Switching one core of Athlon64 X2 off when not necessary?

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gamebox
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Joined: Mon Sep 22, 2014 9:21 am

Switching one core of Athlon64 X2 off when not necessary?

Post by gamebox » Thu Nov 20, 2014 3:19 pm

Hello all, I'm back with some more time-machine questions. :)

I currently have a Soc.939 Athlon 64 3500+ processor on WinFast nForce4 motherboard with 2x1GB Corsair XMS CL2 DDR400 ram. The CPU is 67W TDP one with CnQ enabled. I'm considering upgrading it to Athlon64 X2 4200+ (basically my current core times 2), with TDP 89W. The main heavy-load activity I use my setup for is HD x264 video encoding/transcoding - very high quality, with very slow set of options, which takes 30-40 hours per movie to complete. The new CPU should be a simple and cheap upgrade to bridge some (short) time until I take the next step and go quad-core+ddr3.

My questions are - can one core of A64 X2 be temporarily disabled in a system (Windows XP) when I don't need to run video encoding for a while, in order to save electricity and basically "revert" my new CPU to an old one when higher performance is not necessary? And how much power would an 2.2GHz Athlon64 X2, using single core, consume versus regular 2.2GHz Athlon64 (Full (dual core) consumption of the former is 89W and the latter 67W)?

Thanks in advance! :)

quest_for_silence
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Re: Switching one core of Athlon64 X2 off when not necessary

Post by quest_for_silence » Thu Nov 20, 2014 4:21 pm

Edit the boot.ini file and append

Code: Select all

/numproc=1
right after the end of the other XP boot options.
Or look into the BIOS if it has that feature.

washu
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Joined: Thu Nov 19, 2009 10:20 am
Location: Ottawa

Re: Switching one core of Athlon64 X2 off when not necessary

Post by washu » Thu Nov 20, 2014 5:53 pm

While what quest_for_silence said is correct, it may not actually save power VS leaving it on. The BIOS or the numproc option may leave the 2nd core in an "inaccessible" state instead of a "turned off" state. Without the OS issuing HLT commands at idle it may actually use more power. You would need to test all three ways (BIOS, numproc, leave it on) with a power meter to find out the best way.

gamebox
Posts: 27
Joined: Mon Sep 22, 2014 9:21 am

Re: Switching one core of Athlon64 X2 off when not necessary

Post by gamebox » Fri Nov 21, 2014 4:14 am

Thanks for replies. :)

Sadly, washu, I don't have a power meter yet. :\ My "low-load" work involves browsing the internet which causes my current A64's load to oscillate between 10 and 80 percent - two cores might even reduce consumption if load gets split between them so they both remain in lower power states, but I can check it only "on site".

What's interesting for me is that A64x2 is built using the same technology as regular A64 (90nm), but it still requires 89W for 2 cores instead of 67W per core - now, where that big difference comes from, when both cores even have separate L2 caches? My plans are to use the new CPU to perform two single-threaded video encoding tasks simultaneously, and I expect (perhaps unrealistically) to get 200% increase in performance, but performance rating of only 4200+ vs 3500+ bothers me. Would there be some strong bottleneck in the system (the RAM is very fast, runs in dual channel and 2GB should be sufficient even with doubled encoding requirements)?

washu
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Re: Switching one core of Athlon64 X2 off when not necessary

Post by washu » Fri Nov 21, 2014 8:03 am

There are still some shared resources in a dual core CPU which explains some of the TDP difference. The memory controller and HT bus interface are shared. Also the dual core is likely running closer to its TDP rating in real life than the single core. Core voltage on the dual is also slightly lower.

The "speed rating" on Athlons from those days is pretty much useless. There is too much variation in clock speed VS cache to say for sure one model is better than another. I think for the early dual cores like yours they were being conservative with the rating given how little software at the time could really use them.

Video encoding is something that can be very easily parallelized, so running two separate encodes instead of just one is likely detrimental to overall performance. By running two encodes you are wasting cache space by duplicating code, and you are sharing the CPU's memory controller and bus. Also, unless your videos are very low resolution then 2GB is far from enough to do two at once. Depending on the speed of your hard drive you could also get bottlenecked there by causing the access pattern to be more random than sequential and made worse by your low RAM for disk cache.

gamebox
Posts: 27
Joined: Mon Sep 22, 2014 9:21 am

Re: Switching one core of Athlon64 X2 off when not necessary

Post by gamebox » Fri Nov 21, 2014 9:07 am

@washu: Yes, it might be that single core A64s are actually quite a long way away from stated 67W TDP, as they mostly displayed very low consumption in benchmarks at the time, and lower difference between idle and full-load states than expected.

The reason why I want to do my video encodes this way is that multi-threading (slightly) reduces compression efficiency (bringing lower quality or increased file sizes). Parallelization in video encoding was done not because it was a clever thing to do, but out of pure necessity since standard, single-core CPUs, didn't advance in processing power as it was necessary. The videos I re-encode are mostly 720p or 1080p, single encoding process takes about 300mb of ram. The hard drive is not yet the bottleneck as processing crawls at about 1 fps, and even the decoding part of the process (although highly demanding when done in software) doesn't create any processor load worth mentioning at those speeds.

Some other problems I might come across are PSU load (it is capable of supplying about 16A @ 12V and is strained already), mobo support for A64x2, and above all the dual-core in question is being offered for about 20 euros (~25 $) at the moment, second hand, which is a rip-off (though these are very rare). The benefit would be somewhat faster encoding and lower overall consumption (not covering the investment in CPU) and the possibility to do early tests of upcoming codecs, although that would still be unbearably slow for regular use as only quad-core I plan next would bring reasonable speeds.

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