Subject: Raspberry Pi Compute Module 4 has been released

Thanks for posting, it wasn't clear that they were going to do a CM version of the Pi4 when I looked into it a while ago. Interesting change in design.

variant=raspberry-

I wonder how close in size that it to a mini-ITX motherboard and, if similar, do it's hold-downs match the mounting lugs in a mini-ITX case.

It is a great deal smaller than a mini-ITX motherboard at 55mmx40mm. You could pack a pretty large cluster of them in a mini-ITX case with a little ingenuity and a good fan.

CM4 is 55x40mm miniITX is 170x170mm

Or were you thinking of PC/104? Either way you'd only be able to use one mounting hole

In fact that is precisely what TuringPi does (aims to do). For now their ingenuity stops at 4, though:

It seems to be an evolution of their V1 solution for the CM3 in SO-DIMM form where they managed to get seven on the mainboard, now with four CM4's on SO-DIMM-like adapter boards. Probably room for improvement.

Is that just intended as a "fun" project?

If so, then fine, have fun; but if not, I can't see a four x CM4 cluster being faster or cheaper than a workaday PC, even Oracle's one thousand x Pi3 cluster seems kinda pointless ...

Hmm rows of three mounted vertically, probably need about a 2cm pitch (height isn't specified) so a mini-ITX board with 24 compute modules could probably be built. The case would probably need to be mostly fan.

In many such clusters, having multiple nodes is the whole point. They're good for prototyping platformss like Kubernetes, which in production might have dozens or hundreds of servers in a datacentre. A Pi cluster makes that cheap enough to have that a whole cloud on your desk. You can then test the cloud features, like load balancing and resiliency (pull the plug on a Pi and check it fails over correctly). When you're done, deploy your app to the production datacentre (at the cost of $100 per hour or whatever).

Another use case is where you might end up running a pile of VMs on a PC - it can end up more efficient to run one task per Pi instead.

Theo

The connectors are on the bottom, though, not on the side like with the SODIMM-like CM3, so you'd need some sort of adapter board which adds to the height, plus room for up to 200 connections for each CM4. Well, I haven't counted the actual number of signals... From the RPi blog: "two high-density perpendicular connectors (one for power and low-speed interfaces, and one for high-speed interfaces)" and from the datasheet: "The electrical interface of the CM4 is via two 100-pin high density connectors" and "The Module is 4.7 mm deep, but when connected the height will be 5.078 or 6.578 mm depending on the stacking height chosen." It also lists the complete pin-out:

Thanks - I've been thinking on and off about using mini-ITX-type cases to house much smaller servers and more power-efficient desktop stuff rather than the big old whitebox dual-Athlon thats my current house server. The TuringPi setup is interesting, but very much overkill for what I need if/ when the dual Athlon goes bang.

Currently it looks like a single RPi 4 or a plain mini-ITX system would fit my requirements rather better.

OK so 2cm pitch should work - that motherboard is going to need a lot of layers though :)

Gives a whole new meaning to "box fan" ...

Buy six, make a box out of them!

Added points if the thing can levitate and move around.