[Xenomai-help] task management, multi-thread, multi-core

[Xenomai-help] task management, multi-thread, multi-core

[Everett Wang]

Hi All,

I am working on a navigation program that has two realtime tasks. One
is a high priority one executed in a higher frequency. Another task
runs about 1/10th of the first one, but requires substantially more
CPU computations. I am using an atom board which has dual threads
capability. I am worried that the second task can block the execution
of the first one until it finishs all the calculations. Can this
happen in a dual thread and/or dual core CPU? If yes, should I break
the task two into small chunks and let the task one to send a trigger
to
execute one chunk at a time? I know it is not xenomai specific
question and I have tried to read manual or other documents trying to
figure
it out.

I have read that tasks require more resources than threads in Linux.
The inter-threads communication is also more efficient than that of
tasks. Are these also true for xenomai realtime OS? Should I create
threads instead of tasks?


Any suggestions are welcome. Thanks for reading. :-)

Everett

Re: [Xenomai-help] task management, multi-thread, multi-core

[Gilles Chanteperdrix]

Everett Wang wrote:
> Hi All,
> 
> I am working on a navigation program that has two realtime tasks. One
> is a high priority one executed in a higher frequency. Another task
> runs about 1/10th of the first one, but requires substantially more
> CPU computations. I am using an atom board which has dual threads
> capability. I am worried that the second task can block the execution
> of the first one until it finishs all the calculations. Can this
> happen in a dual thread and/or dual core CPU? If yes, should I break
> the task two into small chunks and let the task one to send a trigger
> to
> execute one chunk at a time? I know it is not xenomai specific
> question and I have tried to read manual or other documents trying to
> figure
> it out.

You did not tell us how was the priority of the second task, but since
you told us that the first task was high priority, I have to guess that
the second task is lower priority.

On an uni-processor machine, the priorities are used by the scheduler to
schedule your tasks, so, if a low priority task could prevent a high
priority task from running, the scheduler would need to be fixed.

On a multi-processor machine, you have to set the "affinity" of the
tasks to pin the tasks on a given cpu. On each cpu, tasks are scheduled
following their priorities.

> 
> I have read that tasks require more resources than threads in Linux.
> The inter-threads communication is also more efficient than that of
> tasks. Are these also true for xenomai realtime OS? Should I create
> threads instead of tasks?
> 
> 
> Any suggestions are welcome. Thanks for reading. :-)

task is an informal term which may mean process or thread depending on
the context. So, I guess what you mean is "process" instead of task. The
difference between processes and threads is that changing context
between processes requires changing the memory mapping, which is an
heavy operation. On the other hand, processes provide an isolation that
threads do not provide. So, choosing threads or processes is not only a
matter of performances (otherwise, we would all run Linux without MMU,
since in that case, everything is essentially a thread).

> 
> Everett
> 
> _______________________________________________
> Xenomai-help mailing list
> Xenomai-help@domain.hid
> https://mail.gna.org/listinfo/xenomai-help
> 


-- 
					    Gilles.

Re: [Xenomai-help] task management, multi-thread, multi-core

[Everett Wang]

On Wed, Jul 7, 2010 at 6:21 PM, Gilles Chanteperdrix
<gilles.chanteperdrix@xenomai.org> wrote:
> Everett Wang wrote:
>> Hi All,
>>
>> I am working on a navigation program that has two realtime tasks. One
>> is a high priority one executed in a higher frequency. Another task
>> runs about 1/10th of the first one, but requires substantially more
>> CPU computations. I am using an atom board which has dual threads
>> capability. I am worried that the second task can block the execution
>> of the first one until it finishs all the calculations. Can this
>> happen in a dual thread and/or dual core CPU? If yes, should I break
>> the task two into small chunks and let the task one to send a trigger
>> to
>> execute one chunk at a time? I know it is not xenomai specific
>> question and I have tried to read manual or other documents trying to
>> figure
>> it out.
>
> You did not tell us how was the priority of the second task, but since
> you told us that the first task was high priority, I have to guess that
> the second task is lower priority.
>
> On an uni-processor machine, the priorities are used by the scheduler to
> schedule your tasks, so, if a low priority task could prevent a high
> priority task from running, the scheduler would need to be fixed.
>
> On a multi-processor machine, you have to set the "affinity" of the
> tasks to pin the tasks on a given cpu. On each cpu, tasks are scheduled
> following their priorities.

Thanks Gilles, you are so nice. Yes, the second task is at lower priority. I did
state accurately. If the second task is running already, when the
higher priority
task is waiting for an outside event, then the event comes, the high
priority task
has to wait until the lower priority one to finish?

