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Re: soft real-time on linux

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On 2016-11-12, Randy Yates <yates@digitalsignallabs.com> wrote:

> I was told by someone on the ##kernel channel that if the kernel is
> compiled with "preemptive" enabled, there is a way to set a thread's
> priority to "real-time" and it will not only have priority over other
> user-land threads but also will preempt much of the kernel's internal
> threads. This sounds promising.

In my experience (on a 400MHz ARM9), if you build the kernel with all
the pre-emptable options, and run your task at the highest real-time
priority then latency between an external event that causes an
interrupt and your user task waking up and running is going to be
somwhere in the 100-300 microsecond range.  [Assuming the interrupt
routine and device driver require minimal processing.]

That, of course, depends on the exact set of drivers you have enabled.
All it takes is one long critical section in a poorly written driver
to run that into many hundreds of microseconds.

I would be shocked if it ever got up to 1ms.

Unless you disable the rx FIFO, I would guess that the vast majority
of the latency is going to be in the UART itself.  With the rxFIFO
enabled, there's often a delay of several tens of bit-times between
the last data byte and the receive interrupt.

-- 
Grant Edwards               grant.b.edwards        Yow! Zippy's brain cells
                                  at               are straining to bridge
                              gmail.com            synapses ...

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soft real-time on linux Randy Yates <yates@digitalsignallabs.com> - 2016-11-12 08:53 -0500
  Re: soft real-time on linux Grant Edwards <invalid@invalid.invalid> - 2016-11-12 17:16 +0000
  Re: soft real-time on linux Mel Wilson <mwilson@the-wire.com> - 2016-11-12 17:33 +0000
  Re: soft real-time on linux upsidedown@downunder.com - 2016-11-12 20:34 +0200
  Re: soft real-time on linux Don Y <blockedofcourse@foo.invalid> - 2016-11-12 13:53 -0700
    Re: soft real-time on linux sbattazzo@gmail.com - 2017-02-09 11:06 -0800
  Re: soft real-time on linux Paul Rubin <no.email@nospam.invalid> - 2016-11-12 14:12 -0800
  Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-08 21:17 -0500
    Re: soft real-time on linux Jack <jack4747@gmail.com> - 2017-02-09 00:16 -0800
      Re: soft real-time on linux David Brown <david.brown@hesbynett.no> - 2017-02-09 10:00 +0100
      Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-10 11:12 -0500
    Re: soft real-time on linux George Neuner <gneuner2@comcast.net> - 2017-02-11 01:33 -0500
      Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-10 22:10 -0500

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