As Frank S. stated: what do you mean by realtime? HP sold a realtime operating system based on a proprietary microprogrammable processor for more than two decades called the HP 1000. As mentioned, realtime is usually defined as a deterministic response time to an external event, something virtually no Unix (or Windows) based system can claim. The problem is that the opsystem itself will preempt processes to handle special kernel events such as a core dump for a program or paging or some networking activities.
True, if you open the window of acceptable response time to dozens of seconds, then most Unices will get things started within that time frame, but I would NEVER use a typical Unix system for process control. For instance, the HP 1000 computers could guarentee a maximum response time of 1ms from the initiation of a priority interrupt on a card to the start of the driver to handle the interrupt. And if the driver was coded as a microprogrammed instruction, the response time could be as little as 15 microseconds.
For true realtime applications, programs almost never control devices directly because they are just too slow. Instead, the driver performs the interrupt handling and the program simply changes the behavior of the driver through special interfaces. And filesystems are only useful for slow (100's of ms) activities such as logs or configurations. Typical process control systems will be manipulating giant machines where instant response is not just desirable, it can be a serious safety concern.
So before you look for a realtime opsystem, you must clearly define the expected response times, issues about safety, the ability to collect or transmit data at a specific rate, and most important, the availability of development tools designed for realtime processing.
HP-UX has a realtime processing priority which must be used with care. For instance, setting a shell to a realtime priority and then running this simple script:
while :
do
:
done
will lock up the entire machine, allowing no one to login or run anything.
Bill Hassell, sysadmin
