Editing Cray-2 (section)

=== Packed circuit boards and new design ideas ===
Early Cray-2 models soon settled on a design using large circuit boards packed with ICs. This made them extremely difficult to solder together, and the density was still not enough to reach their performance goals. Teams worked on the design for about two years before even Cray himself "gave up" and decided it would be best if they simply canceled the project and fired everyone working on it. Les Davis, Cray's former design collaborator who had remained at Cray headquarters, decided it should be continued at low priority. After some minor personnel movements, the team continued on much as before.

[[Image:Cray-2 module side view.jpg|right|288px|thumb|Typical logic module, showing the  tight packing. The [[pogo pin]]s connecting the cards together are the gold-colored rods seen between the ICs.]]

Six months later Cray had his "[[Eureka (word)|eureka]]" moment. He called the main engineers together for a meeting and presented a new solution to the problem. Instead of making one larger circuit board, each "card" would instead consist of a 3-D stack of eight, connected together in the middle of the boards using pins sticking up from the surface (known as "pogos" or "z-pins"). The cards were packed right on top of each other, so the resulting stack was only about 30&nbsp; mm high.

With this sort of density there was no way any conventional air-cooled system would work; there was too little room for air to flow between the ICs. Instead the system would be immersed in a tank of a new inert liquid from [[3M]], [[Fluorinert]]. The cooling liquid was forced sideways through the modules under pressure, and the flow rate was roughly one inch per second.  The heated liquid was cooled using chilled water heat exchangers and returned to the main tank. Work on the new design started in earnest in 1982, several years after the original start date.

While this was going on the [[Cray X-MP]] was being developed under the direction of [[Steve Chen (computer engineer)|Steve Chen]] at Cray headquarters, and looked like it would give the Cray-2 a serious run for its money. In order to address this internal threat, as well as a series of newer Japanese Cray-1-like machines, the Cray-2 memory system was dramatically improved, both in size as well as the number of "pipes" into the processors. When the machine was eventually delivered in 1985, the delays had been so long that much of its performance benefits were due to the faster memory. Purchasing the machine really made sense only for users with huge data sets to process.

The first Cray-2 delivered possessed more physical memory (256 [[Word (computer architecture)|MWord]]) than all previously delivered Cray machines combined.  Simulation moved from a 2-D realm or coarse 3-D to a finer 3-D realm because computation did not have to rely on slow virtual memory.  <!-- This inability to trade space (memory) for time (speed) is what defines supercomputation (extreme, high-end computing){{Fact|date=March 2010}}. -->