[mythtv-users] Are there any throttling features in Mythtv?

Yeechang Lee ylee at pobox.com
Tue Feb 3 22:20:50 UTC 2009


Mike Perkins <mikep at randomtraveller.org.uk> says:
> Yup, did that, see [2] above.

Yes, but you had not previously tested three recording streams plus
one playback stream, which is the configuration that got you into
trouble! In other words, your testing was not complete, and therefore
useless except inasmuch it gave you some baseline for a "known to
work" configuration you can retreat to, if desired. I'll address this
point later.

> How long a test do you suggest I make?

A half hour will do.

* Schedule three simultaneous recordings, preferably on commercial
  channels so multiple commflagging can be tested, too.
- mythfrontend being a little sluggish is OK, but does it "stick"
  anywhere while traversing through Watch Recordings?
- Try 'iostat -xk 2'. How is disk utilization?
- Try 'top' on the backend. How much CPU is being used? What is the wa
  percentage?
* Try playing back some recordings, including the three recordings
  under way plus others.
- Are there any playback skips?
- How is jumping to random points within the recordings?
- A few IOBOUND messages appearing in mythbackend.log are OK, but
  having lots and lots is a bad sign for the sanctity of your
  recordings.
- Repeat the iostat and top tests. (Note, though, that the output from
  iostat and top in this sort of test really isn't as important as how
  the system overall "feels.")
* Once the recordings are complete, check them.
- How do their stated durations compare with the actual recording
  times? On my setup, for example, it's normal for a recording to be
  about 5 (due to channel-changing/tuning overhead) to about 90
  seconds (depending on whether framerates vary within it) short.
- There's probably no need to actually sit down and watch each
  recording from start to finish, but again do random jumps within
  each. Any missing frames? Macro blocks? Missing sound chunks?

> As for how is mythtv supposed to know that my system can't handle 3
> (actually 4), but your can handle > 8, that's straightforward. Have
> a limit such that # of input + # of output streams <= # of tuners as
> a default, with it being a backend setup field that can be adjusted
> by the user as experience of the system improves. Again, just a
> thought idea.

Please, no. I'm firmly on Mike Dean's side when he notes (as he has a
couple of times over the years) that there are way, way, way too many
settings already.

That doesn't mean I never want new settings put into MythTV. I'd love
#4242 to make it into the code (come to think of it, it
would likely help someone in your situation), for example. It does
mean that any new setting has to solve a problem that can't be readily
solved in another way. Your situation--of throwing too many tuners at
a system that evidently can't handle all of them at once--is not an
example of such.

You responded to Kevin Kuphal (after he sagely observed that "It's up
to you not to configure your system to exceed it's capacities") with:
> But sometimes, particularly after a reschedule, things can get
> recorded when you least expect it, and if you've had the front end
> going for a while, it's easy to miss a change that might affect the
> load.

Your personality is different from mine. Although I have neither kids
nor spouse I still like keeping the "WAF" high, and part of that means
eliminating any factor that poses a meaningful risk of destablizing
the system.

If I knew that using all of my four tuners at once posed a real risk
of crashing something, *I would disable one of the tuners*. That way I
wouldn't have to worry that the scheduler would ever create a
dangerous sitaution. A big benefit of any MythTV setup is not having
to think about when a program airs, so why would I intentionally
reintroduce that dilemma into my life in any way?

-- 
Frontend:		P4 3.0GHz, 1.5TB software RAID 5 array
Backend:		Quad-core Xeon 1.6GHz, 6.6TB sw RAID 6
Video inputs:		Four high-definition over FireWire/OTA
Accessories:		47" 1080p LCD, 5.1 digital, and MX-600


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