Intel Core i7: Nehalem Launched

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POWER

With all the changes under the hood of the processor and the increased die size, you might assume that the Core i7 chips uses more idle power. This was not the case — idle power was almost identical on all of these system configurations. During playback of a VC-1 video, the power draw was very similar across the board as CPU usage was very low, due to the hardware acceleration capabilities of the nVidia graphics card. The Core i7 system did not exhibit significantly higher power draw until the Prime95 load tests. With half the cores stressed, the Core i7 920 system pulled 10W more than the QX9650 at 2.66Ghz and 3W more at 3.00Ghz. At full load, the Core i7 system used an extra 45-50W. The extra power draw of the 965XE system running Prime95 is directly attributable to the higher clock speed of its cores.

System Power Consumption (AC)
CPU
QX9650
QX9650
i7 920
i7 920
i7 965XE
Clock Speed
2.66Ghz
(UC)
3.00Ghz
(Stock)
2.66Ghz
(Stock)
2.66Ghz
(Stock)
3.20Ghz
(Stock)
System RAM
2 x 1GB
2 x 1GB
2 x 1GB
3 x 1GB
3 x 1GB
Off
2W
2W
2W
2W
2W
Sleep
3W
3W
5W
5W
5W
Idle
102W
103W
102W
104W
104W
VC-1
137W
140W
143W
140W
138W
Prime95 (2/4)
152W
159W
162W
160W
173W
Prime95 (4/4)
171W
177W
216W
215W
236W
Prime95 (4/4) + Furmark06
295W
302W
341W
343W
367W
Note: The number of threads used in Prime95 were doubled for the Core i7 920 due to Hyper-Threading. To stress 2/4 cores, 4 threads must be run on the Core i7 920 while the QX9650 requires only 2.

The extra DIMM in the triple channel memory configured i7 system barely made an impression on our power readings, varying an average of 2W. During some of the tests we noticed the power draw was actually lower. Normally adding an extra stick of memory increases power across the board, but not so for the Core i7. Initially when we were comtemplating overclocking the Core i7 920 to 3.00Ghz to match the QX9650's stock clock speed, we found that it would blue screen with only 2 DIMMs at 3Ghz, yet it was perfectly stable with 3 DIMMs. Rearranging and using different modules did not change this behavior. This particular system seemed to work best with triple channel memory.

ENERGY EFFICIENCY

Timed benchmarks give us an opportunity to analyze power efficiency while keeping performance in mind. Once a task is completed, the system sits idle, and in our case the Core 2 and Core i7 systems in our test setups idle using the same amount of power. So how fast the program takes to finish its task and how much power it draws while doing so ultimately determines power efficiency. With that in mind we calculated the watt-seconds for each benchmark by multipling the time with the average power consumption during the task. W/hr would be easy to obtain from this number, but since the tasks themselves were very short (typically no more than 3 minutes), that seemed unwarranted.

Benchmark Energy Efficiency
CPU
QX9650
QX9650
i7 920
i7 920
i7 965XE
Clock Speed
2.66Ghz
(UC)
3.00Ghz
(Stock)
2.66Ghz
(Stock)
2.66Ghz
(Stock)
2.66Ghz
(Stock)
System RAM
2 x 1GB
2 x 1GB
2 x 1GB
3 x 1GB
3 x 1GB
WinRAR
185s @
133W
177s @
135W
153s @
133W
151s @
130W
140s @
136W
24605 Ws
+3%
23895 Ws
100%
20349 Ws
-15%
19630 Ws
-18%
19040 Ws
-20%
iTunes
214s @
125W
189s @
128W
209s @
136W
200s @
133W
175s @
141W
26750 Ws
+10%
24192 Ws
100%
28424 Ws
+17%
26600 Ws
+10%
24675 Ws
+2%
TMPGEnc
210s @
165W
189s @
170W
177s @
188W
178s @
189W
151s @
208W
34650 Ws
+8%
32130 Ws
100%
33276 Ws
+4%
33642 Ws
+5%
31408 Ws
-2%

The QX9650 system at stock 3Ghz clock speed was used as the reference point for each benchmark. The energy consumption of the other systems were scored as needing more (plus %) or less (minus %) energy compared to that used by the stock-clock QX9650 system. The lowest energy consumption is in bold green.

WinRAR used about the same amount of power as the Core 2 configurations, but finished a fair bit faster, resulting in much better energy efficiency, with the 965XE using a substantial 20% less energy than the baseline. The QX9650 2.66Ghz and i7 920/triple channel configuration were about dead even when it came to iTunes encoding efficiency, but the QX9650 at its stock speed of 3Ghz proved to be best, by a small margin over the top i7. Video encoding with TMPGEnc was faster on the Core i7 920 configurations but the extra power it consumed doing so made it slightly less efficient overall. The extra power demanded by the 965XE, however, was more than compensated by the reduced time, which gave it a 2% advantage.

From these limited tests it would seem that Core i7 is fairly close to the power efficiency of Core 2, with variance depending on the application used. However, it seems likely that with highly demanding, multi-threaded applications, the i7 will scale up better than the Core 2s.



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