Background and History
For the room-size computers of the 1960s and 1970s, a cost-effective way to increase compute power was to add a second CPU. Since these computers were already close to the fastest available (near the peak of the price:performance ratio), two standard-speed CPUs were much less expensive than a CPU that ran twice as fast. Also, adding a second CPU was less expensive than a second complete computer, which would need its own peripherals, thus requiring much more floor space and an increased operations staff.
Notable early offerings by computer manufacturers were the Burroughs B5000, the DECsystem-1055, and the IBM System/360 model 65MP. There were also dual-CPU machines built at universities.
The problem with adding a second CPU to a computer system was that the operating system had been developed for single-CPU systems, and extending it to handle multiple CPUs efficiently and reliably took a long time. To fill the gap, operating systems intended for single CPUs were initially extended to provide minimal support for a second CPU. In this minimal support, the operating system ran on the “boot” processor, with the other only allowed to run user programs. The next step towards symmetric multiprocessing (SMP) was to allow the operating system to run on any CPU, but only on one at a time, which could be thought of as a form of symmetric multiprocessing with a single coarse-grained lock. Eventually the operating systems improved their method of using the additional CPUs until they achieved full SMP, in which the operating system and the applications under its control ran on all of the CPUs simultaneously, with finer-grained locking.
Read more about this topic: Asymmetric Multiprocessing
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