[Emc-developers] Difference RTAI 5.0 <--> PREEMPT-rt ?

Discussion:

Karlsson & Wang

2016-01-03 18:06:40 UTC

Do anybode know the difference between RTAI and PREEMPT-rt?

Regards Nicklas Karlsson

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Niemand Sonst

2016-01-03 18:59:41 UTC

Permalink

Yes,

in some words:

RTAI is much faster, but relies on the kernel you have, so it is much
more complicated to be included in software parts.
It is mostly needed only for very high speed applications

PREEMPT-RT is a a little bit slower than RTAI, but can be implemented in
software much easier, just imagine it is a modul.

That is very simple explained, there are more differences. For a normal
CNC opperation both should work fine. But if you use i.e. software
stepgenerator over a LPT, than take the RTAI.

Norbert

Post by Karlsson & Wang
Do anybode know the difference between RTAI and PREEMPT-rt?
Regards Nicklas Karlsson
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Peter C. Wallace

2016-01-03 19:49:57 UTC

Permalink

Date: Sun, 3 Jan 2016 19:59:41 +0100
Subject: Re: [Emc-developers] Difference RTAI 5.0 <--> PREEMPT-rt ?
Yes,
RTAI is much faster, but relies on the kernel you have, so it is much
more complicated to be included in software parts.
It is mostly needed only for very high speed applications
PREEMPT-RT is a a little bit slower than RTAI, but can be implemented in
software much easier, just imagine it is a modul.
That is very simple explained, there are more differences. For a normal
CNC opperation both should work fine. But if you use i.e. software
stepgenerator over a LPT, than take the RTAI.
Norbert

Depending on the hardware the differences between RTAI and Preempt-RT may be
minimal:

Loading Image...

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Both about 1/2 a day of YouTube videos (I favor the crackling fireplace ones)
and 10 glxgears each

I'll try some Skylake hardware when I can figure out how to build the latest
(4.4-rc6-rt1) Preempt-RT kernel

Note that both the latency test and latency histogram mainly display
_dispatch_ latency. Actual latency to _do_ anything (allocate memory, do I/O,
access the PCI bus, etc) will be significantly worse

Peter Wallace
Mesa Electronics

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Neil Whelchel

2016-01-03 19:24:17 UTC

Permalink

Hello,
To cut to the core, RTAI is basically an interrupt dispatcher that traps
interrupts and re-routes them up either through the RTAI (realtime)
modules, or for non realtime tasks back to the Linux kernel. RTAI also
provides an API for realtime functions, and floating point operations.
The general intent is to run realtime processes in the kernel.
PREEMPT-RT is a configuration option for the kernel to add preemption to
many more places in the kernel, in particular, it makes the spinlocks
preemptable, and it adds a priority tree (priority inheritance). It also
includes changes to the high resolution timer code. It does not introduce
any API, however all timers in user code are replaced with high resolution
timers. There are some issues with PREEMPT-RT and the VGA text console that
have not yet been corrected. (Anything that writes to the VGA text console
such as printk() introduce large unpredictable latencies.) This patch is
intended to run realtime processes in user space, but it also allows
realtime kernel modules to be used.
-Neil-

On Sun, Jan 3, 2016 at 10:06 AM, Karlsson & Wang <

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