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This varies. On a multicore CPU it's possible that processes will actually be pinned to a core and those processes will have exclusive access to the CPU core (i

This varies. On a multicore CPU it's possible that processes will actually be pinned to a core and those processes will have exclusive access to the CPU core (i.e nothing else will be scheduled on it). Real-time programs tend to avoid syscalls, etc. which cause a context switch so it is indeed possible to run a realtime program indefinitely without the OS stepping in. It's tricky though because even things like dynamic memory allocation can have non-realtime behavior.

zepearl 8y ago
>>Real-time programs tend to avoid syscalls, etc. which cause a context switch so it is indeed possible to run a realtime program indefinitely without the OS stepping in. I guess that context switches would have to happen if e.g. 2 realtime programs, with the same priority etc..., would run concurrently on the same CPU - or would "the running program" (the one that managed to start first) be allowed by the OS(/HW?) to take total ownership of the CPU until it finishes running?
- wtallis 8y ago
  That "until it finishes running" bit is why you can't tackle realtime at just the kernel level or just within the process. You need to also consider the whole system—ensuring that the services you're trying to run as realtime processes actually fit within your hardware's performance envelope. If you have processes that need to be pinned to a CPU core and have exclusive use of that core, then you need to ensure that you don't try to run more of those on the system than you have CPU cores available to dedicate. If you're timesharing cores between multiple processes, you need to know that the processes sharing cores have timing requirements that can be satisfied by your available resources. The human in charge of the overall system design needs to be figuring out things like how many time slices per second each realtime process needs and how many microseconds of CPU time it needs in each time slice to complete its work.
kllrnohj 8y ago
> Real-time programs tend to avoid syscalls, etc. which cause a context switch That's not why they avoid syscalls. If you are running on a real time schedule there won't be a context switch in response to a syscall. They may, however, get blocked on kernel mutexes. So syscalls are avoided for the same reason user-space mutexes are avoided - being real-time doesn't help if you're blocked on a mutex.
shaklee3 8y ago
At least in Linux, there are still os threads that will run on pinned, isolated CPUs. There's work being done to allow only the user thread to run, but last I checked it wasn't complete.

zepearl 8y ago

>>Real-time programs tend to avoid syscalls, etc. which cause a context switch so it is indeed possible to run a realtime program indefinitely without the OS stepping in. I guess that context switches would have to happen if e.g. 2 realtime programs, with the same priority etc..., would run concurrently on the same CPU - or would "the running program" (the one that managed to start first) be allowed by the OS(/HW?) to take total ownership of the CPU until it finishes running?

wtallis 8y ago
That "until it finishes running" bit is why you can't tackle realtime at just the kernel level or just within the process. You need to also consider the whole system—ensuring that the services you're trying to run as realtime processes actually fit within your hardware's performance envelope. If you have processes that need to be pinned to a CPU core and have exclusive use of that core, then you need to ensure that you don't try to run more of those on the system than you have CPU cores available to dedicate. If you're timesharing cores between multiple processes, you need to know that the processes sharing cores have timing requirements that can be satisfied by your available resources. The human in charge of the overall system design needs to be figuring out things like how many time slices per second each realtime process needs and how many microseconds of CPU time it needs in each time slice to complete its work.

kllrnohj 8y ago

> Real-time programs tend to avoid syscalls, etc. which cause a context switch That's not why they avoid syscalls. If you are running on a real time schedule there won't be a context switch in response to a syscall. They may, however, get blocked on kernel mutexes. So syscalls are avoided for the same reason user-space mutexes are avoided - being real-time doesn't help if you're blocked on a mutex.

shaklee3 8y ago

At least in Linux, there are still os threads that will run on pinned, isolated CPUs. There's work being done to allow only the user thread to run, but last I checked it wasn't complete.