SDRAM vs. RDRAM, Facts and Fantasy

Two technology trends should increase the gap in performance between systems that use SDRAM and RDRAM. The first is that faster, more advanced processors will increase the importance of memory systems in overall performance. For a given program, as CPUs get faster the amount of time the CPU takes to do its computation drops, but the time it spends idle waiting for data from the memory hierarchy stays about the same. Overall execution time (the sum of CPU busy time and CPU idle time) drops, but the fraction of time spent idle waiting for the memory hierarchy increases. In the future, as CPU clock speeds continue to rise much faster than memory system clock speeds, the aggressive use of techniques such as prefetching, predication, out-of-order execution, branch prediction, and speculative execution will require significantly higher memory bandwidths in order to better tolerate memory system latency.

A second important trend is that as hardware evolves, software inevitably evolves as well. Even office applications like Word and Excel stress CPUs and memory systems more today than previous versions did five years ago. Advances in CPU and memory technology allow incorporation of features like dynamic spelling and grammar checking, multimedia, and interactive graphic utilities. At the time of the transition from EDO to SDRAM, several studies showed almost no performance benefit for SDRAM-based systems on benchmarks prominent at the time. However, faster CPUs and the benefits of SDRAM technology allowed software to become more ambitious. Running today's software on EDO-based machines and SDRAM-based machines produces more noticeable benchmarking differences than did applications designed four years ago. This will be true again as PC main memory transitions from SDRAM to RDRAM, allowing software writers to exploit new features that enhance productivity and allow products to distinguish themselves from previous versions and the offerings of competitors.

These trends (especially increasing processor speeds) point to the need for increased processor bandwidth. Today, as processor speeds reach 1 GHz, we are starting to see definite performance advantages for RDRAM-based platforms. Of course, in some of the more bandwidth-intensive applications like AutoCAD and visualization benchmarks, RDRAM-based platforms outperform SDRAM-based platforms by wider margins at processor speeds below 1 GHz.

System performance can also be affected by many BIOS settings, and RDRAM-based motherboards are no exception. In fact, there are several such settings in the 820 chipset. One group that can have a dramatic impact on performance relate to 'device pools.' RDRAMs were designed to be used in many environments, including workstations, desktops, and portables. The portable market in particular is sensitive to the power consumption of all components, as it affects battery life. Desktops and workstations are less sensitive, however. RDRAMs have four power states that balance power consumption and access latency to meet the needs of all three of these markets. The lowest-latency access modes are called Active and Standby, while the lowest power-consuming modes are called Nap and Powerdown. Active and Standby consume more power than Nap and Powerdown, but the latter two states have higher access latencies.

RDRAM-based motherboards that use Intel's 820 chipset allow RDRAMs to be split into multiple pools (A,B), with all devices in a pool being placed in similar power states. Pool A devices can be placed in Active or Standby, and Pool B devices can be placed into Standby or Nap. Many motherboards allow BIOS programmability of Pool B devices. In order to obtain the highest performance, the maximum number of devices should be placed in the Active state in Pool A, and Pool B devices should be set to 'Standby.'

While allowing Pool B devices to be placed into Standby instead of Nap increases power consumption, it is a common misconception that that this will readily cause the RDRAMs to overheat, necessitating special cooling. But systems from Dell, for example, do not ship with any special RDRAM cooling, yet achieve some of the best benchmark scores reported to date by reviewers such as PC Magazine and Maximum PC. Some BIOSes default to placing Pool B devices into Nap instead of Standby, which unnecessarily hinders the performance of RDRAM-based platforms. In the next section, the power consumption of RDRAMs will be explored further.

Skip To Page 1 Introduction 2 Rambus Direct RDRAM 3 Conventional Memory Systems 4 RDRAM Benefits 5 Reducing System Cost 6 RDRAM Pricing 7 RDRAM Pricing Continued 8 RDRAM Performance 9 RDRAM Performance Continued • 10 System Performance 11 RDRAM Power Consumption 12 Benchmark Applications 13 BAPCo SYSmark 2000 14 Benchmark Setup 15 Benchmark Results Intel 440BX 16 Benchmark Results VIA 694X Apollo Pro 133A 17 Benchmark Results Intel i820 18 Benchmark Evaluation 19 Conclusion

Tools:

Add hardwarecentral.com to your favorites Add hardwarecentral.com to your browser search box IE 7 | Firefox 2.0 | Firefox 1.5.x