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Hit frequencies

December 2nd, 2016
tech  [html]

When load testing server-side software there are two straight-forward options:
  1. Stress all endpoints evenly
  2. Stress one endpoint in isolation
For example, if you're testing a dynamic website with siege you could (1) give it a list of all the urls on your site and have it rotate among them, or you could (2) pick a representive url and just hit that one repeatedly. Like any synthetic test this isn't completely realistic, but in this case you're missing something pretty important: if you want to test something that uses caching, you need to approximate the distribution of a real workload.

Why? How well caching works depends on your hit rate, which depends on what the distribution of your requests looks like. Situation (1) is a caching worst-case, (2) is a caching best case, and your real situation is somewhere in the middle. For example, from my access logs over the past couple years, here's the distribution of requests I've seen:

1275358 /news.rss
562369 /
437312 /favicon.ico
406583 /index
168296 /robots.txt
162728 /p/
116353 /simple_piano_recordings/piano_chords.svg
94048 /icdiff
92680 /p/mercury-spill
63728 /wsgi/json-comments-cached/gp/i6WrmjSHXLg
[snip ~1k entries]
1179 /news/2011-11-02
1178 /wsgi/json-comments/gp/YTnbXQoRPRN
1178 /fiddle-clip-on/pictures/11-top.jpg
[snip ~10k entries]
113 /news/back_from_564.rss
113 /news/2012-07-24
113 /news/2012-07-09.html
[snip ~100k entries]
3 /nextbus/omnitrans/82/7387/
3 /nextbus/art/1135/759763/
3 /nextbus/mbta/39/6460/
[snip ~1M entries]
1 /ngx_pagespeed_beacon?ets=load:906&rload=1605...
1 /nextbus/jtafla/17/1433/next/
1 /news/all/trillion-dollar-platinum-coin

This is kind of a mess, but the main idea is that we have a few "hot" endpoints, and then a long tail of less popular entries. I can use this distribution in load testing, to get something between uniform sampling (option 1) and singleton sampling (option 2) that better represents what real load on the site would look like.

In case this is useful to other people, here's the frequency list (hits-frequency.txt.gz) and a short script to sample from it (generate_urls.py).

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