GSN testing with ORNL IBM RS/6000 p660-6H1

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The p660 series has a new I/O architecture that sounds particularly interesting. It has fourteen I/O slots, ten of which support 66 MHz 64-bit interfaces. This made us hope that this machine could drive our GSN interface at new record rates (perhaps as high as 400 megabytes/second).

Unfortunately that proved optimistic. Our testing saw only about 100 megabytes/second. Further investigation and conversations with IBM technical staff revealed the following:

• the p660 was designed for high *aggregate* throughput, not high individual-slot ("burst") rates



• there are configuration options available that would increase throughput to as much as 200 megabytes/second



• the I/O architecture involves various layers and latencies and is unlikely ever to support full bus bandwidth.

This concludes our GSN testing. We now know what to expect from current GSN technology - that no current PCI-bus machine can achieve more than about one quarter of the available GSN bandwidth. We will have to deal with this limitation when structuring visualization applications.