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Comment by meindnoch | Hacker News Reader

What is exactly the use case for hot-swapping CPUs?

Swapping a failed device for a working one, without powering down the machine.

Curious to see how you imagine swapping all the registers and caches of a currently broken CPU (so, unable to do anything including sending those registers somewhere).

Because if you don't, sure, you didn't technically power down your machine, you just fucked up your entire stack, stack pointer goes to the wrong place, checks based on hardware are now wrong... In effect, you powered down the machine because your OS will just stop working, your running processes are fucked, etc.

Mainframes has been doing hot-swapping everything for decades. OS and motherboard is just supposed to tolerate multiple CPU crashes. Technical feasibility of that is a long-solved problem, only economically it's dubious.

The equivalent to flushing your register state is more comparable to a context switch than a complete shut down. Also Thanks to PCIe CPU swapping could be done in pure software at the OS level. No further hardware changes required. Simply copy the contents of your RAM to the other CPU card and manage the hand-off. This is clearly different from a complete shutdown e.g. all devices on the PCIe bus would be able to keep their state.

A good programmer familiar with the target OS and inner workings of PCIe could deliver a working prototype within a week, production ready within less than a year.

As a web developer unfamiliar with the hardware stack, I was surprised by this part...

> A good programmer familiar with the target OS and inner workings of PCIe could deliver a working prototype within a week, production ready within less than a year.

Could you please explain why it'd be normal to take a year to go from prototype to production in this case? What would happen in that one year, starting from a working prototype?

Most of the time will be spend on undocumented corner cases of third party hardware and dealing with various race conditions. Even though the PCIe protocol is reasonably simple the devices attached to it are not. Since this feature is only viable if it works 100% reliable the rule of thumb is the last 20% of problems will cost you 80% of the time.

But why would you do that on a Mac workstation?

Uptime [0]

———

[0]: http://bash.org/?search=Uptime&sort=0&show=25

Fault-tolerant systems with five nines uptime requirements.

How many people would run such fault-tolerant systems on Mac workstations?

None, you also need a fault-tolerant OS like Stratus VOS, but the question was not limited to Macs. I doubt they will sell many Mac Pros, the Studio’s rendered it irrelevant.

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