priority of VIs

[Max Weiss]

I have two independent VIs running on my PC.
One for DAQ and one for displaying my data.
The DAQ-VI should be very fast, because I do not want to have any
timeouts longer than 20ms -> I want to sample with 50Hz without using a
DMA or a Fifo. The other VI is just displaying the data in four charts
(transfered by a queue).
I set the priotity of the DAQ-VI to "time critical priority (highest)"
and the priority of the other VI to "normal priority".

When the two VIs are running and I display no data in the VI with
"normal priority" it works well. But when I display data (updating only
every 2th second), I have timeouts with more than 20ms.
I set the priority of the DAQ-VI to "normal priority" and I had not much
more timeouts.
How powerful is t
he priority setting?
Can I make it faster with changing the setting in "Preferred Execution
System"?

My comupter works with Windows ´95 and I´m using LV 5.0 . I know that
Windows has those timeouts, but can´t I do something to reduce them to
20ms or even 10ms??

It becomes faster with only 800x600 points and 256 colours, but I want
to use the full screen.
--
Ciao
Max


* Max Weiss*Adlerstr.22*76133 Karlsruhe*0721/3842835*Germany *
* max@mvmpc9.ciw.uni-karlsruhe.de*DB8MWE *

[Walter Petermann]

Max Weiss wrote:
>
> I have two independent VIs running on my PC.
> One for DAQ and one for displaying my data.
> The DAQ-VI should be very fast, because I do not want to have any
> timeouts longer than 20ms -> I want to sample with 50Hz without using a
> DMA or a Fifo. The other VI is just displaying the data in four charts
> (transfered by a queue).
> I set the priotity of the DAQ-VI to "time critical priority (highest)"
> and the priority of the other VI to "normal priority".
>
> When the two VIs are running and I display no data in the VI with
> "normal priority" it works well. But when I display data (updating only
> every 2th second), I have timeouts with more than 20ms.
> I set the priority of the DAQ-VI to "normal priority" and I had not much
> more timeo
uts.
> How powerful is the priority setting?
> Can I make it faster with changing the setting in "Preferred Execution
> System"?
>
> My comupter works with Windows ´95 and I´m using LV 5.0 . I know that
> Windows has those timeouts, but can´t I do something to reduce them to
> 20ms or even 10ms??
>
> It becomes faster with only 800x600 points and 256 colours, but I want
> to use the full screen.
> --
> Ciao
> Max
>
> * Max Weiss*Adlerstr.22*76133 Karlsruhe*0721/3842835*Germany *
> * max@mvmpc9.ciw.uni-karlsruhe.de*DB8MWE *


Max,
I'm new to Labview, but it has been my experience that 15 ms sampling
is about the limit with _nothing_ else going on. This is with a
regular DAQ card. Maybe it would work with the specialized signal
processing card (I forget the actual name of the card).

Walter

[Greg_McKaskle2]

> I have two independent VIs running on my PC.
> One for DAQ and one for displaying my data.
> The DAQ-VI should be very fast, because I do not want to have any
> timeouts longer than 20ms -> I want to sample with 50Hz without using a
> DMA or a Fifo. The other VI is just displaying the data in four charts
> (transfered by a queue).
> I set the priotity of the DAQ-VI to "time critical priority (highest)"
> and the priority of the other VI to "normal priority".
>
> When the two VIs are running and I display no data in the VI with
> "normal priority" it works well. But when I display data (updating only
> every 2th second), I have timeouts with more than 20ms.
> I set the priority of the DAQ-VI to "normal priority" and I had not much
> more timeouts.
> How powerful is the priority setting?
> Can I make it faster with changing the setting in "Preferred Execution
> System"?
>
> My comupter works with Windows �95 and I�m using LV 5.0 . I know that
> Windows has those timeouts, but can�t I do something to reduce them to
> 20ms or even 10ms??
>
> It becomes faster with only 800x600 points and 256 colours, but I want
> to use the full screen.

Your priority implementation is only as good as the OS that you, and
LV are implementing it on. Win95 priorities work OK, but will they
be able to interrupt GDI calls to draw a bitmap? Apparently not.

The separate VIs are communicating with each other, and that is also
causing overhead. You say that with 256 colors and a smaller screen,
that you can do this. I'd suggest that you move the DAQ and display
into the same loop and run both at an elevated priority, but not
time-critical. I don't think that you will have any problem doing
this change, and since LV already makes the screen update be asynch
with the diagram, the diagram gets priority.

If you want to keep the architecture the way you currently have it,
you may also want to run some tests to see how long the data transfer
is taking between the two subVIs. Is it through a subVI, a global,
or a queue.

Greg McKaskle

[Max Weiss]

> The separate VIs are communicating with each other, and that is also
> causing overhead. You say that with 256 colors and a smaller screen,
> that you can do this. I'd suggest that you move the DAQ and display
> into the same loop and run both at an elevated priority, but not
> time-critical. I don't think that you will have any problem doing
> this change, and since LV already makes the screen update be asynch
> with the diagram, the diagram gets priority.
>
Why not "time-critical"? When do you have to use this setting?
I will make some tests with one and with two loopes...

