955X Memory V2.Indd

APPLICATIONS NOTE AN504: Memory Options and Performance on the Intel 955X Express Chip Set John Beekley, VP Applications Engineering, Corsair Memory, Inc. Introduction This white paper will examine memory options for the Intel 955X Express chip set. We will give a brief summary of chip set features. A benchmark setup to test memory for this chip set will be described, and BIOS parameters and frequency settings will be explained. We will then evaluate several memory modules that are suitable for use with this chip set. We will examine performance for these modules, and provide guidance for which memory types should be used based on target performance and cost of the system. Finally, we will overclock the platform and explore its maximum performance capabilities. Chip Set Overview The Intel 955X Express Chip set (abbreviated throughout the article as “955X”) is Intel’s highest performance desktop chip set. A block diagram is shown in Figure 1. The 955X enables Intel’s highest performance platforms, with support for dual core processors with hyperthreading technology, an 8 GB of memory address space, and other advanced features. The 955X utilizes DDR2 memory. As the chip set uses a dual channel architecture, memory should be used in pairs of similar modules to obtain optimal performance and stability. The 955X is Intel’s primary enthusiast chip Figure1. 955X Block Diagram (source: Intel Corp.) set; most motherboards that use the chip set provide excellent flexibility in adjusting memory latency settings. See Corsair’s AN501 application note for more information on the impact of latency settings on performance. Test Setup We decided to construct a test setup which would highlight the performance and features of the chip set and memory. With this goal in mind, an aggressive test platform was constructed using May, 2005 Page 1 the following components: • Asus P5WD2 Premium motherboard, BIOS version 0408 • Intel Pentium 4 550, 3.4GHz, 800MHz front side bus • GeFORCE 6800 Ultra PCI Express video card • Western Digital “Raptor” SATA hard drive As CPU speed has a substantial impact on benchmark results, we attempted to keep the internal CPU speed constant throughout the testing. BIOS Settings - Overview and Terminology The Asus P5WD2 BIOS uses the following settings to control memory and CPU operating characteristics: • CPU Frequency - Found in the “Advanced/Jumper Free” submenu, this frequency is used as the base frequency for the Front Side Bus (“FSB”) and the Memory Bus. FSB frequency is set to CPU Frequency times four. Memory bus is a selectable, but is based on a multiple of CPU Frequency • DRAM Frequency - This parameter is also found in the “Advanced/Jumper Free” submenu. It determines memory bus frequency; the BIOS will force you to select values which are a valid multiple of the CPU frequency. • AI CPU Lock Free - This parameter allows you to determine the internal speed of the CPU. For the P4 550, if “Enabled” is selected, the internal CPU speed will equal fourteen times the CPU Frequency. If “Disabled” is selected, the internal CPU speed will equal seventeen times the CPU Frequency. Also found in “Advanced/Jumper Free”. • Memory Latency Settings - These are configured in the “Advanced/Chipset” submenu. While a general discussion of latency settings is outside the scope of this document, we will note that “RAS# Activate to Precharge” (known as TRAS) was determined to have no real impact on performace, regardless of value selected. Also, we will also note that the P5WD2 does not allow the user to manually set a value for Command Rate; it is automatically set to 2T. Optimum Front Side Bus Selection Some of the memory frequencies that we were interested in testing could be achieved in more than one way, based on the multipliers available in the P5WD2 BIOS. For example, DDR2- 667 could be selected by using a CPU frequency of 200MHz and a multiplier of 3.33, as well as with a CPU Bus speed of 250 MHz and a multiplier of 2.667. Similarly, DDR2-800 could be reached with a CPU bus of either 200 MHz or 240 MHz, with 200MHz actually providing a slightly higher CPU speed (3.4 GHz vs. 3.36 GHz). Our findings were somewhat surprising. Benchmark results for the two configurations are shown in Table 1. As you can clearly see, the 250MHz CPU frequency provided a sizeable jump in performance across all the benchmarks. Spot checks at other speeds confirmed these results. So, May, 2005 Page 2 the 250 MHz CPU frequency was used in fCPU = fCPU = Increase Test Name 250 MHz 200 MHz the balance of the testing. PCMark2004 - Memory 5810 5287 9.8% Benchmark Parameters SiSoft Sandra 2005 - Int 5823 MB/s 4923 MB/s 18.3% SiSoft Sandra 2005 - Float 5875 MB/s 4933 MB/s 