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Comment by infinite8s | Hacker News Reader

Actually concurrency makes it worse. Say your site has an median latency of < second but a p99 of 10 seconds for any particular request, but your site needs to make 100 requests (not unheard of with modern sites) for any particular page. This means that now almost 99% of your users will be affected by the p99 latency. This is usually a big issue because most systems are designed with median or mean latency in mind, so the p99 is usually orders of magnitude greater.

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I agree that this should be kept in mind, but I think your specific conclusion requires a lot of assumptions. The primary one is that all those requests are "independent and identically distributed". If instead your server gets laggy one minute per hour when some background update runs and trashes the file cache, the percentage of affected users would be completely different.

I think the blog author is on the right track to suggest that you at least need to look at the full distribution of the latencies. I'd go farther and say that if the metric you care about is total load time for a page with 100 components, lumping together all the individual component requests is probably never going to be able to provide all the information you actually need.

But as 'sp527' pointed out below, I may not have been precise enough in my use of "concurrency". From the Little's Law perspective I was emphasizing, we really care about the "number of requests in flight". But what constitutes a request depends on what we are measuring. If if each "page" consists of 100 pipelined HTTP requests, we probably need to define "in flight" as the number of simultaneous users we can support if we want to calculate the metric we actually care about.

Mathmatically, 1-(1-0.99)^100 = ~1.

Not quite. What you calculated is the probability that visitors will not hit the p99 case on all their requests. In other words, the chance of getting a p99 response on all 100 requests is infinitesimal.

The chance of getting at least one p99 response out of a hundred is 1 - 0.99^100 ≈ 63%.

https://latencytipoftheday.blogspot.com/2014/06/latencytipof...

This is only if it's Markovian - but it almost certainly is not.

If you need X requests served for a full page-load (or whichever for the primary functionality of the page), you should be totaling those request times into a single entry for your histogram, otherwise you are calculating the distribution of something potentially useless (e.g. 50% of page_is_broken_without_it.js might actually be in p99 even if that file is 1% of your requests and the generic histogram might look deceptively good).

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