How to Measure Latency

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How to Measure Latency?
Heinrich Hartmann
Principal Engineer @

@HeinrichHartmann
Motivation and Background
■ I have been talking about Statistics and Latency for the last years
State of...

"It's slow" is the hardest
problem you will ever debug.
Theo Schlossnagle @postwait

@HeinrichHartmann
What is Latency?

@HeinrichHartmann
How to Measure Latency?
t_start = time.now()
#
# operation you want to measure
#
latency = time.now() - ...

@HeinrichHartmann
Things to watch out for
■ Capture early returns / exceptions
● Use: try/catch/ﬁnally or defer
■ Which cl...

@HeinrichHartmann
Measuring Latency over Time

The End

Where to Measure Latency?

@HeinrichHartmann
Hidden Queues

@HeinrichHartmann
Hidden Queues

@HeinrichHartmann
A practical Queuing Model

@HeinrichHartmann
Response Time vs. Service Time

@HeinrichHartmann
Response Time vs. Service Time
Image Credits: ŠJů, Wikimedia Commons, CC BY-SA 3.0 , via Wikimedia Commo...

@HeinrichHartmann
Where to Measure Latency?
- Only get Service Time
+ Easy to implement
+ Most meaningful
- Hard to implem...

You can't measure Response
Time on the Server.
@HeinrichHartmann
SAD BUT
TRUE

Request Time vs. Service Time
An Experiment

@HeinrichHartmann
Simulation Setup
■ Request Rate (~ constant)
■ Concurrency (Active Requests)
■ Response Time
■ Arrival R...

@HeinrichHartmann
Queuing System at 50% Capacity

@HeinrichHartmann
Queuing System at 90% Capacity

@HeinrichHartmann
Queuing System at 99% Capacity

@HeinrichHartmann
Queuing System at 100% Capacity

@HeinrichHartmann
Queuing System at 101% Capacity

@HeinrichHartmann
A Hockey Stick

@HeinrichHartmann
A Stalled System

@HeinrichHartmann
Coordinated Omission in Load Testing
Def. Coordination between Load Generator (Client) and Server that l...

How to Analyze Latency Data?

@HeinrichHartmann
Best Practice: Histogram (Metrics)
More Details:
- Latency SLOs done right @ FOSDEM 2019
- State of The ...

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Further Reading
- HeinrichHartmann.com/latency
- @HeinrichHartmann
Thank you!

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