Context Switch in Real Time Linux Vs VxWorks (or any other RTOS)

Context Switch in Real Time Linux Vs VxWorks (or any other RTOS)

[Arun Prasad Velu]

Hi,

I am in the process of analyzing the feasibility of migrating an existing
VxWorks system (MPC860 target) to Linux (RTOS).

I'd like to compare the features of Linux and VxWorks with respect to the
Real Time capabilities. I am little concerned about Linux's Context
Switching.

I'd like to get your opinion on this. Even if I want to move to Linux, I'll
have to convince my management. So, any pointers on comparison of Linux Vs
VxWorks with respect to Real Time capabilities would be highly appreciated.

Thanks and Regards
Arun


Arun Prasad Velu,
Team Lead (Systems Software),
Omnee System Solutions India Pvt. Ltd.,
Abundance Networks Division,
E-401, Crystal Plaza, New Link Road,
Andheri (West), Mumbai - 400 053
Tel - +91-22-5692 5159 / 2673 3299
E-mail : arun@ossi.co.in
http://www.abundancenetworks.com
http://www.osscorporation.com


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Re: Context Switch in Real Time Linux Vs VxWorks (or any other RTOS)

[Oliver Korpilla]

Arun Prasad Velu wrote:
>
>I am in the process of analyzing the feasibility of migrating an
>existing VxWorks system (MPC860 target) to Linux (RTOS).
>
>I'd like to compare the features of Linux and VxWorks with respect to
>the Real Time capabilities. I am little concerned about Linux's Context
>Switching.
>
>I'd like to get your opinion on this. Even if I want to move to Linux,
>I'll have to convince my management. So, any pointers on comparison of
>Linux Vs VxWorks with respect to Real Time capabilities would be highly
>appreciated.

I hardly guess context switching will be your main problem, because the
stock Linux kernel does not support hard realtime!

You either have to use RTAI (free kernel modification that requires
drivers to be rewritten for realtime), RTLinux (nearly the same,
non-free, patented) or modified Linux kernels from TimeSys or MontaVista
(who both add real-time capabilities in a more stock-Linux-compliant way).

Depending on your target, eCos (a free embedded operating system with a
focus on minimal code) may be the better choice for you, especially if
your platform is already supported.

Be warned: Not all PowerPC processors are supported in eCos (last time I
checked 74xx and 7xx were missing).

With kind regards,
Oliver Korpilla

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RE: Context Switch in Real Time Linux Vs VxWorks (or any other RTOS)

[Kevin P. Dankwardt]

There are a number of approaches to real-time in Linux. There are also
several different task/thread/process models from which to choose. Some
involve kernel space and some allow for user space. Some are open source and
some are proprietary. One has to decide on a number of factors before
choosing the solution best suited.

If you are comfortable with the flat memory model of VxWorks then a kernel
space solution may be acceptable. Kernel space solutions such as RTAI have
quite low "context switch" and quite fast interrupt response time.

For more information I'd recommend starting by reading (my) articles from
Embedded Linux Journal:

http://www.linuxdevices.com/articles/AT5997007602.html
http://www.linuxdevices.com/articles/AT5503476267.html
http://www.linuxdevices.com/articles/AT6320079446.html

Best of luck,
Kevin Dankwardt




>I'd like to compare the features of Linux and VxWorks with respect to the
>Real Time capabilities. I am little concerned about Linux's Context
>Switching.

>I'd like to get your opinion on this. Even if I want to move to Linux, I'll
>have to convince my management. So, any pointers on comparison of Linux Vs
>VxWorks with respect to Real Time capabilities would be highly appreciated.


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Re: Context Switch in Real Time Linux Vs VxWorks (or any other RTOS)

[Gary Thomas]

On Fri, 2004-07-16 at 00:50, Oliver Korpilla wrote:
> Arun Prasad Velu wrote:
> >
> >I am in the process of analyzing the feasibility of migrating an
> >existing VxWorks system (MPC860 target) to Linux (RTOS).
> >
> >I'd like to compare the features of Linux and VxWorks with respect to
> >the Real Time capabilities. I am little concerned about Linux's Context
> >Switching.
> >
> >I'd like to get your opinion on this. Even if I want to move to Linux,
> >I'll have to convince my management. So, any pointers on comparison of
> >Linux Vs VxWorks with respect to Real Time capabilities would be highly
> >appreciated.
>
> I hardly guess context switching will be your main problem, because the
> stock Linux kernel does not support hard realtime!
>
> You either have to use RTAI (free kernel modification that requires
> drivers to be rewritten for realtime), RTLinux (nearly the same,
> non-free, patented) or modified Linux kernels from TimeSys or MontaVista
> (who both add real-time capabilities in a more stock-Linux-compliant way).
>
> Depending on your target, eCos (a free embedded operating system with a
> focus on minimal code) may be the better choice for you, especially if
> your platform is already supported.

Thanks for the plug :-)  [n.b. I am one of the main eCos developers]
FYI, look to http://sources.redhat.com/ecos for details.

>
> Be warned: Not all PowerPC processors are supported in eCos (last time I
> checked 74xx and 7xx were missing).

Ah, but they are simple to add (and have been, but not necessarily
public) as they are effectively PPC60x processors with some differences.

--
Gary Thomas <gary@mlbassoc.com>
MLB Associates


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