If Pi Clusters were actually cost competitive for performance there would be data centres full of them.
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Faith in the perfect efficiency of the free market only works out over the long term. In the short term we have a lot of habits that serve as heuristics for doing a good job most of the time.
For those like me that don't know the joke:
Two economists are walking down the street. One of them says “Look, there’s a twenty-dollar bill on the sidewalk!” The other economist says “No there’s not. If there was, someone would have picked it up already.”
The current RPi 5 makes no sense to me in any configuration, given its pricing.
If one just wants a cheap desktop box to do desktop things with, then they're a terrible option, price-wise, compared to things like used corpo mini-PCs.
But they're reasonably cost-competitive with other new (not used!) small computers that are tinkerer-friendly, and unlike many similar constructs there's a plethora of community-driven support for doing useful things with the unusual interfaces they expose.
My realization in ordering the Rock-2Fs is I really only need an MMU (that is, an SBC instead of something like an ESP32) when I'm running something with a graphical desktop, which is, outside my workstation, never (except for kiosks, which I use Android tablets for). -OR when I want to plug something into a bloated SBC board which saves me from having to solder a connector on, which is sometimes.
I use one for running a timelapse camera (camera is USB) while another is a portable mp3 player I can put in shirt pocket and which has aux port (tho its aux line is noisy). -So that's two of the four Rock-2F boards in use.... but it took me far less time to think up uses and deploy 25/25 of seeedstudio's ESP32C3 boards I ordered a couple years ago, and have used ~5/25 of the ESP32C6s I ordered early this year. They're so cheap, and use so much less energy than ARM boards, that it's difficult to justify using the SBCs anymore.
I think they're asking $50 for a base 2GB Pi4B, now -- that's 10 ESP32C3 boards (with integrated WiFi and BMS, btw!) -- and the Pi5 is even less competitive except in what I'd characterize as a very unusual scenario where you need high compute at edge (where it's both needed AND the latency of computing at the edge is lower than sending it to central server for processing), OR you need the security of protected memory, OR you have no central server and an ESP32 isn't going to cut it (I'll say, though, that one can run a thermostat with multiple WiFi-connected thermometers, and run a web server interface just fine.).
One day my primary Raspberry Pi broke (turned out to be a PSU issue), and I thought of having an old laptop running 24/7 as a home server. While being not very power hungry, it’s still wants much more energy (plus it has fans). For a casual usage (I forgot to mention Pi-Hole) it feels like an overkill. So, while a Raspberry Pi isn’t the best, it has its niche, and I’m happy of having one (actually, a few).