[TUHS] Re: Early multiprocessor Unix

On 11/28/22, Clem Cole <clemc(a)ccc.com> wrote: SBI (Synchronous Backplane Interconnect), the data bus for the 11/780. The KA780 CPU plugged into it, as did the memory controller, the UBA (UNIBUS adapter), the MBA (MASSBUS adapter), and the CI (Computer Interconnect, the high-speed network used for VAXcluster communication). Normally only one KA780 CPU was attached to the SBI, but there was nothing physical or electronic to prevent plugging in a second one. IIRC there were a couple of places in the 11/780 microcode that assumed there was only one CPU on the SBI, but that was easily tweaked. DEC productized the two-processor asymmetric multiprocessor configuration as the VAX-11/782. The software development tools group that I worked in was one of the two beta test sites for the 782 (the other was MIT). I was the system manager for that machine. Ours ran VMS, of course, not Unix, but with the same idea that only user mode code was scheduled to run on the second processor. All I/O and kernel mode code was done on the primary. It scaled pretty well for compute-intensive loads. It actually ran slower than a single processor under high I/O loads, though, and it gave no advantage to real-time workloads. I don't know if Ultrix ever supported the 11/782. The product had a rather short lifetime, as it was soon superseded by VAX models that were designed for symmetric multiprocessing from the ground up, and full SMP support had been implemented in VMS and Ultrix. -Paul W.