30 Mac Minis recreated a Cray-1 and hit 1.3 teraflops

The replica copies the look of the Cray-1, its curved tower, green panels and glass, at 1:1 scale, but not its insides. The museum spent about 18 months on the project and unveiled it in September as a 50th-anniversary tribute to both the Cray-1 and Apple’s founding.

Inside are 30 Intel Mac minis. Five have quad-core Core i7 chips and 25 have dual-core Core i5 chips. All but two date from 2012; the other two are 2014 models. Together that’s 70 processor cores. Half of the tower is still empty.

A separate Mac mini hidden in a custom control terminal runs the show. The terminal borrows its style from 1970s machines such as the DEC VT05 and the Sol-20, with an analog control panel. The team planned to use old CRT monitors but swapped in flat Samsung screens after compatibility trouble connecting vintage displays to newer computers.

Both are, at different moments. With all 70 cores running, the museum measured about 1.5 teraflops. One machine later went offline, leaving 64 active cores, and the most recent tests show about 1.3 teraflops.

A teraflop is a trillion calculations with decimal numbers every second. The Cray-1’s theoretical peak was roughly 160 megaflops, or 160 million. On paper that makes the Mac cluster thousands of times faster.

There’s a big caveat. The museum’s results come from its own Python script that multiplies large grids of numbers (matrix multiplication), while the Cray-1 figure is a theoretical peak. The comparison shows how far hardware has come. It doesn’t mean the two machines were tested the same way.

Most home-built clusters run Linux. This team picked macOS Mojave (version 10.14) because it already supported MPI, short for Message Passing Interface. MPI is the software layer that lets each computer in a cluster hand work to the others and collect the results, so 30 separate Macs can act as one machine.

It also suits the teaching goal. The original Cray-1 used vector processing: one instruction applied to a long row of numbers at once. The replica works the modern way, splitting a job into pieces and giving each to an independent processor. Visitors can watch the same math run on a single core and then across the full cluster, and see the speed jump.

The tower adds no cooling fans of its own. The minis stand upright on aluminum trays in a steel frame, with their bases removed. Warm air rises through the tower like a chimney and leaves through grilles at the top, while cool air comes in at the bottom.

The museum says you have to stand close to hear it. The loudest parts are two gigabit network switches at the base, at 30 to 33 decibels, and the team plans to replace them with silent ones. The real Cray-1, by contrast, needed heavy refrigeration to stay cool.

The lesson for the rest of us is that old hardware still has real value. When Apple moved from Intel chips to its own Apple silicon, a lot of perfectly working Intel Mac minis lost their place on desks. Networked together, they now handle serious math in silence.

Clusters like this are not exotic anymore. People build them from Raspberry Pis, and current Mac minis are grouped together to run AI software. If you have an old computer in a closet, it may not be a supercomputer on its own, but it is far more capable than the machines that once filled entire rooms.

The museum wants to fill the empty half of the tower with 70 to 80 new M4 Mac minis, possibly through crowdfunding, and get the machine onto the TOP500 list of the world’s fastest supercomputers. That is still only an ambition, and qualifying takes strict benchmarks and an official submission, not just more processors.

More vintage details are on the way, including original lamps from an IBM System/3, 3.5-inch and 5.25-inch floppy drives and a 3D-printed keyboard shell. The museum is in Majadas de Tiétar, in Spain’s Cáceres province, and is open for guided tours on Saturdays.