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AMD Athlon X2 3800+ and Athlon 3800+ |
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Same Numbers, Different Results |
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by Josh Walrath |
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The caches on these cores are still separate though, but the internal connections of the X2 series are a lot more efficient than what Intel currently has with their desktop dual core products. In the 800 and 900 series of Pentium D products, the two cores are connected through a pseudo front side bus, which in turn is connected to the real front side bus that communicates to the chipset and memory controller. AMD’s crossbar is faster and a much more elegant solution than Intel’s. Now, this will probably not be the case when it comes to Conroe and its family, which was truly designed from the ground up to be dual core.
When Hector's happy, everyone's happy. When Hector ain't happy... The crossbar does a very good job with what it is designed to do, but it does have a slightly negative effect to overall performance. Because there is one more functional arbitrator between the cores and the memory controller, overall bandwidth and latency for dual core Athlon 64’s takes a small performance hit as compared to a single core Athlon 64. Not only that, but because the processors must follow the MOESI protocol, bandwidth used by the caches and the crossbar controller to insure cache coherency. These negatives are usually offset by the greater overall performance that a dual core product can provide in most applications. Even in single threaded applications, modern operating systems can do some load balancing with the cores, so background tasks can be assigned to one core while the other core handles the application. Users that typically run more than one application at a time will notice a “smoother” experience with a multi-processor setup. Applications that do use a multi-threaded approach can see dramatic performance increases from a single core, as we will soon see. Hi-Level Architectural Diagrams
In this simplistic diagram we see how the single core Athlon 64 processor is basically laid out.
Here we see how the SRI and Crossbar Switch help the two cores communicate with both the system and the integrated memory controller. The Competitors The X2 3800+ is a 2 GHz Manchester based part with each core featuring 64 KB + 64 KB L1 cache and 512 KB L2 per core. The Athlon 64 3800+ is a 2.4 GHz Venice based part with 64 KB + 64 KB L1 cache and 512 KB L2 cache. It is interesting to see AMD using the same numerical value for each product, though there is a 400 MHz difference between the two. Though the X2 3800+ is comprised of two cores, as well as the extra transistors that the crossbar interface requires, it pulls only slightly more power than the single core yet faster clocked A64 3800+. Heat differences between the two are pretty minimal, and it seems the X2 actually pushes out slightly less heat than the 2.4 GHz part. Though the X2 3800+ does pull more power at 2 GHz than a single core at 2.4 GHz, the extra die space does help to spread out heat more and acts as a better conductor (more surface area equals better heat dissipation). Also add to the fact that the latest Venice cores at 2.4 GHz require 1.35 to 1.4 volts to work, while the X2 3800+ only requires from 1.30 to 1.35 volts. In terms of processing power, heat production, and power consumption the Venice 3800+ is really the sweet spot for AMD. As such they have really focused their process management to hit that speed with that class of product with good yields, and yet still remain in a favorable power envelope. Add onto that AMD’s APM (Advanced Process Management) software and hardware suite, it is relatively easy for AMD to hit the numbers they want for this given product yet still retain excellent yields. The side effect of this effort is that lower clocked processors have very favorable power and heat characteristics. This is very apparent with the X2 3800+. AMD ships this chip with a stock aluminum heatsink, as it produces very little heat by itself at idle, and not a whole lot more at full load. It also does not require a massive power supply to function properly. The online prices for these products are also very close together. The X2 3800+ retails for around $297, while the Athlon 64 3800+ comes in at around $284. $13 separates these two products, and as such it makes for a tough decision for users. Here are two similarly priced products that can have a world of performance difference between the two depending on the application. In this article I hope to spotlight the differences, as well as the advantages and disadvantages of each.
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