> If you want to keep the architecture the way you currently have it,
> you may also want to run some tests to see how long the data transfer
> is taking between the two subVIs. Is it th
rough a subVI, a global,
> or a queue.
I start the DAQ-VI with the VI-Server and I´m using a queue for data
transfer.
So the DAQ is absolutely independent and has not to wait until the other
VI has taken the last data.


--
Ciao
Max


* Max Weiss*Adlerstr.22*76133 Karlsruhe*0721/3842835*Germany *
* max@mvmpc9.ciw.uni-karlsruhe.de*DB8MWE *

[Greg_McKaskle2]

> > The separate VIs are communicating with each other, and that is also
> > causing overhead. You say that with 256 colors and a smaller screen,
> > that you can do this. I'd suggest that you move the DAQ and display
> > into the same loop and run both at an elevated priority, but not
> > time-critical. I don't think that you will have any problem doing
> > this change, and since LV already makes the screen update be asynch
> > with the diagram, the diagram gets priority.
> >
> Why not "time-critical"? When do you have to use this setting?
> I will make some tests with one and with two loopes...
>

You can use time-critical, but only in pretty important situations
where you are sure to give time back to the OS by using a wait
or a timeout function.
If you don't give time back to the OS,
then the other LV and OS threads will starved out. This can lead
to not being to abort the VI, not being able to see any displays,
change any buttons, move windows, the start button not working, etc.

So, use time-critical only when needed, and only when you have a
definite way of controlling it.

Greg McKaskle

[Martin_Henz]

I never tried to do a timed data aquisition in a simple loop without
using a hardware timed aquisition. That's not the right way to do it.

But I made a view tests for this timing issue, because sometimes
others have similar questions.

The tests are running on a 450MHz PIII machine. No cpu consuming
tasks are running during the tests.
I aquired 1 sample of one channel with the AI Read One Scan.vi
in each iteration. I don't had any delay's inside the loop. In
addition to the data aquisition, the loop has to calculate the
loop-execution time and has to find out the maximum loop-execution-time.
The two-VI solution passes the data through LabVIEW queue-VIs.
The data was displayed on a standard-sized waveform chart (I didn't
change the size of the chart after opening it from the palette).

The data aquisition VI is running in the data aquisition thread, the other
VI in the standard thread. I found out, that only the average execution
time was different when using higher or lower priorities. The maximum
execution time was always (nearly) the same. So I decided to use normal
priority for both VIs.

If I use the flatten to string function to get a string for the queue-VIs,
the
maximum time was around 15ms. The loop-time was most of the time
below 2ms. Only a view loops are rising up to 15ms. I executed the VIs
for a one minute period.

I made some additional tests to optimize the timing.
flatten to string -> 15ms
type-cast (instead of the flatten to string) -> 12ms
one-VI solution ->12ms

The loop times are much higher, when I moved a window with the mouse.
The max. time rised up to 50ms and more.

I moved to a 2x 500MHz PIII machine running Windows NT. This PC
didn't actually have a DAQ board, so I used a remote DAQ device over
network (a RDA-Server solution). But with this solution the loop time rised
up in a view cases (about each 15seconds) to 50ms and more. But in most
cases, the execution time was 3ms or below.
I moved the DAQ board to this machine and tried again... and got a
max. loop time of 4ms.
By the way, the two VI solution was running smother but not faster or
slower.
On this machine, it doesn't change the timing, when moving a window
during the aquisition.


Martin


Martin Henz Systemtechnik
Dipl. Ing. (FH) Martin Henz
Walchensee Str. 3
70378 Stuttgart
Tel. ++49-711-5302605
Fax ++49-711-5302605
http://www.mhst.de

Max Weiss schrieb in im Newsbeitrag:
383E82A6.F0E731C1@mvmpc9.ciw.uni-karlsruhe.de...
> I have two independent VIs running on my PC.
> One for DAQ and one for displaying my data.
> The DAQ-VI should be very fast, because I do not want to have any
> timeouts longer than 20ms -> I want to sample with 50Hz without using a
> DMA or a Fifo. The other VI is just displaying the data in four charts
> (transfered by a queue).
> I set the priotity of the DAQ-VI to "time critical priority (highest)"
> and the priority of the other VI to "normal priority".
>
> When the two VIs are running and I display no data in the VI with
> "normal priority" it works well. But when I display data (updating only
> every 2th second), I have timeouts with more than 20ms.
> I set the priority of the DAQ-VI to "normal priority" and I had not much
> more timeouts.
> How powerful is the priority setting?
> Can I make it faster with changing the setting in "Preferred Execution
> System"?
>
> My comupter works with Windows �95 and I�m using LV 5.0 . I know that
> Windows has those timeouts, but can�t I do something to reduce them to
> 20ms or even 10ms??
>
> It becomes faster with only 800x600 points and 256 colours, but I want
> to use the full screen.
> --
> Ciao
> Max
>
>
> * Max Weiss*Adlerstr.22*76133 Karlsruhe*0721/3842835*Germany *
> * max@mvmpc9.ciw.uni-karlsruhe.de*DB8MWE *
>