Share lang nako for those who have queations about RAM (especailly about Bandwith & Latency)
link:
http://xbitlabs.com/articles/memory/...ide.html#sect0
Choosing the Right Memory for Core 2 Duo Platform
by Ilya Gavrichenkov (09/08/2006 | 05:05 PM)
Do you want to get the maximum out of your system with a Core 2 Duo processor? Read this review to learn what memory suits that purpose best!
Intel's new processors with the Core micro-architecture remain in the focus of PC enthusiasts' attention. Numerous tests have proved that Core 2 Duo processors deliver unrivalled performance at their default frequencies as well as at overclocking. No wonder that various modifications of Core 2 Duo and Core 2 Extreme are on the wish list of many users who are upgrading their computers with their own hands. A mass transition to the new Core 2 platform is just about to begin if it hasn't begun already. Our intent now is to review the Core 2 Duo infrastructure to see what is the best environment for that processor.
In this first article we are going to talk about system memory with respect to the Core 2 Duo, particularly we will try to find out which memory parameter has a bigger effect on performance of Core 2 Duo systems, bandwidth or latency. As a result, we will come up with some general conclusions as to what exactly memory out of the variety of DDR2 SDRAM available today suits best for the new platform. Besides, we will give you our own recommendations on purchasing DDR2 SDRAM for systems with new Intel Core 2 Duo processors.
Intel Core 2 Duo and System Memory
Before proceeding to discuss the results of our tests which may give us exhaustive answers to all the questions asked in the introduction, we want to say a few words on why Core 2 Duo processors may put forth specific requirements to the memory subsystem to achieve maximum performance. After all, these CPUs are compatible with the same LGA775 platforms (with minor variations in electric characteristics) that long- and deep-studied processors from the Pentium 4 and D families were used on earlier. But as a matter of fact, the Core 2 Duo has a dramatically different micro-architecture which is the main reason for its different way of working with system RAM.
First of all, the Core 2's innovative dual-core design with a shared L2 cache comes to mind. As opposed to separate L2 caches, a shared L2 cache frees the front-side bus and the memory bus from data transfers required to maintain cache data coherency. Dual-core Pentium D processors used to utilize the front-side and memory buses to exchange data between the execution cores whereas the Core 2 Duo achieves this by means of its shared L2 cache alone. As a result, the Core 2 Duo can use the CPU-memory link to more effect, freeing it from auxiliary data transfers.
The second thing that turned out to have a positive effect on memory performance in Core 2 Duo systems is the increased frequency of the Quad Pumped Bus which connects the CPU and the chipset's North Bridge. The resulting frequency of this bus is now 1067MHz which provides a bandwidth of 8.5GB/s. It also means that Core 2 Duo platforms have an opportunity to fully utilize the bandwidth provided by a dual-channel memory subsystem with DDR2-533 SDRAM modules. Installing even higher-frequency modules can give a chance to additionally reduce memory access latencies.
We shouldn't also forget that the Core micro-architecture features a number of technologies that improve the CPU's memory-accessing capability like the memory disambiguation technique and the data pre-fetch algorithms which are much better than those employed in the Pentium 4.
Although Core 2 Duo processors still use an external memory controller, located in the chipset's North Bridge, the mentioned features of the new micro-architecture help them challenge Athlon 64 X2 CPUs, which have a memory controller integrated into their core, in terms of memory performance. The graphs below show you the results of our measuring the effective memory bandwidth and latency in systems based around an Intel Pentium D 960, Intel Core 2 Duo E6700 and Athlon 64 X2 5000+. DDR2-800 SDRAM with 4-4-4-12 timings was used in each case:
As you see, the Core 2 Duo platform features higher memory performance in comparison with the Pentium D system in practice, not only in theory. Although the two processors from Intel access memory through the same memory controller integrated into the chipset's North Bridge (this test was performed on an i975X-based mainboard), the choice of the CPU affects the result greatly. The Core 2 Duo can ensure a 10% higher memory bandwidth and much lower data access latency (from 20% to 40% depending on how efficient the data pre-fetch algorithms are in a particular application). The Core 2 Duo is obviously superior to previous-generation NetBurst processors when it comes to using the memory subsystem efficiently.
It is quite interesting to compare the real-life performance of the Core 2 Duo platform with that of the Athlon 64 X2 platform especially since they use different approaches to placing the memory controller. Contrary to the Core 2 Duo, the Athlon 64 X2 (in Socket AM2 design) has an internal DDR2 SDRAM controller, integrated right into the CPU core. The integrated controller provides a very high memory bandwidth. The substantial advantage over the Core 2 Duo platform is not to be wondered at considering that in Intel's systems the speed of data transfers between the CPU and memory is limited by the bandwidth of the FSB. As a result, the DDR2-800 SDRAM memory subsystem is about 40% efficient on the platform with the new Intel CPU and 55-60% efficient on the Athlon 64 X2 platform.
As for the memory latency parameter, two out of three test utilities show that the Core 2 Duo platform is capable of achieving lower memory latency than the Athlon 64 X2 system. This result is obviously due to the data pre-fetch algorithms employed by the Core micro-architecture. Those algorithms prove to be very helpful in many cases. So, even though with an external memory controller, Core 2 Duo processors have high performance in applications sensitive to memory speed.
Bandwidth vs. Latency
We first measured the bandwidth and latency of the memory subsystem in synthetic benchmarks.
As you can see, memory types with different theoretical bandwidths do not differ much in practice. For example, there is a 100% difference in theoretical bandwidth between DDR2-533 and DDR2-1067 whereas the difference between the practical results obtained with those memory types is 17% at maximum.
This poor performance of fast DDR2 SDRAM is due to the architecture of Core 2 Duo systems in which memory is connected to the CPU via the chipset and two sequential buses. In this design it is not the bandwidth of dual-channel high-frequency memory that becomes the bottleneck, but the Quad Pumped Bus that connects the CPU with the chipset's North Bridge. Its maximum theoretical bandwidth is 8.5GB/s in Core 2 Duo systems, which only equals the bandwidth of dual-channel DDR2-533 SDRAM. That's why we don't see a really big performance growth if we use memory faster than DDR2-533.
It seems it doesn't make any sense to use memory faster than DDR2-533 on the Core 2 Duo platform. This is not quite so. Memory access latency decreases along with frequency, which can be seen in practical tests.
Here, the results differ much more. Like the bandwidth, the latency should have a considerable effect on system performance in many applications and may justify the use of high-frequency memory in a computer with a Core 2 Duo processor.