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Old 09-19-2010, 11:40 AM   #1
trist007
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A question about tickless system...


By enabling this kernel option does this make my machine a realtime machine?

What does this really mean?

In theory, when a process runs that process runs in realtime meaning it has full control of the system. So theoretically, the process would release control once the process end.

However, what would happen if that process running in realtime crashes? Then it would never release the command to stop and hand the CPU over to another process ending up in a total freeze up

Is this understanding correct?

What are some applications of realtime kernel? Midi sequencing? Audio synching?
 
Old 09-19-2010, 01:16 PM   #2
salasi
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Quote:
Originally Posted by trist007 View Post
By enabling this kernel option does this make my machine a realtime machine?

What does this really mean?

In theory, when a process runs that process runs in realtime meaning it has full control of the system. So theoretically, the process would release control once the process end.

However, what would happen if that process running in realtime crashes? Then it would never release the command to stop and hand the CPU over to another process ending up in a total freeze up

Is this understanding correct?

What are some applications of realtime kernel? Midi sequencing? Audio synching?
No, you have misunderstood, probably both real time and tickless. Essentially, tickless is (strictly, enables) a lower power idle state, although this old testing doesn't show much difference. And this is unconnected with real time.

(Although this RH document seems to show them unconcerned about whether it is advantageous or not.)

On the other hand, real time is all about deadlines; can you guarantee that a certain thing will occur by some specified deadline and how important is it, if that does not occur (do people die. which should really be avoided at all costs or is it just a bit undesirable, so it is worth taking measures to minimise, but if it happens infrequently, it might not be a big problem?).

Sometimes people think that realtime is equivalent to fast, and that is a mistake.

Realtime is often relevant to embedded control applications and (oddly) to enterprise database applications in which deadlines for (and coherency of) transfer of numbers with lots of zeros at the end is relevant. And, music, as you mentioned...although choppy music reproduction doesn't kill all that many, on an annualised basis.
 
Old 09-19-2010, 01:21 PM   #3
tommylovell
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Nope. salasi stated it very well and I agree, not a realtime machine.

Tickless kernel is more about sampling/accounting accuracy, cpu power conservation,

From Redhat,
http://www.redhat.com/docs/en-US/Red...ss-kernel.html
Quote:
3.4. Tickless Kernel
Previously, the Linux kernel periodically interrupted each CPU on a system at a predetermined frequency 100 Hz, 250 Hz, or 1000 Hz, depending on the platform. The kernel queried the CPU about the processes that it was executing, and used the results for process accounting and load balancing. Known as the timer tick, the kernel performed this interrupt regardless of the power state of the CPU. Therefore, even an idle CPU was responding to up to 1000 of these requests every second. On systems that implemented power saving measures for idle CPUs, the timer tick prevented the CPU from remaining idle long enough for the system to benefit from these power savings.
The kernel in Red Hat Enterprise Linux 6 runs tickless: that is, it replaces the old periodic timer interrupts with on-demand interrupts. Therefore, idle CPUs are allowed to remain idle until a new task is queued for processing, and CPUs that have entered lower power states can remain in these states longer.
You need a realtime kernel. Google for "Linux realtime kernel". You'll see a number of articles
https://rt.wiki.kernel.org/index.php...inux_kernel.3F is a good one.
But many have argued that Linux really is incapable of being a true realtime system and at best is a near-realtime system. Many very subtle points are argued that I don't pretend to understand.
 
Old 09-19-2010, 09:11 PM   #4
jefro
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Aren't there or isn't there already real time option in the latest kernel? For some reason I think I read that.
 
Old 09-20-2010, 06:56 AM   #5
salasi
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Quote:
Originally Posted by trist007 View Post
B
What does this really mean?
You might like these articles on realtime. maybe, you'll find that these are from another perspective, but they could still be helpful:

http://www.eetimes.com/design/automo...me-performance

http://www.eetimes.com/design/other/...-Simple-Tasker

http://www.eetimes.com/design/automo...rmance--Part-1
 
  


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