HTPC strategy - undervolt /underclock or not?

[babstar]

I have a mythtv HTPC box (AMD XP 1600+) running for approx 2 years and looking to replace it. I have observed the following:
CPU Load (All recording SD DVB-T & playback SD):

• *~3.5% idle or recording only.
  * 17~25% Playback only (Content Dependant)
  * 100% transcode or transcode/playback or transcode/playback/record. Transcoding takes approx 2x show length (no simultaneous playback)

Typical usage pattern:

• * Typically 4 hours of recordings per day requiring transcoding, therefore 8 transcoding hours.
  * Playback ~ 4 hours per day.
  * Playback will almost always occur simultaneously with transcoding.
  * Machine will be on 24/7

In other words, this box is effectively quantized to only 3 states: idle, 20% or 100%, and would spend approx 10 to 14 hours at 100%.

From this it follows that a faster machine will still peg the processor(s) at 100% but accomplish the transcoding more quickly (I would expect 3200+ would transcode approx 1:1).
...So the question is, is it better to go with an undervolt/ underclock system that spends longer at 100% or a standard system, which will finish the transcode quicker & then sit at idle. I also understand that the heat peak produced will be lower, but last longer on the under spec system.
In other words does undervolting and or underclocking have any effect on the global efficiency of the system, given that a fixed amount of work needs to be accomplished & the machine will spend the balance of the day at idle?
Next, which is the most efficient CPU (watts per unit work) for this type of application?
The reason I ask is that here in AU the range if motherboards is more limited. I cannot always get the recommended motherboards (my preferred supplier only has 1 socket 754 board ASUS K8V-VM & 1 socket 939 board). Therefore I may not be go with an undervoltable board.
Which would be a better choice, standard 754 sempron or undervoltable AM2 (which CPU)? Will I be able to use passively cpu cooling?

[Max Slowik]

I'd say it depends on the amount the processor can be undervolted by. Often Cool'N'Quiet saves as much or more than undervolting; of course AM2+ w/its split power plane will be even more efficient for HTPCs when it's available.

Next, which is the most efficient CPU (watts per unit work) for this type of application?

None as much as the video cards/ TV tuners that can replace them. Good expansion cards take over 90+ % of the load, meaning that what's left for the CPU is just keeping the rest of the system going. I think I logged CPU usage of my HTPC for a week and the only time it hit 100% was when it was installing Windows Updates.

As another reference, recording a show pegs the CPU to about 20%; recording two shows brings the stress up to about 35%, watching a show: another 10%. Add to it an overzealous idle of 5%, and the machine peaks at 70%. I can do that and rip a CD and still have some spare CPU using an Athlon 64 3200. . .

So get yourself some good co-processing. And AM2 X2 3600 Brisbanes cost, what, $60 on a bad day? I spent so much more when I bought that 3200 and it was one of the best. . .

[Max Slowik]

Oh, and the TV tuner converts to .dvr-ms (mpeg 2 and other junk) real-time.

[babstar]

Max, thanks for your posts.
We already get mpeg-2 streamed directly over-the-air (OTA) with DVB-T, so on the input side their is no processing.
However, mythtv has an option to further shrink the mpeg2 stream (OTA SD runs at ~1.8 Gb/hour) to ~700Mb/hour for high quality & ~350Mb/hour for medium - this is what I mean by transcoding. I am not aware of any DVB-T cards that do this transcoding on-the-fly so it must be done by the CPU post recording.
Transcoding is great for storing more hours on the HD - but at a computational cost.
My understanding is that Windows Media Centre doesn't actually do this transcoding natively (third part tools will) thats why your unlikely to see it hit 100% (please correct me if I am wrong).
Playback can partially be offloaded to the GPU with XvMC, but even without it a 3200 should use take 5-10% CPU use.
So really, its the transcoding that is the big computational task.

[Spare Tire]

Cool n' Quiet + undervolting is always good without any evident performance drawback. I all my computers by default. Underclocking, i'm not sure just how much it saves, but in my head the machine will always consume as much at a baseline, i haven't seen any numbers for underclocking and there is one thing for sure and that is it's does have a performance impact so for me i never underclock.

[merlin]

Most cpus can actually both undervolt and overclock at the same time in moderation, just a matter of balance. I would not recommend underclocking unless you're going for extremely low voltages, it does not reduce power much in comparison to undervolting and you'll just have a slower system. The easiest thing to do is to keep your cpu at stock clock and find the lowest stable voltage.

[Max Slowik]

I'm know that the NVIDIA DualTuner does the transcoding and compressing on the fly, and also, if you have an ATI X1K or higher video card, you can offload transcoding to it; yeah, there's still a little overhead, but realistically. . .

Unfortunately, I can't say what the Linux support is for those; I'm pretty much an MCE kind of guy.

I can tell you that Intel does much better at transcoding than AMD right now. If you can wait for Phenom you'll see AMD doing a little better.

There is one aBit motherboard that uses socket M chips. That will no doubt give you the best possible transcode units/watt, if you don't mind spending $200 on a processor. Furthermore, the playback is on a 945GM chipset, that is to say, not great. No full-size PCI-Express, either.

[babstar]

Thanks Max,
I saw your good info on the concurrent HTPC thread.

[bean1975]

There is one aBit motherboard that uses socket M chips. That will no doubt give you the best possible transcode units/watt, if you don't mind spending $200 on a processor. Furthermore, the playback is on a 945GM chipset, that is to say, not great. No full-size PCI-Express, either.

Asus N4L and MSI 945GT Speedster A4R both sport full sized PCI-E x16 slots and work with Yonah/Merom. Prolly there are others.

[Max Slowik]

Hey, thanks, that's good to know. I've been wanting to do a MoDT article, but have had to back-burner a lot, and I guess I haven't stayed in the loop.

[Steep]

babstar, just as a speed reference...

My HTPC is running a 65W amd x2 4600+ (1 gig/ram) with cool 'n quiet, and it converts mpeg2 videos into compressed Xvid in 2:1 time for standard definition content. (30 minute show takes about 15 minutes to transcode). Note, however, that my mpeg2 source is 480x480 SVCD format rather than DVD resolutions that some recorders use.

For people that do transcoding and/or commercial skipping, I definitely recommend getting the beefiest low-watt cpu you can find. When the horsepower isn't needed, the speed and voltage will be throttled down anyway.

[babstar]

Steep,
I assume that your system still pegs at 100% when transcoding. What sort of temps do you see & what cooling setup do you have?

[Steep]

Steep,
I assume that your system still pegs at 100% when transcoding. What sort of temps do you see & what cooling setup do you have?

Yes, 100%

Here is a description of the system. xp-120 heatsink, temps reported under multiple configurations. Only change is that now both case fans are attached to the system controlled fan header via a splitter.

Picture in this thread.