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LOSTCIRCUITS
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| AMD's Phenom Processor - Beyond Erratum 298 | |
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(Author: Michael Schuette, January 1, 2008) |
NorthBridge Frequency and System Performance
A major difference between the Phenom 9700/9900 and the lesser speed grades is that in the latter offerings, the Northbridge, including memory controllers and L3 cache, is running at 10% lower speed, that is 1.8GHz instead of 2 GHz. As mentioned earlier, the NB is running at fixed frequency and also has its own dedicated power supply controlled by the fifth phase of the hybrid voltage regulator. The whole idea is somewhat intriguing, assuming the memory controller runs at a more constant load than a CPU, one wouldn't need as elaborate voltage regulation. Moreover, it becomes possible to uncouple the NB voltage from the CPU supply voltage and, even if the core voltage is increased for overclocking purposes, it would not affect cause an additional voltage-induced power drag on the level of the memory controller. This is important since, as we showed in several earlier articles, the memory controller does eat quite a bit of power and if the same principle is applied to yet another power hog in form of the L3 cache, then one might be able to achieve some significant savings in energy consumption and heat dissipation. Our power measurements show that this is at least worth a discussion. Unfortunately with our methodology it is not possible to dissect core and NB power consumption but the fact that the 9600 at the same core frequency and core supply voltage draws about 25% less power than a 9900 under the same conditions makes a good argument for our claims.
Power and energy efficiency are one side of the coin but what kind of trade-offs have to be expected with respect to performance. We are trying to address this question by repeating the same benchmarks we ran throughout this article on the Phenom 9600 overclocked to 2.6 Ghz. Functionally, the only difference is the NB running at the 1.8GHz frequency as opposed to the 2.0 GHz setting. All benchmarks were run with the memory bus clocked at 1066 MHz and the Turbo button set to red.
The blue bars indicate where the Phenom 9900 performs better than the Phenom 9600, both running at the same clock speed. The red bars indicate where the 9900 performs worse than the 9600. The power numbers show higher (worse) energy consumption by the 9900 compared to the 9600. Across the bench, there is really no difference beyond the margins of systematic measurement errors.
It may be a moot point but you never know until you try. To be on the safe side, we repeated the same benchmarks with the memory clocked at 800 MHz. Again, both CPUs were clocked at identical core frequency, that is 2.6 GHz
In the same fashion as above, the blue bars indicate where the Phenom 9900 performs better than the Phenom 9600, both running at the same clock speed. The red bars indicate where the 9900 performs worse than the 9600. The power numbers show higher (worse) energy consumption by the 9900 compared to the 9600. Again, we fail to see any significant difference in performance.
 (AMD Phenom 9600 2.3GHz (HD9600WCGDBO)) |
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