8.5. Multicast Considerations

When multiple applications listen to a multicast group, the kernel code that handles multicast frames is required by design to duplicate network data for each individual socket. This duplication is time-consuming and occurs in the softirq context.

Adding multiple listeners on a single multicast group therefore has a direct impact on the softirq context's execution time. Adding a listener to a multicast group implies that the kernel must create an additional copy for each frame received for that group.

The effect of this is minimal at low traffic volume and small listener numbers. However, when multiple sockets listen to a high-traffic multicast group, the increased execution time of the softirq context can lead to frame drops at both the network card and the socket queue. Increased softirq runtimes translate to reduced opportunity for applications to run on heavily-loaded systems, so the rate at which multicast frames are lost increases as the number of applications listening to a high-volume multicast group increases.

Resolve this frame loss by optimizing your socket queues and NIC hardware buffers, as described in Section 8.4.2, “Socket Queue” or Section 8.4.1, “NIC Hardware Buffer”. Alternatively, you can optimize an application's socket use; to do so, configure the application to control a single socket and disseminate the received network data quickly to other user-space processes.