[mythtv-users] MythTV backend running on NVIDIA Ka-el, 35W replaced by 1W
Raymond Wagner
raymond at wagnerrp.com
Tue Mar 8 22:00:16 UTC 2011
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.
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