Real World Performance, Users and Impact
The STAR-100's architecture meant that its real world performance was a fraction of its peak performance. This was due to a number of reasons. Firstly, the vector instructions, being memory-to-memory, had a relatively long startup time, since the pipeline from the memory to the functional units was very long. In contrast to the register-based pipelined functional units in the 7600, the STAR pipelines were much deeper. The problem was compounded by the fact that the STAR had a slower cycle time than the 7600 (40 ns vs 27.5 ns). So the vector length needed for the STAR to run faster than the 7600 occurred at about 50 elements; if the loops were working on data sets with fewer elements, the cost of setting up the vector pipeline was higher than the savings provided by the vector instruction(s).
When the machine was released in 1974, it quickly became apparent that the general performance was nowhere near what people expected. Very few programs can be effectively vectorized into a series of single instructions; nearly all calculations will rely on the results of some earlier instruction, yet the results had to clear the pipelines before they could be fed back in. This forced most programs to hit the high setup cost of the vector units, and generally the ones that did "work" were extreme examples. Making matters worse was that the basic scalar performance was sacrificed in order to improve vector performance. Any time that the program had to run scalar instructions, the overall performance of the machine dropped dramatically. (See Amdahl's Law.)
Two STAR-100 systems were eventually delivered to the Lawrence Livermore National Laboratory and one to NASA Langley Research Center. In preparation for the STAR deliveries, LLNL programmers developed a library of subroutines, called STACKLIB, on the 7600 to emulate the vector operations of the STAR. In the process of developing STACKLIB, it was noticed that STACKLIB-based applications could run even faster on the 7600 than they had prior to the integration of the vector library. This discovery placed further pressures on the performance problems of the STAR.
The STAR-100 was a disappointment to everyone involved, and Jim Thornton, the chief designer, left CDC to form Network Systems Corporation. An updated version was later released in 1979 as the Cyber 203, followed by the Cyber 205 in 1980, but by this point systems from Cray Research with considerably higher performance were on the market. The failure of the STAR led to CDC being pushed from its former dominance in the supercomputer market, something they tried to address with the formation of ETA Systems in September 1983.
Read more about this topic: CDC STAR-100
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