[rlpb]

> He pauses and says give me a dollar value.

He asked you for "how much is google's spend" and you finished your estimations without giving him a dollar value? Did you forget his question?

> He doesnt want to understand the logic behind the calculations.

From your description that sounds like a false assumption to me. It sounds like despite your estimations you didn't give him an answer to his actual question, and so he had to prompt you.

[phaus]

>He asked you for "how much is google's spend" and you finished your estimations without giving him a dollar value? Did you forget his question?

He didn't say he finished without ever giving him an answer. He was simply explaining that he tried to give an explanation of calculating the amount and the interviewer cut him off and asked for a number. Its absolutely stupid that the interviewer for such a question would only be interested in the number and not the candidate's thought process for arriving at that number. For a question like this its also absolutely reasonable for the candidate to assume that the interviewer wanted his thought process, because from an objective standpoint, that's the only scenario where this specific question wouldn't be a complete waste of time.

>From your description that sounds like a false assumption to me. It sounds like despite your estimations you didn't give him an answer to his actual question, and so he had to prompt you.

This sounds like an assumption on your part. You might be right, but you could also easily be wrong.

[alehul]

I'm fairly sure that he was speaking his calculations aloud in the process of reaching the dollar value when the interviewer stopped him and asked for an answer.

It's a great interview question in the sense of a Fermi problem (I was once asked by a startup "how many window washers are there in [CITY]?"). The premises you choose and how you evaluate them, however, are so much more important in a Fermi problem than a correct answer. I remember being about 5x off but it was only because of a drastic underestimation on the time it takes to clean a window.

[vonmoltke]

Fermi problems are terrible questions when asked about things the questionee has no concept of. The point of a Fermi problem is to make an educated guess, and doing so requires knowledge of the problem space the guess is being made in. Pulling random constraints out of your ass for a domain you do not know does not demonstrate this ability.

[alehul]

If you're saying that in regards to the window washer question, I actually enjoyed it!

The point of a Fermi problem is to arrive at a reasonable estimation for a fairly unknown value by extrapolating and connecting from known values (by known, I mean there's a more narrow lower and upper bound).

I wouldn't say that a Fermi problem about how many window washers are in a city is a terrible question; it is more challenging than something in which you already have domain expertise, but that just makes you have to extrapolate further, which is the real point of the Fermi problem. In fact, pretty easy (and knowable) starting points are the population of the city, windows per individual, etc.

Doesn't a Fermi problem without domain expertise display more critical thinking and reasoning style, while a Fermi problem with domain expertise is less of a Fermi problem, and more of a knowledge test?

[vonmoltke]

> Doesn't a Fermi problem without domain expertise display more critical thinking and reasoning style, while a Fermi problem with domain expertise is less of a Fermi problem, and more of a knowledge test?

A Fermi problem is both a test of knowledge and a test of reasoning skills. The types of estimates Enrico Fermi was known for were only possible because he had the domain knowledge for his reasoning to leverage. If you remove the domain knowledge from the problem you remove a significant amount of the signal from trying to concoct the estimate. It is a much easier problem if you can just make up numbers rather than infer accurate guesses from the domain.

[C1sc0cat]

And how would an outsider have any idea of the cost do you just mean the plant costs how much does google pay per MW in each locale how much does labour costs what allowance for accrued pension rights.

[dpark]

Costs to Google are not magically different. You can make estimates without insider knowledge, but as in the window cleaner example, your estimates will be as bad as your assumptions.

You can also make estimates for compute, network, and storage costs based on the prices Google charged its Cloud customers for the same.

[C1sc0cat]

Ah and exactly where in a CS course do you get into the economics of large scale telecoms / networking infrastructure pray?

Let alone the economics of personnel costs or the non standard way google builds its infrastructure.

[dpark]

You don't. The exercise is in estimation. This is specifically not a case of the interviewer looking for you to get the "right" answer. The interviewer likely doesn't even know what the right answer is. They want to see if you can make back-of-the-envelope calculations and if you're capable of making sane (if inaccurate) assumptions.

Make a guess at total cost for an hour of compute time and how long it might take to transcode the average video. Guess at how many videos are uploaded on a typical day. Guess at how much the typical SRE costs Google and how many SREs YouTube employs. Do the same for software engineers, or explicitly exclude R&D. Guess at networking, storage, etc. Then roll all that together with some hours of video * (cost to transcode + cost to storage + cost to upload + cost to playback * average viewers) + sre cost +.... Bonus points if you can account for elasticity and peak load instead of just averages.

The point is to show that you can think through the problem. If all you can say is "I don't know what your networking costs are", then you come across as useless.

[ummonk]

He's not a new grad, he was a director of PM. He should have a feel for ballpark figures regarding infrastructure and personnel costs, which don't vary by that big a factor from company to company.

The question is perfectly reasonable (and it sounds like the interviewee was providing a reasonable answer). The issue is the way the interviewer ran the interview, not with the particular question itself.

[AnimalMuppet]

Wow. Feel free to use some punctuation here and there. That's annoying to parse as written.

[gjm11]

I wonder whether there were some line breaks that C1sc0cat was hoping would remain as line breaks instead of being treated as spaces...

[albedoa]

Be straight with us, is that really how you read his story?

[gjm11]

I can't speak for rlpb, but it's how I read it, albeit not with much confidence. Why? Because "he pauses and says ..." doesn't fit with the interviewer interrupting and not waiting for the answer to be finished -- there must have been enough of a gap for the interviewer to pause and then ask that question.

[EDITED to fix a typo.]