Comment by forrestthewoods
20 hours ago
Here’s an older blog post of mine on roughly the same topic:
https://www.forrestthewoods.com/blog/memory-bandwidth-napkin...
I’m not sure I agree with the data presentation format. “time per element” doesn’t seem like the right metric.
Great post, thanks for the link! I think you and I were just focusing on different things. You gave a broader discussion of the topic from a few different angles, with more specific basic numbers about CPUs as well as more realistic benchmarks than what I have here. I just wanted to focus on the simplest example I could think of, and run it on as wide a range of different array sizes as I could.
The reason I chose "time per element" then follows from that different goal, because I was comparing across vastly different array sizes, so no other metric I could think of would have really worked for the charts I was drawing.
What are your qualms with time per element? I liked it as a metric because it kept the total deviation of results to less than 32 across the entire result set.
Using something like the overall run length would have such large variations making only the shape of the graph particularly useful (to me) less so much the values themselves.
If I was showing a chart like this to "leadership" I'd show with the overall run length. As I'd care more about them realizing the "real world" impact rather than the per unit impact. But this is written for engineers, so I'd expect it to also be focused on per unit impacts for a blog like this.
However, having said all that, I'd love to hear what your reservations are using it as a metric.
It’s not wrong per se. I’m just very wary of nano-scale benchmarks. And I think in general you should advertise “velocity” not “time per”.
Perhaps it’s a long time inspiration from this post: https://randomascii.wordpress.com/2018/02/04/what-we-talk-ab...
I also just don’t know what to do with “1 ns per element”. The scale of 1 to 4 ns per element is remarkably imprecise. Discussing 1 to 250 million to 1 billion elements per second feels like a much wider range. Even if it’s mathematically identical.
Your graphs have a few odd spikes that weren’t deeply discussed. If it’s under 2ns per element who cares!
The logarithmic scale also made it really hard to interpret. Should have drawn clearer lines at L1/L2/L3/ram limits.
On skim I don’t think there’s anything wrong. But as presented it’s a little hard for me as an engineer to extract lessons or use this information for good (or evil).
There shouldn’t be a Linux vs Mac issue. Ignoring mmap this should be HW.
I dunno. Those are all just surface level reactions.
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.
From your blog post:
> Random access from the cache is remarkably quick. It's comparable to sequential RAM performance
That's actually expected once you think about it, it's a natural consequence of prefetching.
Heh. That line often gets called out.
Lots of things are expected when you deeply understand a complex system and think about it. But, like, not everyone knows the system that deeply nor have they thought about it!
If that wasn't the case the machine would have to prefetch to register file. I don't know of any CPU that does that.
Whats most misleading is the data for the smaller sizes (1k)