Test 2: Overclocking
In the default mode the speed and timings of system memory have the same influence on the Socket AM3 platform as on the speed of Intel platforms we had tested earlier. Perhaps we’ll see some difference at overclocking?
Our Phenom II X6 1090T comes from the Black Edition series and thus can be overclocked via both the clock generator and the frequency multiplier. The second method doesn’t affect the memory controller, so we are not interested in it for now. Instead, we sped up our CPU for the second part of our tests by raising the base clock rate, leaving the frequency multiplier intact at 16x. We reached a frequency of 4.0 GHz, which is the clock rate you are likely to get with most Phenom II processors using air cooling.
Increasing the base clock rate raises the rest of system frequencies set up by means of multipliers. So, when the base clock rate is increased from 200 to 250 MHz, the processor begins to support another set of memory frequencies namely DDR3-1000, DDR3-1333, DDR3-1667 and DDR3-2000. Therefore, mainboard makers can claim that their products are compatible with memory faster than DDR3-1600.
Well, such marketing claims are sometimes too far-fetched. It is next to impossible to make system memory work in DDR3-2000 mode on a Socket AM3 platform. In fact, no one had ever mentioned such an opportunity before the release of the Phenom II X6 processor with the E0 stepping (this stepping features an optimized memory controller) and the AMD 890FX chipset. Then AMD said that this new CPU plus chipset combination could make DDR3-2000 possible if you’ve got a good mainboard, a lucky sample of the Phenom II X6 with an overclockable memory controller, and memory modules capable of working at such a high clock rate.
Mainboard makers were quick to spread out this news, omitting to note all the prerequisites, so some users may think that DDR3-2000 SDRAM is always an option with new mainboards if you’ve got a six-core CPU.
It’s not so in practice. Although there are accounts of successful experiments of this kind on the Web, they should be viewed as exceptions from the general rule. It is indeed a problem to make memory work at 2000 MHz on a Socket AM3 platform.
For example, we used an ASUS Crosshair IV Formula mainboard, presumably compatible with DDR3-2000, and a suitable Phenom II X6 1090T processor but could not make DDR3-2000 stable in our testbed. Therefore, we had to limit ourselves to DDR3-1000, DDR3-1333 and DDR3-1667 modes at a base clock rate of 250 MHz.
When the CPU is overclocked by increasing the base clock rate, the frequency of the CPU-integrated North Bridge (also referred to as Uncore; it includes L3 cache and a memory controller) may grow up along with the available memory frequencies. In our earlier overclocking guide we recommended to reduce the appropriate multiplier to keep the Uncore frequency as close as possible to the default 2.0 GHz. However, if you want to get the maximum from your configuration, you can try overclocking it as well. AMD hints at that, too. For example, the North Bridge of the server-oriented Istanbul processor, which is analogous to the desktop Thuban, is clocked at 2.2 GHz by default.
As we learned in our practical experiments, the North Bridge of our Phenom II X6 1090T processor could be easily overclocked by 50%, i.e. to 3.0 GHz, by changing the appropriate multiplier. Unfortunately, you can only increase this multiplier above the default 10x with Black Edition series processors. If you’ve got an ordinary CPU, you will have to put up with what overclocking you can get by increasing the base clock rate.
Does it make much difference, anyway? To check out the influence of the CPU-integrated North Bridge on system performance, we carried out a separate test of our Phenom II X6 1090T processor overclocked to 4.0 GHz with its North Bridge clocked at its default 2.0 GHz as well as at an overclocked 3.0 GHz. To keep the rest of the test conditions identical, the system memory worked in the same mode in both cases: DDR3-1333 with 9-9-9-27-1T timings. The next table shows the difference in performance we could observe in benchmarks and real-life applications:
The CPU-integrated North Bridge might be expected to affect system performance because it incorporates not only a memory controller but also L3 cache. Thus, increasing its operating frequency cannot but speed up the whole memory subsystem. On the other hand, the performance growth we can see in our tests is far smaller than the frequency growth. The 50% increase in frequency leads to but a 5% increase in performance, and not even in all applications. And if our CPU didn’t have unlocked multipliers, we’d see an even smaller performance growth while overclocking through the base clock rate.
In other words, there is an effect from changing the frequency of the Uncore part of a CPU but it is too small to affect the overall system performance much. You are likely to get much better results by simply overclocking your system memory. That’s what the next diagrams show. The results were obtained at an Uncore frequency of 3.0 GHz.
First, let’s check out the practical bandwidth and latency.
The results are roughly the same as in the previous section. Everest CacheMem seems to be indifferent to our overclocking of the CPU and memory controller, its results being almost the same as in the previous section of this review. However, there are notable changes which you can see in the test of copying data in memory. Overclocking makes the dependence of bandwidth at copying on the memory frequency more pronounced, so we can expect some changes in tests that are based on real-life applications and algorithms.
Well, these diagrams look much better than those that we had with the non-overclocked system. The overclocked processor calls for more data while the higher frequency of the CPU-integrated North Bridge lowers the latency of the CPU – memory thoroughfare. As a result, the frequency of DDR3 SDRAM begins to affect system performance even in such memory-indifferent tasks as final rendering and video encoding. You can get an average 5% performance boost by adjusting the memory parameters then. As for memory-sensitive applications, they can even speed up by 10-15%. This means that overclockers shouldn’t be as careless about choosing system memory as ordinary users.