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Subject: Re: Increasing scan rate to 10 kHz
From: Till Straumann <[email protected]>
To: Andrew Johnson <[email protected]>, Evgeniy <[email protected]>
Cc: EPICS Tech Talk <[email protected]>
Date: Wed, 08 Jan 2014 08:36:26 -0800
It should be noted that with RT_PREEMPT the real-time threads
all execute in *user-mode*. They can run side-by-side with other
low-priority stuff in the same process and still get scheduled as
needed, preempting any other (lower-priority) thread or process
with low latency. Hence it is not necessary to play special tricks
with putting high-priority code into kernel modules etc.

- Till


On 01/08/2014 08:20 AM, Till Straumann wrote:
On 01/07/2014 02:19 PM, Andrew Johnson wrote:
Periodic scans are quite complicated, since they rely on the
underlying operating system to implement a delay timer and reschedule
the scan thread after the specific delay has elapsed. Most OSs don't
provide high accuracy delay scheduling, so I don't expect we will ever
be able to run periodic scan threads at 10KHz. - Andrew
As a matter of fact I don't think that 10kHz periodic scans are a big
problem for something like RTEMS
or linux (with RT_PREEMPT) on a reasonably fast CPU.

I just used a simple 'clock_nanosleep' based loop (based on code by
windriver, modified by myself)
which does something like

while ( 1 ) {
      /* get current time */
        clock_gettime( CLOCK_MONOTONIC, &wakeup );
      /* compute wakeup time 100us from now   */
        timespec_add_100us( &wakeup );
      /* sleep until wakeup time      */
        clock_nanosleep( CLOCK_MONOTONIC, TIMER_ABSTIME, &wakeup, NULL );
      /* see when we actually woke up */
        clock_gettime( CLOCK_MONOTONIC, &now );
      /* missed = now - wakeup        */
        timespec_diff( &missed, &now, &wakeup );

      compute_statistics( &missed );
}

for a bunch of threads (using the SCHED_FIFO scheduler).

On my laptop (vanilla kernel, no RT_PREEMPT) I observe worst-case
'missed' deadline of several milli-seconds
but even on that system the running average is between 5us..50us.

On a kernel patched with RT_PREEMPT I get a worst-case missed deadline
by ~10us (average around 3-4us)
after running for a few minutes (on a not heavily loaded intel core
i7-2655LE, 2.2 GHz).

I would expect RTEMS/vxWorks to even perform a bit better.

- Till


Replies:
Re: Increasing scan rate to 10 kHz Andrew Johnson
References:
Re: Increasing scan rate to 10 kHz Andrew Johnson
Re: Increasing scan rate to 10 kHz Till Straumann

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