Intel Xeon 1.7GHz: King of the High End?

Memory Bandwidth Comparison

As usual, we'll start off our evaluation of the Intel Xeon with some memory bandwidth benchmarks.  The Pentium 4 has already proved to be extremely appreciative of large amounts of memory bandwidth that is provided to it by the i850 chipset with its dual Rambus channels.  The Intel Xeon should be no different since the CPUs are identical and the available memory bandwidth is identical as well. 

The Linpack performance graph provides us with the answer we already knew; as the size of the data being worked on increases past what the CPUs can hold in their caches, the FPU throughput is determined by the amount of available memory bandwidth.

Here we can see exactly what will give the Xeon the edge over the Intel Pentium III (and the Pentium III Xeon) in memory/FSB bandwidth intensive server applications.  We will get into examples of these applications later on, but remember that video encoding isn't the only bandwidth intensive application out there.

The fluctuations in the Dual Xeon graph were not present if only a single processor was used.  Unfortunately we were unable to explain this phenomenon beyond relating directly to the juggling of tasks between the two processors. 

Earlier we mentioned that regardless of whether you have a point-to-point bus or a shared FSB, your CPUs are still going to have to contend with one another for the same amount of memory bandwidth through the same bus. 

A good way of illustrating this is by running two instances of Linpack simultaneously on the DP Intel Xeon system and note the drop in memory bandwidth available to each processor individually. 

By looking at the drop in performance after the matrix size exceeds the cache size of the individual processors you notice that the performance of the two processes is much less than the performance of a single process running on the system.  The reason for this being that the amount of memory bandwidth the i860's dual RDRAM channels are able to deliver still isn't enough to feed the extremely hungry processors. 

It is because of this that we see the need for the ServerWorks Grand Champion HE chipset with its 6.4GB/s of memory bandwidth when the Xeon is eventually placed in quad processor systems.