Mailinglist Archive: opensuse-factory (498 mails)

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Re: [opensuse-factory] Real Time on openSUSE
  • From: Jason Newton <nevion@xxxxxxxxx>
  • Date: Fri, 27 May 2016 19:56:45 -0400
  • Message-id: <CAGou9MhNezGQoPXLpy=JDOk-6Fq+1o4oJwPhcyCuy2qnoz8pRg@mail.gmail.com>
On Fri, May 27, 2016 at 10:27 AM, Todd Rme <toddrme2178@xxxxxxxxx> wrote:

On Thu, May 26, 2016 at 6:02 PM, Jason Newton <nevion@xxxxxxxxx> wrote:
As a former heavy user of real time preempt patch and RTAI, I found for many
tasks I no longer needed it in the last few years. I'd recommend putting in
some fine grained auditing for time of computations/io stages and measure
the timing/jitter/stats as time series to analyze system behavior. You
might find it works perfectly fine for you with the desktop kernel.
Basically the timing jitter we're talking here, assuming you've set your
schedulers, split things up appropriately, and tuned the slow/problematic
spots, down to a millisecond or under. I've had IO jitter, through use of
tmpfs or carefully written IO to XFS be very low jitter as well - but this
is a larger discussion (which the RT patch wont solve) if you have large
datarates (several hundred MiB/s).

Millisecond timing jitter is a problem for what we are doing.


Well my point is that YMMV and it very much depends on what you are
doing, but you can get submillisecond jitter - I was trying to be
conservative in what you might see but I don't think I ever found a
clear lower limit, I'd guess in the 10s of microseconds. It will
depend on the hardware you have too, and then you need to swallow the
fact that you're not on a hard real-time OS still, so going
submillisecond is risky buisiness and you should at least consider
Xenomai for those cases. I guess I can only say measure to make sure
you need such a patch... and really you should measure even when you
use the patch, too.



I honestly don't think it's worth the effort to maintain it in a repo with
all the other kernels since the versions it works with are out of sync with
their stable release.

RTAI is out-of-sync with stable releases, but RT-Preempt is kept in
sync. For example the 4.6 patch set was released before the 4.6
kernel was released.


My point was on the RT-Prempt patch and it didn't always follow so
dutifully, nor is it required to. I haven't kept up it so I don't
know in the last few years how deterministic they are anymore with
releases, but I recall them missing kernel releases in the past and
then there was the stable series patches and sort of next gen patches.
That's alot of extra complexity to track and deal with for a handful
of users who have within their capability to simply compile the
kernels themselves. Further, these users usually stick with the
kernel they get working for a fairly long time.



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