[Xenomai-help] How to avoid a tight real time loop

[Xenomai-help] How to avoid a tight real time loop

[Daniele Nicolodi]

Hello. In my application I need to read data from an ADC card in blocks
of a given number of samples. For efficiency I'm using an mmapped buffer
to exchange data with the ADC driver. I'm setting up the acquisition,
and then calling a loop like this:

unsigned int required = <compute required number of bytes>;
unsigned int read = 0;
while (1) {
  while (read < required) {
    read = a4l_pool(...);
  }
  process(buffer, ...);
}

However if I run this loop in a real time thread the tight while loop is
an obvious cpu hog: the watchdog kicks in and kills the thread.

There is not an analogy API that can be used to require a certain amount
of data to the ADC driver. How can I code this loop to let other
processes to run?

Thanks. Cheers,
-- 
Daniele

Re: [Xenomai-help] How to avoid a tight real time loop

[Gilles Chanteperdrix]

Daniele Nicolodi wrote:
> Hello. In my application I need to read data from an ADC card in blocks
> of a given number of samples. For efficiency I'm using an mmapped buffer
> to exchange data with the ADC driver. I'm setting up the acquisition,
> and then calling a loop like this:
> 
> unsigned int required = <compute required number of bytes>;
> unsigned int read = 0;
> while (1) {
>   while (read < required) {
>     read = a4l_pool(...);
>   }
>   process(buffer, ...);
> }
> 
> However if I run this loop in a real time thread the tight while loop is
> an obvious cpu hog: the watchdog kicks in and kills the thread.
> 
> There is not an analogy API that can be used to require a certain amount
> of data to the ADC driver. How can I code this loop to let other
> processes to run?

You should define a period, and how much data you want to process during
this period. Then make your task periodic, and wait for the next period
at the beginning of the loop.

> 
> Thanks. Cheers,


-- 
					    Gilles.

Re: [Xenomai-help] How to avoid a tight real time loop

[Alexis Berlemont]

Hi,

Daniele Nicolodi wrote:
> Hello. In my application I need to read data from an ADC card in blocks
> of a given number of samples. For efficiency I'm using an mmapped buffer
> to exchange data with the ADC driver. I'm setting up the acquisition,
> and then calling a loop like this:
> 
> unsigned int required = <compute required number of bytes>;
> unsigned int read = 0;
> while (1) {
>   while (read < required) {
>     read = a4l_pool(...);
>   }
>   process(buffer, ...);
> }
> 
> However if I run this loop in a real time thread the tight while loop is
> an obvious cpu hog: the watchdog kicks in and kills the thread.
> 
> There is not an analogy API that can be used to require a certain amount
> of data to the ADC driver. How can I code this loop to let other
> processes to run?

As a matter of fact, there is something in the API which could have
helped you in an unmapped configuration. If you set the flag
A4L_CMD_BULK in the field "flags" of the command structure, a call to
a4l_async_read() only returns when the data size specified as argument
is available. 

Unfortunately, I have not done the same thing with a4l_poll, I might
have added the requested size as an additional argument to a4l_poll. 

Any idea on how to extend the API without breaking it ?

> 
> Thanks. Cheers,
> -- 
> Daniele
> 
> _______________________________________________
> Xenomai-help mailing list
> Xenomai-help@domain.hid
> https://mail.gna.org/listinfo/xenomai-help

-- 
Alexis.

Re: [Xenomai-help] How to avoid a tight real time loop

[Daniele Nicolodi]

On 16/06/10 00:11, Alexis Berlemont wrote:
>> There is not an analogy API that can be used to require a certain amount
>> of data to the ADC driver. How can I code this loop to let other
>> processes to run?
> 
> As a matter of fact, there is something in the API which could have
> helped you in an unmapped configuration. If you set the flag
> A4L_CMD_BULK in the field "flags" of the command structure, a call to
> a4l_async_read() only returns when the data size specified as argument
> is available.

Unfortunately copying memory data around would decrease too much the
performance of my data acquisition code. By the way, I think that using
the O_NONBLOCK fcntl flag on the device descriptor file descriptor would
be the best way to activate this feature.

> Unfortunately, I have not done the same thing with a4l_poll, I might
> have added the requested size as an additional argument to a4l_poll. 
> 
> Any idea on how to extend the API without breaking it ?

I do not see any way rather than augmenting the function signature with
an additional parameter. If you want to maintain API compatibility the
only thing to do is to add a different function.

Cheers,
-- 
Daniele

Re: [Xenomai-help] How to avoid a tight real time loop

[Gilles Chanteperdrix]

Alexis Berlemont wrote:
> Hi,
> 
> Daniele Nicolodi wrote:
>> Hello. In my application I need to read data from an ADC card in blocks
>> of a given number of samples. For efficiency I'm using an mmapped buffer
>> to exchange data with the ADC driver. I'm setting up the acquisition,
>> and then calling a loop like this:
>>
>> unsigned int required = <compute required number of bytes>;
>> unsigned int read = 0;
>> while (1) {
>>   while (read < required) {
>>     read = a4l_pool(...);
>>   }
>>   process(buffer, ...);
>> }
>>
>> However if I run this loop in a real time thread the tight while loop is
>> an obvious cpu hog: the watchdog kicks in and kills the thread.
>>
>> There is not an analogy API that can be used to require a certain amount
>> of data to the ADC driver. How can I code this loop to let other
>> processes to run?
> 
> As a matter of fact, there is something in the API which could have
> helped you in an unmapped configuration. If you set the flag
> A4L_CMD_BULK in the field "flags" of the command structure, a call to
> a4l_async_read() only returns when the data size specified as argument
> is available. 
> 
> Unfortunately, I have not done the same thing with a4l_poll, I might
> have added the requested size as an additional argument to a4l_poll. 
> 
> Any idea on how to extend the API without breaking it ?

Maybe it would be possible to use the standard "select" call, with a
threshold configurable with an ioctl ?


-- 
					    Gilles.

Re: [Xenomai-help] How to avoid a tight real time loop

[Alexis Berlemont]

Gilles Chanteperdrix wrote:
> Alexis Berlemont wrote:
> > Hi,
> > 
> > Daniele Nicolodi wrote:
> >> Hello. In my application I need to read data from an ADC card in blocks
> >> of a given number of samples. For efficiency I'm using an mmapped buffer
> >> to exchange data with the ADC driver. I'm setting up the acquisition,
> >> and then calling a loop like this:
> >>
> >> unsigned int required = <compute required number of bytes>;
> >> unsigned int read = 0;
> >> while (1) {
> >>   while (read < required) {
> >>     read = a4l_pool(...);
> >>   }
> >>   process(buffer, ...);
> >> }
> >>
> >> However if I run this loop in a real time thread the tight while loop is
> >> an obvious cpu hog: the watchdog kicks in and kills the thread.
> >>
> >> There is not an analogy API that can be used to require a certain amount
> >> of data to the ADC driver. How can I code this loop to let other
> >> processes to run?
> > 
> > As a matter of fact, there is something in the API which could have
> > helped you in an unmapped configuration. If you set the flag
> > A4L_CMD_BULK in the field "flags" of the command structure, a call to
> > a4l_async_read() only returns when the data size specified as argument
> > is available. 
> > 
> > Unfortunately, I have not done the same thing with a4l_poll, I might
> > have added the requested size as an additional argument to a4l_poll. 
> > 
> > Any idea on how to extend the API without breaking it ?
> 
> Maybe it would be possible to use the standard "select" call, with a
> threshold configurable with an ioctl ?

Thanks. I added your idea in my TODO list.

> 
> 
> -- 
> 					    Gilles.

-- 
Alexis.