>>
>> I have read that tasks require more resources than threads in Linux.
>> The inter-threads communication is also more efficient than that of
>> tasks. Are these also true for xenomai realtime OS? Should I create
>> threads instead of tasks?
>>
>>
>> Any suggestions are welcome. Thanks for reading. :-)
>
> task is an informal term which may mean process or thread depending on
> the context. So, I guess what you mean is "process" instead of task. The
> difference between processes and threads is that changing context
> between processes requires changing the memory mapping, which is an
> heavy operation. On the other hand, processes provide an isolation that
> threads do not provide. So, choosing threads or processes is not only a
> matter of performances (otherwise, we would all run Linux without MMU,
> since in that case, everything is essentially a thread).
>

You are right again. Sorry for the confusing. I meant process instead
of the task.
Do you mean that if I need memory isolation, I use process, if I am sure my code
has no memory problem, I should choose thread for better performance? Or it
is more complex than this?

Have a nice day,

Everett
>>
>> Everett
>>
>> _______________________________________________
>> Xenomai-help mailing list
>> Xenomai-help@domain.hid
>> https://mail.gna.org/listinfo/xenomai-help
>>
>
>
> --
>                                            Gilles.
>

Re: [Xenomai-help] task management, multi-thread, multi-core

[Gilles Chanteperdrix]

Everett Wang wrote:
> If the second task is running already, when the
> higher priority
> task is waiting for an outside event, then the event comes, the high
> priority task
> has to wait until the lower priority one to finish?

No. Xenomai scheduler is preemptive:
http://en.wikipedia.org/wiki/Preemptive_multitasking

> Do you mean that if I need memory isolation, I use process, if I am sure my code
> has no memory problem, I should choose thread for better performance? Or it
> is more complex than this?

You can never be sure that code has no memory problem. It is more
complex than that, for instance, whether the threads are inter-dependent
should be taken into account.

-- 
					    Gilles.

Re: [Xenomai-help] task management, multi-thread, multi-core

[Everett Wang]

On Wed, Jul 7, 2010 at 7:24 PM, Gilles Chanteperdrix
<gilles.chanteperdrix@xenomai.org> wrote:
> Everett Wang wrote:
>> If the second task is running already, when the
>> higher priority
>> task is waiting for an outside event, then the event comes, the high
>> priority task
>> has to wait until the lower priority one to finish?
>
> No. Xenomai scheduler is preemptive:
> http://en.wikipedia.org/wiki/Preemptive_multitasking

That is why I thought to cut the lower priority but more CPU comsuming
task into smaller pieces and only let it run for one piece at a time
then wait so
higher priority task don't have to wait. Once the higher priority task has
done its thing, then send a trigger to the let the lower priority task to run
one more chunk. Is this the right thing to do or I am totally off the track?

Everett

>> Do you mean that if I need memory isolation, I use process, if I am sure my code
>> has no memory problem, I should choose thread for better performance? Or it
>> is more complex than this?
>
> You can never be sure that code has no memory problem. It is more
> complex than that, for instance, whether the threads are inter-dependent
> should be taken into account.
>
> --
>                                            Gilles.
>

Re: [Xenomai-help] task management, multi-thread, multi-core

[Gilles Chanteperdrix]

Everett Wang wrote:
> On Wed, Jul 7, 2010 at 7:24 PM, Gilles Chanteperdrix
> <gilles.chanteperdrix@xenomai.org> wrote:
>> Everett Wang wrote:
>>> If the second task is running already, when the
>>> higher priority
>>> task is waiting for an outside event, then the event comes, the high
>>> priority task
>>> has to wait until the lower priority one to finish?
>> No. Xenomai scheduler is preemptive:
>> http://en.wikipedia.org/wiki/Preemptive_multitasking
> 
> That is why I thought to cut the lower priority but more CPU comsuming
> task into smaller pieces and only let it run for one piece at a time
> then wait so
> higher priority task don't have to wait. Once the higher priority task has
> done its thing, then send a trigger to the let the lower priority task to run
> one more chunk. Is this the right thing to do or I am totally off the track?

You are off the track. As I said, the scheduler is preemptive.

-- 
					    Gilles.

Re: [Xenomai-help] task management, multi-thread, multi-core

[Everett Wang]

On Wed, Jul 7, 2010 at 8:51 PM, Gilles Chanteperdrix
<gilles.chanteperdrix@xenomai.org> wrote:
> Everett Wang wrote:
>> On Wed, Jul 7, 2010 at 7:24 PM, Gilles Chanteperdrix
>> <gilles.chanteperdrix@xenomai.org> wrote:
>>> Everett Wang wrote:
>>>> If the second task is running already, when the
>>>> higher priority
>>>> task is waiting for an outside event, then the event comes, the high
>>>> priority task
>>>> has to wait until the lower priority one to finish?
>>> No. Xenomai scheduler is preemptive:
>>> http://en.wikipedia.org/wiki/Preemptive_multitasking
>>
>> That is why I thought to cut the lower priority but more CPU comsuming
>> task into smaller pieces and only let it run for one piece at a time
>> then wait so
>> higher priority task don't have to wait. Once the higher priority task has
>> done its thing, then send a trigger to the let the lower priority task to run
>> one more chunk. Is this the right thing to do or I am totally off the track?
>
> You are off the track. As I said, the scheduler is preemptive.
>

Sorry. my mistake. This makes my life so much easier though. :-) Thanks
Gilles, I have no more question. Have a nice day.


>                                            Gilles.
>