Intel’s Core i5-655K & Core i7-875K: Overclocked and Analyzed
by Rajinder Gill on May 28, 2010 5:00 AM ESTNow that we’ve got a couple of baselines out of the way using Intel’s stock heat-sink, it’s time to peg thermals back a bit on and see how the i5-655K substrate responds to lower load temperatures:
Switching over to our water-cooling loop decreases load temperature deltas by up to 35 degrees Celsius on this sample (as reported by on-die DTS). The upshot of this is seen quite clearly in the graph - lower load temperatures decrease processor power consumption at equivalent frequencies due to a positive temperature co-efficient (the hotter the CPU runs the more power it leaks - all else being equal). With the end result being that we’re able to boost our previous 3.77GHz stock VID overclock using the stock Intel cooler by almost 200MHz to 3.953GHz.
The improved thermal dissipation allows us to ramp voltages and takes us all the way to 4.55GHz before we hit the wall of diminishing returns. The demands on voltage ramping past this point are such that we start to see a minimum 5~10 watt rise in power consumption for every 20MHz increase in frequency. Also note, that we’re about 40MHz away from doubling up on stock processor power consumption; 4.25GHz seems to be the sensible operating point, coming in at 16 Watts over stock power draw to VCC and VTT with a 900MHz boost in full-load core frequency - and that’s with all performance related domains overclocked.
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Rajinder Gill - Tuesday, June 1, 2010 - link
As the graphs state - this is VCC/VTT power only, the two major power rails of this architecture. The 12V ATX fan headers and PCIe 12V only on the E659 motherboard. Power to DDR3 is not something I focused on but may do in a future piece (there will be a frequency proportional rise in power provided timings are not changed). A very crude guess - I'd expect the rise over stock to be around 5 watts on the DRAM side in this frequency band (and total draw to be no more than 10~15w). Although figures would differ according to the scaling capabilities of various modules.Other than that, there's not much else aside from CPU PLL which is specified at around 1.1 amps at 1.8V (around 2-3 watts tops).
Rajinder Gill - Tuesday, June 1, 2010 - link
EDIT: The 12V ATX fan headers and PCIe 12V only on the E659 motherboard.That should read the 12V ATX line supplies fan headers and PCIe 12V power only. The 3.3v and 5V rails supply DDR, CPU PLL, IOH (and all derivatives such as IOH PLL, SB~IOH termination voltage etc).
Later
Raja
DanNeely - Tuesday, June 1, 2010 - link
Time permitting I'd be interested in seeing those numbers as well. I understand your desire to measure power consumption closer to the source. My concern is that increased power consumption from the secondary items you're not measuring is a black box; while the AC-DC conversion loss in the PSU from measuring power at the wall can be mostly corrected away by looking at what the efficiency rating of the PSU used in the test setup is.DanNeely - Tuesday, June 1, 2010 - link
Time permitting I'd be interested in seeing those numbers as well. I understand your desire to measure power consumption closer to the source. My concern is that increased power consumption from the secondary items you're not measuring is a black box; while the AC-DC conversion loss in the PSU from measuring power at the wall can be mostly corrected away by looking at what the efficiency rating of the PSU used in the test setup is.Rajinder Gill - Tuesday, June 1, 2010 - link
I prefer to keep things at the DC level. There are plenty of articles covering wall level consumption with your standard kill-a-watt type unit (and they also state the PSU used so users can factor out the losses if they know the effective efficiency curve). I think you are worrying too much about the lesser rails. Sure they will make interesting reading at some point - but there is nothing that pulls more than a couple of amps so the effects on power consumption will not be huge.Regards
Raja
Rajinder Gill - Tuesday, June 1, 2010 - link
Ok "DanNeely", this is for you,I just ran tests on the 3.3V and 5V rails. At stock the combined power consumption of these two rails on the E659. Bear in mind this is an enthusiast level board (higher switching losses due to higher switching speeds on VDIMM, plus using an NF200 for PEG multiplexing):
Running DDR3-1333 CAS 8-8-8-24 with 4GB of memory. (3.3v + 5V rails combined).
20.2 watts idle
Linpack load = 26.36 Watts.
That's a change of 6 Watts between idle and load.
At 4.551GHz, now running DDR3-1820 (1.60 VDIMM):
Idle = 22.86 Watts (2 Watt idle increase)
Load = 27.86 Watts.
That's a 1 Watt increase over stock speeds under load with an overclock of 1GHz on the CPU (running QPI over 4GHz). Hardly worth writing about. Do note - the effective change will vary from board to board according to VRM switching efficiency (which is coming into play if you look at the deltas between idle and load). Of course, I am not including things like HDD's etc although, some of the static 3.3V and 5V rail consumption is due to the GPU (GTX 275) which also draws a little power from that rail.
Hope that answers your questions.
Regards
Raja
DanNeely - Tuesday, June 1, 2010 - link
yes it did. thank you.tno - Friday, May 28, 2010 - link
Setting aside typos, I know wall of text is to be avoided but this felt almost like the opposite problem. Additional clicks for additional adviews. Clearly you didn't like what you saw out of the chips despite the voltage improvements evident in the more modest chip. Yet in the end despite devoting 1/9th of the coverage to it, you reward your recommendation to the pricier chip just one page after showing it severely underperforming its non-K analogue in both overclocking and voltage.I have said it before and I think its worth mentioning again, clearly there is a lo of passion for tech in the growing AnandTech team, but maybe adding a team member whose passion is writing and across whose hands every article will pass would give the site that extra polish that elevates it from other tech sites.
Jason
7Enigma - Friday, May 28, 2010 - link
At first I thought the same thing (saying nothing at all in the final page about the 655K does leave me puzzled as it is good), but Rijinder did clarify his recommendation for the 875K by saying PRICE. He is saying for the price of these chips and where they fall in line with the rest of the offerings from AMD and INTEL, the 875K is in a sweet spot. Remember the 655K is a dual-core,4 thread chip for $215 (lots of competition from both camps), while the 875K is a quad-core, 8 thread chip for under $350.In the end (and after re-reading the conclusion and article) I think the last page needs to have a bit more meat behind it. The data in the article itself is very detailed, but the final wrap-up needs some work. But honestly, since it's a free site, I'll take the good data and sketchy conclusions (I tend to make my own).
Thanks for the article!
troun - Friday, May 28, 2010 - link
"Past 3.9GHz, we’re already looking at a 10W increase in power consumption for every 20 MHz rise in CPU frequency"
But I read 20W for 100Mhz (or 10W for 50Mhz), with ~160W @ 3.9Ghz and ~180W @ 4Ghz...
However very interesting article, a similar curve (W/Mhz) would be also appreciate for an i7 9x0 comparison, 32nm Vs 45nm (980 Vs 930?).