10 ms·

Not sure what you mean by testing intuitions. Which intuitions? It sounds more like testing whether you know the speed of DDR5 without looking it up, and doing

by hnfong 3y ago

Not sure what you mean by testing intuitions. Which intuitions? It sounds more like testing whether you know the speed of DDR5 without looking it up, and doing a bunch of basic unit conversions. And your analysis doesn't even cover latency (which GP mentioned), which apparently according to my first google result is ~90ns. So unless your data is already in a cache it might not even be fetched within the requisite time. Your question sounds like a trick question because there are so many nuances and gotchas, and if you're asked this question without a concrete context and the problem one's trying to solve, it's basically a check of whether the interviewee's mental model of computation matches the interviewer's. And as this thread shows, even among (presumably) competent people, the factors to consider can differ quite a bit.

cogman10 3y ago
Precisely my point. What I’ve seen, intuition wise, on where slowness is in computation it’s usually along the terms of “how many FLOPs can this do?” or “How much bandwidth does this have?”. But that’s a tiny fraction of the story. Very little computation is boring math operations. It’s true, CPUs are absurdly fast at multiplying 2 numbers together. That’s not what trips them up. What trips them up is getting those two numbers and determining when those 2 numbers should be multiplied. Those two problems are so hard that CPUs are often guessing, running the calculation, and then backtracking if they guessed wrong. And that action has lead to side channel attacks (spectre, meltdown) which can leak information about data to malicious software. This is why such a question doesn’t have either an intuitive or non-intuitive answer. It’s very strictly “it depends”. And if you boiled it all the way down to “no, what’s the absolute max it can do” you are now talking about software problems that don’t exist/aren’t valuable. Consider, what execute the add instruction faster, a 3.4 ghz CPU from 2004 or a 3.4ghz CPU from 2023? The answer is they are the same. Both do the add in 1 cycle. Did you learn anything about the performance of a 2004 CPU vs a 2023 CPU? Heck no. That’s because how fast the CPU can execute instructions has almost nothing to do with how fast it can process data. (Yes, I’m aware that even the above statement is somewhat incorrect as x86 CPUs do weird things with adds where they’ll run multiple additions per cycle with some clever reordering… Not even that example can be simple. Assume they didn’t). The trivia question is a boring question. The actual interesting discussion is “so WHY aren’t we doing multiplications that fast?”. That is what can actually lead to deeper understandings about what slows things down in computing. There’s a reason the 10ghz CPU never surfaced. It’s not because we couldn’t make it, it’s because we couldn’t keep it busy.

by hnfong 3y ago

cogman10 3y ago

Precisely my point. What I’ve seen, intuition wise, on where slowness is in computation it’s usually along the terms of “how many FLOPs can this do?” or “How much bandwidth does this have?”. But that’s a tiny fraction of the story. Very little computation is boring math operations. It’s true, CPUs are absurdly fast at multiplying 2 numbers together. That’s not what trips them up. What trips them up is getting those two numbers and determining when those 2 numbers should be multiplied. Those two problems are so hard that CPUs are often guessing, running the calculation, and then backtracking if they guessed wrong. And that action has lead to side channel attacks (spectre, meltdown) which can leak information about data to malicious software. This is why such a question doesn’t have either an intuitive or non-intuitive answer. It’s very strictly “it depends”. And if you boiled it all the way down to “no, what’s the absolute max it can do” you are now talking about software problems that don’t exist/aren’t valuable. Consider, what execute the add instruction faster, a 3.4 ghz CPU from 2004 or a 3.4ghz CPU from 2023? The answer is they are the same. Both do the add in 1 cycle. Did you learn anything about the performance of a 2004 CPU vs a 2023 CPU? Heck no. That’s because how fast the CPU can execute instructions has almost nothing to do with how fast it can process data. (Yes, I’m aware that even the above statement is somewhat incorrect as x86 CPUs do weird things with adds where they’ll run multiple additions per cycle with some clever reordering… Not even that example can be simple. Assume they didn’t). The trivia question is a boring question. The actual interesting discussion is “so WHY aren’t we doing multiplications that fast?”. That is what can actually lead to deeper understandings about what slows things down in computing. There’s a reason the 10ghz CPU never surfaced. It’s not because we couldn’t make it, it’s because we couldn’t keep it busy.