SMP and Embedded Real Time

As embedded real-time applications start to run on SMP systems, kernel issues emerge.

No article mentioning the -rt patchset would be complete without a note of thanks to Ingo Molnar, Thomas Gleixner, Sven Deitrich, K. R. Foley, Gene Heskett, Bill Huey, Esben Neilsen, Nick Piggin, Steven Rostedt, Michal Schmidt, Daniel Walker and Karsten Wiese. I also owe a debt of gratitude to Ted Ts'o, Darren Hart, Dinakar Guniguntala, John Stultz, Vernon Mauery, Jennifer Monk, Sripathi Kodi, Tim Chavez, Vivek Pallantla and Hugh Miller for many valuable real-time-Linux words and deeds. I am likewise grateful to David Bacon and his real-time-GC-research team and to Boas Betzler for many productive conversations. We all owe Bruce Jones, John Kacur and Mark Brown many thanks for their invaluable service rendering this article human-readable. Finally, many thanks go to Daniel Frye for his unstinting support of this effort.

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Paul E. McKenney is a Distinguished Engineer with IBM's Linux Technology Center. He has worked on NUMA, SMP and real-time algorithms and, in particular, RCU for longer than he cares to admit. In his spare time, he jogs and supports the usual house-wife-and-kids habit.



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a question

hpare's picture

I have a question about that "interrupt" discribed in figure 6-8.
Could you tell me if this kind of interrupt happens on one CPU, from cpu catch a INTn do tophalf instructions to deal with the blue rectangle(maybe a softirq() of bottomhalf),do all of these was executed by one CPU?
waiting for your explanation!
thank you!

Threaded interrupts

paulmck's picture

There is a small portion of code that happens in the "top half", or hard irq context. On a non-PREEMPT_RT system he actual interrupt handler code would also execute in hard irq context. However, in PREEMPT_RT, the handler instead executes at process level in a kernel thread executing at real-time priority.

If this handler uses a bottom half, or softirq, then the softirq will be scheduled as another kernel thread, also executing at real-time priority.

The softirq interface is such that the softirq handler executes on the same CPU where the raise_softirq() request ran, Normally the system would be configured so that the hard irq and irq handler ran on the same CPU as well. (I believe that it can be configured otherwise, but I don't know of a good reason to do so.)

Great article, really interesting stuff

Adeel's picture

In addition, there are real-time audio systems, SIP servers and object brokers...

Can you give an example of rt audio/sip/object broker software/projects?

Also, has the -rt patch set had any impact on networking in linux? e.g. latency, iptables traversal time, etc

Would a standard program, e.g. X11, have a performance benefit on -rt compared to a non-rt system?

Examples of RT audio, SIP, object brokers...

paulmck's picture

There are a number of open-source audio projects. Two that come to mind immediately are Jack and Pulse audio, both of which were enthusiastic about testing out the -rt patchset. The only RT SIP servers that I am aware of are proprietary, ditto with object brokers.

There has been some effect of -rt on networking, but many real-time applications use lower-level protocols (such as UDP) or special transports (such as Infiniband) in order to retain greater control over latency. That said, there are special real-time protocols, such as the DDS suite.

Usually, real-time operating systems are designed for responsiveness, and usually give up throughput performance in favor of responsiveness. For one look at this issue, see my recent OLS paper on real time vs. real fast.


szkolenia biznesowe's picture

Nice article

thanks :)

Glad you liked it!

Paul E. McKenney's picture