19.1% Benchmarking this memory was somewhat problematic. The BIOS of Lavalys Everest - Read 6399 MB/s 5710 MB/s 12.1% the P5WD2 allows the CPU multiplier Lavalys Everest - Write 2139 MB/s 2085 MB/s 2.6% to be set to either 14 or 17. Most of the Lavalys Everest - Latency 82.4 ns 90.1 ns 8.5% desired memory bus frequencies could be Super Pi 2M digits 37.75 sec 38.81 sec 2.7% opbtained by overclocking the front side 5374.28 4565.79 bus to 250MHz and setting the multiplier ScienceMark2 Membench 17.7% MB/s MB/s to 14. The only clock speed that could Doom3 demo1 640 x 480 97.8 fps 93.8 fps 4.3% not be achieved using these settings Front Side Bus (4*fCPU) 1000 MHz 800 MHz 20% was an 800MHz memory bus. With a 3.50 GHz 3.40 GHz CPU frequency of 250 MHz, memory CPU Speed 2.9% (14*fCPU) (14*fCPU) frequencies of 750 MHz and 833 MHz 667 MHz 667 MHz Memory Bus 0% are allowed, as 800 MHz would require a (2.667*fCPU) (3.33*fCPU) multiplier of 3.2. Latency Settings 5-5-5-15 5-5-5-15 0% In order to achieve a memory bus speed Table 1. Optimum Front Side Bus Testing of 800 MHz, two combinations could be used: either [1] CPU frequency of 200MHz and a multiplier of 4.0, or [2] CPU frequency of 240 MHz and a multiplier of 3.33. Since our front side bus testing (above) showed that a higher CPU frequency caused dramatically improved performance, we selected option [2], a CPU frequency of 240 MHz and a multiplier of 3.33. This configuration was confirmed to be the higher performance of the two. However, this results in a somewhat lower front side bus speed (960 MHz vs. 1000 MHz), and more importantly slows down the CPU from 3.50 GHz to 3.36 GHz, a slowdown of four percent. This had a significant impact on the XMS6400, as you will see in the discussion of results. Five Corsair memory pairs were benchmarked in this system using the closest spec possible to their guaranteed speed settings (see Table 2) with the constraints described above. As with all benchmarks, system settings and component characteristics can greatly affect the benchmark scores measured. These tests represent results achieved in our lab under the conditions outlined in the paper. Your own results, of course, may vary substantially. Benchmark Descriptions The following benchmarks will be used to measure system performance: • PCMark 2004 - Memory test suite. PCMark is designed to measure relative performance in general computing functions. The PCMark memory test suite focuses on system memory, so it makes a good measure of memory subsystem performance. • SiSoft Sandra 2005 - This system diagnostic has a memory benchmarking tool that is designed to measure memory bandwidth. It provides two output values; one for integer processing, and one for floating point processing. May, 2005 Page 3 • Lavalys Everest Ultimate Edition - This program is a powerful system diagnostic utility, and provides a suite of memory benchmarking tools. Everest provides three output values; memory READ bandwidth, memory WRITE bandwidth, and memory latency. The latency measurement represents the typical delay between the time that the CPU requests memory data and the time that data is available. • Super Pi - Super Pi is a simple application which calculates pi to a specified number of digits. The one million digit calculation was used for these benchmarks, using a modified version of Super Pi which measures to a precision of 0.001 seconds. Benchmark results indicate the time in seconds required to calculate pi to one million digits. • ScienceMark 2 Membench - This is another synthetic memory performance benchmark, which tests a series of different memory bandwidth algorithms. It provides a single memory bandwidth measurement score. • Doom 3 timedemo, demo1 - This demo is included with the retail version of Doom 3, and provides a measurement of frames per second. By setting display resolution to 640x480 pixels, the benchmark score focuses on CPU/memory performance, rather than video card performance. This is a real-world benchmark, completely based on a retail game that is available to the public. Memory Overview Corsair offers a wide variety of DDR2 memory modules, all of which are completely compatible with the 955X chip set. Since the chip set has dual memory channels, matched memory pairs are ideal for use on boards using this chip set. Table 2 summarizes Corsair’s dual channel DDR2 product offerings. Product Features are summarized below: • TWIN2X1024-8000UL: This XMS memory is optimized for extremely high clock speed. Operation is guaranteed at 1 GHz with 5-4-4-12 latency settings at an operating voltage of 2.2 volts. The part also typically operates at 800MHz with 4-3-3-8 latency settings.

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