Comparison between real time kernels

hello

I am newcommer to the world of linuxcnc. I have read some articles, and I am in confusion about somethings.

I understood that most of the available binaries of linuxcnc versions are based on RTAI realtime core, while in some articles, they referred that the main development in coming releases will stop for RTAI and will continue for Realtime preemption patch .
on the other hand, I have read a paper that claims that Xenomai has the best and fastest realtime core.

I am in some confusion. If I will use linuxcnc for revamping an old CNC milling machine, what will be the best choice?
initially, I downloaded RTAI core to discover how can I do the needed task (as I said I am new so I am just testing the waters currently), but I discovered that RTAI with ethernet-master only supports certain ethernet cards, while on the other hand the RT preemt supports any ethernet card that is supported by linux itself, so this made me change to RT preemt. And now I heared about Xenomai, and I have no idea if it ha same problem of supporting certail ethernet cards or not.

can anyone help me to stop this confusion?

Thank you for your answer.
i have several questions in fact:

1) how was your experience with RT-Preempt? can you tell me more about the specs of the application that you used RT preempt in ?

2) I have read about RT-Preempt with Linux Uspace , but I do not understand , what is "Uspace" ?
I have read a paper that makes comparison between "RT-Preempt with Linux Uspace" and "RT-Preempt with Linux Kernel" , what is the difference between them??
I have downloaded debian-7.11.0-i386-netinst.iso and I will install and patch it with the RT-Preempt patch, this is considered which type of what i mentioned?


3) I am intending to use it with ethernet-master to use it with ethercat drives, so this operation will interface with the ethernet card, can Xenomai be used to communicate with generic ethernet cards and to have realtime behavior as RT-preempt ?

4) you said that Xenomai support is very recent, this is drawback, but it is real that it is way more fast and real time than RT-Preement and RTAI ??

thank you very much

I think all the LinuxCNC Ethercat work is being done with RTAI, so if you want ti use Ethercat, I would use RTAI

Note that for relatively smart external hardware the real time portion of LinuxCNC need not run faster than
a 1 KHz or so thread rate so all three real time kernel options work well

Very fast thread rates are typically only needed for software step generation or very specialized machines

Its quite PC dependent but very roughly latency times are RTAI < Xemomai < Preempt-RT