Re: Anyone using Ada with Real-Time Linux?

[dmitry@elros.cbb-automation.de (Dmitry A. Kazakov)]

On Tue, 27 Nov 2001 17:06:01 +0100, Florian Weimer <fw@deneb.enyo.de>
wrote:

>dmitry@elros.cbb-automation.de (Dmitry A. Kazakov) writes:
>
>> [We are considering which OS platform to choose for an embedded "soft"
>> real-time system. Our requirements are: 5 ms for the DAQ & control
>> task and almost "no matter" for the TCP/IP tasks]
>
>If the 5ms are a *requirement*, it's not "soft" real time. ;-)

It is a "requirement" (:-)). [ You know how it works, there are two
ways to deal with problems: 1) an obsolete one is to solve them, 2) a
biz one is to change the requirements. ]

>(And you shouldn't use a standard Linux version.)

Standard Linux has obvious advantages when the hardware configuration
should remain open. I mean driver support etc.

>>>Whether this would fit the requirement I don't know; there might be
>>>unavoidable delays, as Dmitry says.
>>
>> I heard that the Linux kernel is (or will be) preemptive. If so, there
>> [I suppose] should be no big problem with I/O?
>
>Usually, I/O isn't the problem.  Some FS operations and console
>switching are major offenders, IIRC.

The word "usually" is the major concern. Under WinNT it is also
usually no problem, but sometimes, some driver may block the whole
system for 100-200ms.

>Perhaps you don't need a preemptive kernel.  For 2.2.x, there are
>patches which add additional scheduling points to lower average
>latency (but I don't know if you get an exact 5ms timing, perhaps with
>a kernel module using the real-time clock interrupt).

About 2-3 years ago I played with i486, 2MB under Linux + gnat (I do
not remember which version it was). It easily reached "soft" 1ms with
parallel console output. By "soft" I mean that the "real-time" task
becomes active within each 1ms interval.

Regards,
Dmitry Kazakov