Back in the 1980s, a personal computer was loaded with an operating system that could only support a single user. A "mainframe" was a system that could support more than one user at a time, and you'd connect to it using telnet.
Today, your Linux PC loaded with SSH effectively functions as a mainframe. Heck, you can load virtual machines that themselves can pretend to be mainframes from the 1970s. Heck... your cell phone uses this feature and pretends that apps are different users for maximum security.
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In contrast: GPUs are simply the 1980s style SIMD super-computers. It turns out SIMD Supercomputer style was the fastest way to calculate where pixels go on the screen (Conceptually: each pixel-shader basically gets to run its own thread: so a 1920x1080 screen has 2073600 threads to run per frame)
The best architecture to run millions of simple threads is a GPU or SIMD-computer (See https://en.wikipedia.org/wiki/Connection_Machine)
> The CM-1, depending on the configuration, has as many as 65,536 individual processors, each extremely simple, processing one bit at a time.