[mythtv-users] MythTV backend running on NVIDIA Ka-el, 35W replaced by 1W

[Brian Long]

On Tue, Mar 8, 2011 at 5:00 PM, Raymond Wagner <raymond at wagnerrp.com> wrote:

> On 3/8/2011 15:40, mythtv wrote:
> > I'm sure there are others who have their backend on 24/7.  Is anyone else
> > (esp the devs) intrigued by the idea of replacing a 35W CPU  (+
> additional
> > system power) with a 1W CPU with higher performance?
> >
> http://blogs.nvidia.com/2011/02/tegra-roadmap-revealed-next-chip-worlds-first-quadcore-mobile-processor/
>
> Take a close look at those pictures.  The ARM builds are done using
> heavily optimized settings and GCC 4.4.1.  The Intel builds are done
> using typical settings and GCC 3.4.6.  Just building with the (much
> much) newer 4.4.1 puts the Core 2 ahead of the ARM, and by using
> similarly optimized settings, the Core 2 is a good 50% more powerful
> than the ARM.  Also understand that this test is only performing integer
> math, and the ARM platform has traditionally had pathetic FPUs.
>
> So 66% of the performance at only 1W consumption, that's pretty good,
> right?  Well you're still not getting the whole story.  That is one
> measurement using a synthetic benchmark, which as clearly shown can be
> falsified, and it is done using a quad core part.  The scheduler is
> single threaded.  The independent backend and MySQL bits are done
> sequentially, and not in parallel.  The backend code is not parallel,
> and the sql calls are not something that can be broken into multiple
> threads well by the MySQL server.  On a single threaded workload, the
> ARM is now only 33% the performance of the Core 2.
>
> Commercial flagging is going to be a bit different, because the decoding
> and detection can be handled in independent threads.  Video decoding in
> North America is still either going to be MPEG2, which is single
> threaded, or H264 out of an HDPVR, which is single sliced and thus still
> single threaded.  The T7200 at 2.0GHz won't quite be capable of handling
> full bitrate HDPVR output in real time, so the ARM at less than half the
> performance per core won't come close.  If you intended to live with the
> scheduler constraints on an under powered backend, you would still want
> to have a separate machine (maybe your frontends) do your video
> processing for you.
>
> Now lets look at power consumption.  The Core 2 is rated at 34W TDP, but
> that's both cores at full speed, plus heavy cache use, it's absolute
> worst case scenario.  More realistically since this is largely single
> threaded, it's going to be closer to 20-25W while running the
> scheduler.  When it's finished in 1/3 the time of the ARM, it will drop
> back to low power mode, and being a laptop part, it will be well under
> 10W.  10W run non-stop, at average North American utility rates, equates
> to around $10/yr in power consumption.  Even the desktop processors can
> be downclocked when idle such that the entire system will run under 25W
> at the wall.  Do understand that a significant portion of the power
> consumption is going to be from the attached tuners, hard drives, and
> STBs (if you need analog capture), which are going to be the same
> regardless of what CPU you're using, and will likely end up consuming
> far more than that 10W idle power of a mobile Core 2.
>
> Let's take this a bit further.  This T7200 chip they're comparing
> against was a release part.  It's one of the original 65nm processors
> released in mid-2006 when the Core 2 line was first launched.  They're
> comparing their brand new not-yet-available processor, to one that's
> nearly five years old, and several generations passed on both the
> microarchitecture, and fabrication techniques.  I doubt they're even
> still available for purchase.  I'd like to see the comparison between a
> modern dual core Sandy Bridge part, with a 17W TDP and turbo speed of
> 2.7GHz, or quad core part with a 45W TDP and turbo speed of 3.4GHz
> (turbo being where one or more cores shut down to allow others to run at
> higher speed in the same power envelope).  It will be even more
> interesting the AMD Bulldozer parts due out in a few months, where the
> cores physically have a trench dug around them, with gating to allow
> whole chunks of the chip to be completely powered down, and even the
> high end eight and sixteen core parts are expected to have an idle
> consumption under 10W
>
> > I was surprised that I couldn't find any discussion of the backend
> running
> > on ARM but then again ARM CPUs have never had this kind of horsepower
> > before.
>
> There have been, but they've all come to the same conclusion that ARM is
> not sufficient high performance to recommend for a backend.  Individual
> users claimed it was 'good enough', but they had limited channel count,
> with one or few tuners, and were willing to put up with the minute or
> longer scheduler runs.
>
>
Nice evaluation, Raymond.

/Brian/
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