In order for the EtherCAT bus to comply with all real-time time limits, the Linux operating system of the master must also provide a certain real-time performance. The RT performance of the master is measured with the maximum jitter time in microseconds.

The requirements vary depending on the system. Basically the maximum jitter should be less than 100us. Depending on the system, a larger jitter can be tolerated or a smaller jitter is necessary.

In networks with distributed clocks (DC) which are operated in MasterShift mode, all clocks are synchronized with a clock from a slave. The master only has to send the EtherCAT packet early enough. The exact timing is not relevant. Fundamentally the following applies:
Calculation time of the control algorithm + Calculation time of the EtherCAT master + Maximum jitter < Period length

If the network is operated in BusShift mode or without DC, the RT requirements are higher. In such systems the OS provides the clock for the whole system.
max. jitter OS = min jitter EtherCAT bus

See Distributed Clocks for more information.

The Cyclictest is a software that can be used to easily measure the maximum jitter of a system. This page explains how to install and use the software.