[PD] PD and latency on kernel

IOhannes m zmölnig zmoelnig at iem.at
Sat Nov 19 09:09:33 CET 2005

Christian Klippel wrote:
> hello iohannes and all,
> Am Samstag 19 November 2005 00:13 schrieb IOhannes m zmoelnig:
> [...snip...]
>>please note, that the word "latency" is nowhere mentioned. it has
>>nothing to do with "realtime". (you can have a hard realtime system that
>>has a delay of 1 week).
>>so pd is by design a realtime system.
> ouch! that comment about hard realtime is plain wrong.
> hard realtime is used in systems where you need minimal delay, like engine 
> controls, medical systems, etc.

i guess i was unclear (at least i was cryptic)
"real-time" does say _nothing_ about the "amount of latency".
however, the important thing is, that "realtime" guarantees processing 
within a "given latency" (however large that may be).

> imagine your life support systems would use hard realtime that can delay up to 
> a week .....
> the most important aspect of hard realtime systems (like rtlinux) is that they 
> have the lowest latency possible!

no, the most important thing is, that they have a maximum latency aka 
"deadline" that must not be exceeded.

> also, see here: http://en.wikipedia.org/wiki/Real-time_computing

quoting this artile:

"It is important to note that hard versus soft real-time does not 
necessarily relate to the length of time available. A machine may 
overheat if a processor does not turn on cooling within 15 minutes (hard 
real-time). On the other hand, a network interface card may lose 
buffered data if it is not read within a fraction of a second, but the 
data can be resent over the network if needed, without affecting a 
critical operation, perhaps without a delay noticeable to the user."

but then i am no real time specialist.


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