Rsyslog is heavily threaded to fully utilize modern multi-core processors. However, the imudp module did so far work on a single thread. We always considered this appropriate and no problem, because the module basically pulls data off the OS receive buffers and injects them into rsyslog's internal queues. However, some folks expressed the desire to have multiple receiver threads and there were also some reports that imudp ran close to 100% cpu in some installations.
So starting with 7.5.5, imudp itself supports multiple receiver threads. The default is to use a single thread as usual, but via the "threads" module parameter, up to 32 receiver threads can be configured. We introduced this limit to prevent naive users from totally overruning their system capability - spawning a myriad of threads usually is quite counter-productive (especially when they outnumber the available processor cores). For the same reason, I would strongly suggest that the number of threads is only increased if there is some evidence for this to be useful -- which usually means the imudp thread should require considerable CPU time. In order to aid the decision, I have also added new rsyslog statistics counters which permit monitoring of the worker thread activity.
We will now evaluate practical feedback from the new feature. One of the goals of this new enhancement is to limit the risk of UDP message loss due to buffer overrun, which we hope we have improved even without the need to select realtime priority.
Please note that 7.5.5 is at the time of this writing not yet released, so for the next couple of days the new feature is only available via building from the git master branch.