Inter-process communication for real-time

On ARM you should use one process with more than one thread for this job.
Changing between processes hurts the ARM CPU as it needs to throw away its
cache content for *each* change (cache works on virtual address lines
instead physical ones)!
For example on an Xscale running at ~500MHz you can wake up one process in
your system every 100us (with HRT and RT Preempt). But with only one
additional process in the system it does not work anymore. The time
increases to 300us ... 500us (changing priority in this case does not help,
as this is a hardware restriction).

JB

So what about sharing data between the threads? In the past I have used
message queues, but I suspect they use more processor time than say, a
shared memory or pipe. I'd like to start off with the right architecture
since I don't have time to test all the possible configurations.

Sharing data between threads is always the most efficiency way. The POSIX
message queues are also part of the POSIX realtime library. But: Don't use
the IPC implementations of sharing memory and message queues!

Note: To benefit from the new kernel features you need a very recent EABI
toolchain for ARM. OABI toolchains do not help.

JB

This is true for "full" MMU Linux. If you use µCLinux, this ARM problem
is not triggered and the (soft) realtime performance will be better.

-Michael

Of course with µCLinux sharing data between processes is really simple :).

-Michael

If you can accept a "might be best" solution, this is soft realtime and
not hard realtime. Thus you are fine with Linux.

If you need a "must happen before..." solution this is hard realtime and
Linux will not suite you.

-Michael