A mainframe is just a very large server, with lots of reliability features (RAID-like memory, fault detection and mitigation, redundant components, etc) and lots of intelligent peripherals that offload work from the CPU so that it can spend as much time as possible running application code (and don't waste time handling interrupts, assembling network packets, dealing with IO, etc). A lot of these offload functions are baked into the ISA, making it a VERY CISC machine.

I believe Unisys still makes x86-based mainframes running MCP.

And loads of IP that IBM will defend vigorously with any infringement.

They invest a colossal amount of money creating those patents. There are lots of bullshit patents in this space, but IBM is not playing that kind of game.

Modern cloud environments are basically virtualised infinitely scalable mainframes.

Modern cloud environments tend to be aimed at running multiple independent workloads well on a huge server. Mainframes are generally aimed at running a smaller number of large workloads well on a huge server. Sort of analogous to multithreaded vs singlethreaded performance in CPU benchmarks.

My personal take:

The typical x86[1] is a sports car. Gets going fast, reaches most destinations fast, not great for driving for several hours, and not great at moving lots of cargo.

A mainframe is a freight train. Somewhat slow to get going, but can haul large amounts of cargo without breaks for a long time.

Mainframes weren't built for an interactive, highly variable, query-response workload; they were built for the classic overnight/monthly batch job that streams through a large amount of data.

[1]: It's not about the CPU, it's about the architecture around it, like this article talks about cache, expanded to I/O etc concerns.

That’s how I had always thought about mainframes before, but the focus on low latency here seems to suggest a different purpose (more sports car like than any x86 server cpu) is this a different kind of mainframe?

The freight train part of my thinking is admittedly dated .

This is an impressively fast design, but you'll get much more x86 for the same money -> x86 wins when you can scale out.