrong! But that’s what I’d expect.
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