DRAM Performance: Latency vs. Bandwidth

How Can I Get Fast Latency DRAM

Several types of special fast latency DRAM will be appearing on the scene over the next year. The most immediate of these is ESDRAM (Enhanced SDRAM) from Enhanced Memory Systems of Colorado Springs (subsidiary of Ramtron Inc). ESDRAM has been approved by JEDEC as a superset of the SDRAM standard. It is compatible with standard SDRAM and can be used in existing systems with plug compatible DIMM and SO DIMM modules.

It operates at bus speeds up to 133 MHz, and offers better latency than ordinary DRAM at all speeds. When its special features are properly supported in a chip set, it improves latency by a whopping 35-50% depending on the bus speed. Be warned - this does not translate directly into the benchmarkable performance delta. The benchmarkable performance advantage will always be less than this, depending on how heavily an application actually uses main memory. I will post a follow up article very soon that lays this out in detail (based on extensive performance modeling).

This kind of DRAM will carry a price premium, but it's performance potential should prove interesting for many users and applications - particularly for overclockers.

If Someone Offers Me Massive Bandwidth, Should I "Just Say No" ?

Never reject better performance. But don't get fooled into buying slower memory that is hiding behind a smokescreen of "Bandwidth Overkill ", particularly if someone expects you to pay a significant price premium for it.

Ultra high burst bandwidth can be needed in multiprocessor servers that more fully utilize the DRAM bus. Usually, this need is satisfied nicely using wide configurations of standard DRAM. If DDR, SLDRAM or Rambus become available at no cost premium, these memory types could also be used in the server market.

Using a 5x clock multiplier, SDRAM at 133 MHz can get us up to 667 MHz CPU speeds. It is challenging, but not impossible to migrate to higher bus speeds using SDRAM, but at that point it may become necessary to evaluate new DRAM solutions that combine faster latency and higher burst speeds.

Until we get to that point, fast latency SDRAM running at bus speeds between 66 and 133 MHz will be able to deliver a satisfactory performance enhancement for most types of systems. This will easily take us into the next century.