Important note: both wult and ndl are research, debugging and tracing tools which require superuser privileges and must not be used in production environment.

Table of Contents

1 Introduction

Wult stands for "Wake Up Latency Tracer", and this is a project providing tools for measuring C-state latency in Linux. The project provides 2 tools:

  • wult - measures CPU C-state exit latency in Linux. This is the main deliverable of the project.
  • ndl - measures the memory access latency observed by a PCIe network card when the CPU is is in a C-state. However, this tool is rarely used, and it is documented separately here.

2 Documentation

These web pages are the primary source of wult and ndl. In addition to this, man pages include many details:

Here is an old wult presentation video recording by Artem Bityutskiy at Linux Plumbers Conference in 2019. The presentation begins at "2:17:50" and it gives a high level wult introduction.

The presentation focuses on wult kernel drivers, but some of the details are out of date.

  • The presentation is focused around the I210 NIC measurement method, but nowadays wult supports the hrt method, which is the recommended method. This method does not require any special hardware, such as a NIC. Use hrt unless you have a good reason using the NIC method.
  • Nowadays we measure both wake latency (WakeLatency) and (IntrLatency). The latter is the time it takes to the CPU to exit the C-state and execute the interrupt handler.

And finally, if you do not find the information you are looking for, feel free to ask questions by filing GitHub issues or sending an e-mail to Artem Bityutskiy <>.