Date: Sat, 22 Apr 2006 20:22:54 -0500 (CDT)
Subject: real time in user space
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The robot is back together with what appears to be a stable control 
system. It is not perfect. But when it works, it seems to work well. And 
when it doesn't work well, then it either is completely inoperative or it 
is obvious that things are in a bad state.

What I had thought to be new is really a very conventional approach. The 
Linux HOWTO for Yodaiken's RTLinux (now a commercial product) devotes a 
section to this technique. Here is what it looks like.

   user space process  ----realtime FIFO---->  special hard RT process

What I'm doing is very similar. The difference is that all scheduling is 
soft real time and entirely in user space. Robert Love asserts that this 
is often sufficient. He wrote the pre-emptive kernel patch for Linux. This
experience with the robot control system validates his assertion although 
I would add that significant system tuning and testing is an additional 
cost of this approach. The event processing loops and device I/O must be 
carefully managed in concert with process scheduling and priority in order 
to have good real time performance. This can be complicated. In a big 
system, it might be difficult or impossible to know that the system works 
in all situations.

I'm prepared for driving the robot tomorrow morning. I want to be out 
there when the sun rises with a full night's sleep. I've learned my lesson 
from last week. If you have to rush and lack confidence that the system 
will work properly, hold off and spend more time sleeping, in development, 
or testing.

The long command lines are now in shell scripts. So now it is much easier 
to get everything running. My hope tomorrow morning is to have a good run 
with sensor and control data logged against the same timebase with video. 
This will allow for better visualization of what is going on into the 
future. It is also a necessary first step for selecting image processing 
algorithms required for autonomous operation.