John Michael Clemens wrote:
>
> Greetings,
>
> sorry if this winds up a double/triple post, it doesn't seem to have gone
> though the first time, and my mailer's giving me fits...
>
> I've been put in charge of a project to use RT-Linux to do some hard
> real-time image processing at 30-60 frames per second. What I need to do
> is run a userspace program to analyse one image, and somehow interrupt
> that program at every 3 ms (for 30 fps) to do some accounting and book
> keeping, and then interrupt it every 33 ms to tell it to move onto the
> next image. I'm at a loss for how to go about doing it.
>
> I can set up two timers in the kernel (one with a period of 3 ms and the
> other with a period of 33), but how do I get these timered interrupts to
> interrupt the user space process? More importantly, how can I guarantee
> that the user space program has spent at least most of the time actually
> running? (The target system is a 4-way SMP P3 system).. I don't want to
> run the processing code in the kernel because it's a progressive algorithm
> that would never yield (do I understand that correnctly? it will process
> as much of the image as possible and then move on when it's told, so no
> user space processes would ever run??).
>
> Any ideas would be a great help... I'm really at a loss for this one, and
> I'll admit i'm still coming up to speed in the real-time world..
>
> thanks,
> john.c
>
Hi John,
You can't guarantee anything in a normal userspace process regarding
timing, so you do need to run your progressive algorithm in RT-kernel
space. You are correct that would lock out Linux/user space if this
does not yield, so you need to carefully architect your design.
My initial suggestion would be to run the progressive task as a low
priority RT task (without making it periodic), and have a slightly
higher priority manager RT task control this. This would let you
control the progressive task without interfering with with the 3/33 ms
tasks. Using this control, you can force the progressive task to
suspend for a set time in a given period, thus giving Linux/userspace
some time in which it can be run.
There are many other approaches that could be used (such as locking
particular processing tasks to one of the SMP processors). I suspect
other on the list will have some good suggestions.
Regards, Stuart.
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