Hard and soft real time

Posted Nov 4, 2010 13:34 UTC (Thu) by zmower (subscriber, #3005)
In reply to: Hard and soft real time by csimmonds
Parent article: KS2010: Deadline scheduling

I was watching videos from the Ada conference the other day. They were considering multicore processors and it seems that the interaction between the processors and various caches is such that these systems can no longer be considered to be predictable. One speaker even went so far as to say "Hard realtime is dead" which got a few lighthearted boos!

So would you like to change your statement to embedded hard realtime on unicore processors is a very small part of what linux should do?

Hard and soft real time

Posted Nov 4, 2010 15:36 UTC (Thu) by Shewmaker (guest, #1126) [Link] (1 responses)

Research into optimal real-time multiprocessor scheduling is not dead.
Here's a well written paper from this year.

DP-FAIR: A Simple Model for Understanding Optimal Multiprocessor Scheduling
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5562...

A short overview presentation of that paper.
https://systems.soe.ucsc.edu/sites/default/files/webform/...

This same research group is also working on solving the other parts of the problem. Linus is correct, we can't solve these problems with just CPU scheduling.

Efficient Guaranteed Disk Request Scheduling with Fahrrad (2008)
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1...

Work on memory and network resources is in earlier stages, but the result will hopefully be a coherent general theory of real-time performance management.

Hard and soft real time

Posted Nov 4, 2010 20:15 UTC (Thu) by zmower (subscriber, #3005) [Link]

I think this falls into the category of too simple models. In particular I would questions the assumptions that tasks are independent and that there's no cost overhead in context switches or CPU migrations. Would that it were so easy.

As for linux, even if you have optimal scheduling for all the subsystems, the combined effect is still chaotic.

Ada conference videos etc here http://www.disca.upv.es/jorge/ae2010/outcome.html