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Comment by josefx | Hacker News Reader

> are we talking about my application (say, some web app) becoming noticeably slower? Or we would need to be running several million operations per second before anything a user could notice the difference?

If you have an algorithm with quadratic or higher complexity somewhere you will be in the millions even if you only have a few thousand entries to deal with. Now the worst case cache miss (down to ram) is modeled as roughly 1000 cycles. Put that right in the middle of that O(n^2) algorithm and your web app will be able to handle 1 request per second on a good day.

Note: that calculation is extremely simplified

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Maybe you shouldn't be doing O(n^2) inside a request handler in the first place. This has nothing to do with caches.

And even if you do quadratic operations in your handler, how often do you write a new such handler? Most of the time you work on stuff around that, supporting infra, testing, etc. None of those needs to be cache optimized either.

> Maybe you shouldn't be doing O(n^2)

It was mostly meant as a example of millions of operations even at a small scale. Sadly I have seen way too much accidental O(n^3) code and not all of it could be "fixed" trivially.

> This has nothing to do with caches.

Would you rather handle 1000 request per second or 1? Caching has the potential to make an already bad situation so much worse.

> how often do you write a new such handler?

You only need to write one to DoS your server.

> Would you rather handle 1000 request per second or 1?

Would you rather like to approach this problem by optimizing memory layout which, let's be honest, tends to give you more in the region of 1-10% improvements rather than 10-100x, or would you rather like to try to go from O(n^3) to O(n^2) or O(n log n) or similar?

The real meat is in the complexity class. Cache effects are where you go when there is nothing else to optimize and it actually matters. Preemptively making all your data structures 2-3 as big because of not-actually-necessary padding does not sound right to me, but YMMV.

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