# wall clock time

Published articles for wall clock time.

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## A Backward NTP Clock Step Delayed a Python Scheduler's Measurements by Nine Days

DevFeed: [A Backward NTP Clock Step Delayed a Python Scheduler's Measurements by Nine Days](<https://devfeed.tech/articles/the-ntp-clock-jump-that-made-the-scheduler-skip-a-week-of-measurements-34099.md>)

Original publisher: [Read original article](<https://philipptheserver.com/posts/clock-jump-scheduler/>)

Author: Philipp Lehmann (philipp.lehmann@gruppe.ai)

Published: 2026-09-14T07:00:00Z

Content type: tutorial

Language: en

Sources: [Philipp Lehmann](<https://devfeed.tech/sources/philipp-lehmann.md>)

Topics: [Python](<https://devfeed.tech/topics/python.md>), [Linux](<https://devfeed.tech/topics/linux.md>), [systemd](<https://devfeed.tech/topics/systemd.md>), [Single Board Computer](<https://devfeed.tech/topics/single-board-computer.md>)

Tags: [embedded](<https://devfeed.tech/tags/embedded.md>), [linux](<https://devfeed.tech/tags/linux.md>), [ntp](<https://devfeed.tech/tags/ntp.md>), [python](<https://devfeed.tech/tags/python.md>), [reliability](<https://devfeed.tech/tags/reliability.md>), [scheduler](<https://devfeed.tech/tags/scheduler.md>), [single-board-computer](<https://devfeed.tech/tags/single-board-computer.md>), [systemd](<https://devfeed.tech/tags/systemd.md>), [time](<https://devfeed.tech/tags/time.md>), [wall-clock-time](<https://devfeed.tech/tags/wall-clock-time.md>)

### AI overview

A Python scheduler on an embedded Linux device silently stopped running after NTP corrected the system clock backward by about nine days. Because it used wall-clock time to measure elapsed time, the scheduler waited roughly nine days before running again. The article explains why a monotonic clock is the appropriate fix and mentions testing with an injected clock jump.

### Source excerpt

time.time() vs time.monotonic() in a Python scheduler: a backwards NTP step delayed runs by nine days. Monotonic durations and a jump-injecting test.

## Branch Prediction: Why an if Inside a Hot Loop Costs Milliseconds

DevFeed: [Branch Prediction: Why an if Inside a Hot Loop Costs Milliseconds](<https://devfeed.tech/articles/branch-prediction-why-an-if-inside-a-hot-loop-costs-milliseconds-39571.md>)

Original publisher: [Read original article](<https://ankit-rana.com/logs/19-branch-prediction-loop-unrolling/>)

Author: hello@ankit-rana.com

Published: 2026-03-21T00:00:00Z

Content type: tutorial

Language: en

Sources: [Ankit Rana | Mechanical Sympathy](<https://devfeed.tech/sources/ankit-rana-mechanical-sympathy.md>)

Topics: [cpu](<https://devfeed.tech/topics/cpu.md>), [Hardware](<https://devfeed.tech/topics/hardware.md>)

Tags: [branch](<https://devfeed.tech/tags/branch.md>), [branch-prediction](<https://devfeed.tech/tags/branch-prediction.md>), [cpu](<https://devfeed.tech/tags/cpu.md>), [cycles](<https://devfeed.tech/tags/cycles.md>), [loops](<https://devfeed.tech/tags/loops.md>), [optimization](<https://devfeed.tech/tags/optimization.md>), [performance](<https://devfeed.tech/tags/performance.md>), [pipeline](<https://devfeed.tech/tags/pipeline.md>), [pipelining](<https://devfeed.tech/tags/pipelining.md>), [sorting](<https://devfeed.tech/tags/sorting.md>), [systems](<https://devfeed.tech/tags/systems.md>), [wall-clock-time](<https://devfeed.tech/tags/wall-clock-time.md>)

### AI overview

This tutorial explains how CPU pipelining and branch prediction affect performance in hot loops. It describes the cost of mispredictions and suggests sorting data to create predictable branch patterns or using branchless techniques and loop unrolling, while recommending measurement on the target workload.

### Source excerpt

CPUs pipeline instructions and speculate on branch outcomes. A misprediction discards the speculative work and flushes the pipeline at roughly 10 to 20 cycles. In a million-iteration loop with random branch outcomes, those flushes dominate runtime. Sorting the data so the branch resolves the same way for long runs, or removing the branch entirely with a mask or 0/1 multiplier, is what recovers the time.

## Tracking Wall Clock, On-CPU, and Off-CPU Time for Android Performance

DevFeed: [Tracking Wall Clock, On-CPU, and Off-CPU Time for Android Performance](<https://devfeed.tech/articles/the-three-musketeers-of-time-28540.md>)

Original publisher: [Read original article](<https://www.amanjeet.me/three-musketeers-of-time/>)

Author: Amanjeet Singh Gurtatta

Published: 2021-07-18T21:35:43Z

Content type: tutorial

Language: en

Sources: [Amanjeet Singh](<https://devfeed.tech/sources/amanjeet-singh.md>)

Topics: [Android](<https://devfeed.tech/topics/android.md>), [Process](<https://devfeed.tech/topics/process.md>), [cpu](<https://devfeed.tech/topics/cpu.md>)

Tags: [android](<https://devfeed.tech/tags/android.md>), [cpu](<https://devfeed.tech/tags/cpu.md>), [cpu-time](<https://devfeed.tech/tags/cpu-time.md>), [mobile-performance](<https://devfeed.tech/tags/mobile-performance.md>), [off-cpu-time](<https://devfeed.tech/tags/off-cpu-time.md>), [performance](<https://devfeed.tech/tags/performance.md>), [regression](<https://devfeed.tech/tags/regression.md>), [wall-clock-time](<https://devfeed.tech/tags/wall-clock-time.md>)

### AI overview

This Android performance tutorial explains wall-clock, on-CPU, and off-CPU time, including how to measure them at the process or thread level and how to choose an appropriate monotonic clock API.

### Source excerpt

The post talks about the different types of time to look for: wall clock time, on CPU time and off CPU time