[mythtv-users] Raspberry Pi suitability for MythFrontend

tortise tortise at paradise.net.nz
Mon Feb 20 09:42:33 UTC 2012


On 20/02/2012 9:58 p.m., Raymond Wagner wrote:
> On 2/20/2012 03:01, tortise wrote:
>> A frontend typically only runs for the viewing time, the backend runs
>> all the time... I think the most power saving is actually to be gained
>> if the pi ran as a master backend
>
> Oh dear god no!  Read the first post I made at the very top of this
> thread again.  The ARM11 in the Pi, based off the ARMv6 architecture, is
> an in-order superscaler processor, with a relatively short pipeline and
> no speculative operation.  While all that simplicity means very low
> power consumption, it also means very low performance.  Branch
> prediction, memory prefetch, and out of order operation are what let you
> crank up the IPCs on a processor, and make that superscaler
> configuration worthwhile.
>
> In other words, that means on average, with code not heavily optimized
> for that architecture, the 700MHz ARM11 will run somewhere around the
> equivalent performance of a bottom end Pentium II.  Now you can avoid
> the jobqueue, and not run commflagging and transcoding, but you still
> have to deal with the scheduler.  Take my backend for example.  I have a
> fairly average system, with two dozen broadcast channels, three digital
> tuners, and 17 active recording rules.  On my 3.3GHz Phenom II,
> scheduling passes take 1.5-2 seconds typically, spiking to 6-7 seconds
> under load.  The scheduler is single threaded, so multiple cores make
> little difference, but the Phenom is still going to be 10-15x faster
> than the ARM.
>
> Under load, you're going to be looking at scheduler times up to a couple
> minutes.  The system will be under load when a frontend is in use, or a
> slave backend is recording.  If a change in data triggers a rerun of the
> scheduler at such a time, it will be disruptive of in-progress
> recordings, and will delay the start of new recordings.  A small system,
> with limited guide data, limited channels, and few tuners can be managed
> by a low powered machine such as that.  The problem is that MythTV is a
> hobby, and hobbies tend to grow over time.  The low performance of the
> Pi and SheevaPlugs is going to put them below the needs of most MythTV
> users.
>
>> I recall there was hope that frontend processing could be done with
>> vdpau and 256M however I've not seen anything about that recently, has
>> that died a natural death?
>
> That discussion was about VIDEO memory.  The VDPAU implementation in
> MythTV was not happy with video cards that only had 256MB of memory.  It
> wanted video cards with 512MB, twice that found in the higher end Pi,
> not to mention however much memory the PC that housed that 512MB video
> card had.
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I was not thinking the pi would be an effective scheduler. {"its 
relevant to note the scheduler also requires a lot of CPU grunt that the 
pi isn't likely to cut too well either (Shame the video processor cant 
be easily 'borrowed') "}
Anyway I for one appreciate your clarifications, thank you.


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