Latency and Heat Issues

FiringSquad: Everyone says that RDRAM has latency issues. How does RDRAM latency compare to SDRAM? Doesn't RDRAM have a significant delay as it switches between active and standby modes?

This is another misconception. You have to make a distinction between component level and system level measurements. In the worst case when the memory system is underutilized, Rambus system latency is comparable to that of an SDRAM system. When the memory system is heavily utilized, Rambus' high bandwidth architecture that efficiently supports concurrent memory transactions results in far better latency than SDRAMs.

RDRAMs supports 4 power modes that allow performance and power consumption to be optimized. Between the two highest performance modes called active and standby there is no significant delay.

FiringSquad: Let's talk about heat issues. Is it true that the original RDRAM specification called for its own cooling fan?

This is another misconception. Rambus specification never called for its own cooling fan.

FiringSquad: If heat isn't really an issue, why do RIMMs still ship with heat spreaders?

Like SDRAMs, RDRAMs experience a temperature rise as more bandwidth is being pulled out of the memory system. However, it may come as a surprise to you that RIMMs can provide twice the bandwidth at half the power of DIMMs.

The heat spreader serves two functions: Unlike DIMMs where power is dissipated evenly among all devices, RIMMs can have power localized to one device. The heat spreader serves to spread heat across all devices along the RIMM module.

The second function is device protection during shipping and installation. As further evidence that heat is not an issue, the major OEMs ship RDRAM-based systems without any dedicated cooling for the RIMMs, and these systems achieve the highest scores in benchmark tests.