scheduling real-time and non real-time processes

[sundaresan]

Hello All

I have a doubt in linux scheduling algorithm.
How does linux distinguishes a real-time and a non real-time process?
In other words, what are the characteristics that differentiates a real-time process from a non real-time process?

Thanks
Sundar

[matt_kleviar]

linux distinguishes real-time and noe real time based on its priority, if process is assigned realtime priority then it is of course realtime process.

at user perspective while submitting the job, it shjould be always clear that how much is important to be scheduled at its said releast time, meet it relative deadline. if they are strict and very less tolerance can be entertained then it is a soft-real-time process.
if deadline just can not be missed as it results in hazard then it is hard--real-time process.
it all depends on the system, need of the system and process running in it.

[sundaresan]

Matt
Thanks for your reply. Is it possible to run multiple real-time processes with the same priority but different scheduling mechanisms? Since linux maintains processes with same real-time priority in the same list, how does it schedules the processes with different scheduling mechanisms?

[matt_kleviar]

I think what you are refering to is rather called scheduling policy. I am not sure how linux schedules the realtime processes and exactly which algorithm it uses.
i think it may be different for system to system and need of the syetem.

like linux 2.6 kernel uses o(1) scheduling algorithm (for non real time process)
there may be real time patches which will change the scheduling policy of the scheduler.
I have not heard of such scheduler which follow different schedling policy for different priority of real time processes, but for sure the algorithm like rate montonic, dynamic priority algorithms could be used to define scheduling policy at system level.

let me know if you come to know that scheduler can also follow different scheduling policies at the same time for multiple realtime processes, I am not sure about that info.

[sundaresan]

You can set the priority and scheduling policy for the real-time processes using sched_setscheduler().