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Mention coordinated ommision in performance docs
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maxandersen committed Dec 11, 2023
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This guide covers:

* Coordinated omission Problem in Tools
* how we measure memory usage
* how we measure startup time
* which additional flags will Quarkus apply to native-image by default
All of our tests are run on the same hardware for a given batch.
It goes without saying, but it's better when you say it.

== Coordinated Omission Problem in Tools

When measuring performance of a framework like Quarkus the latency experience by users are especially interesting and for that there are many different tools. Unfortunately, many fail to measure the latency correctly and instead fall short and create the Coordinate Omission problem. Meaning tools fails to acoomodate for delays to submit new requests when system is under load and aggregate these numbers making the latency and throughput numbers very misleading.

A good walkthrough of the issue is https://www.youtube.com/watch?v=lJ8ydIuPFeU[this video] where Gil Tene the author of wrk2 explains the issue.

Although that video and related papers and articles date all back to 2015 then even today you will find tools that fall short with the coordinated oission problem

Tools that at current time of writing is known to excert the problem and should NOT be used for measuring latency/throughput (it may be used for other things):

* JMeter
* wrk

Tools that are known to not be affected are:

* https://github.com/giltene/wrk2[wrk2]
* https://hyperfoil.io[HyperFoil]

Mind you, the tools are not better than your own understanding of what they measure thus even when using `wrk2` or `hyperfoil` make sure you understand what the numbers mean.

== How do we measure memory usage

When measuring the footprint of a Quarkus application, we measure https://en.wikipedia.org/wiki/Resident_set_size[Resident Set Size (RSS)]
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