An Idle CPU Used to Be a Sin
This afternoon I ran top on my home server. Load average: 0.00, 0.00, 0.00. Fifteen gigabytes of RAM, barely half a gig in use. The CPU sat at 99.9% idle, as it had for most of the fifteen hours since I last booted it. Admittedly it’s just running a http server, database and some gnarly python code
My first reaction wasn’t satisfaction. It was a small, reflexive twinge of guilt – a feeling with a very specific origin. Somewhere in the back of my mind, a little voice from the VAX days was muttering: that’s a waste of a machine.
When idle meant money burning
If you came up in the era of the VAX, you learned utilisation as a moral value. A MicroVAX cost more than an extension on your house. When the hardware is the single most expensive thing in the building, an idle processor isn’t restful – it’s capital sitting there depreciating while it does nothing.
So you filled it. Overnight batch jobs to soak up the small hours. Timesharing so thirty people leaned on one machine at once. Background work nice-d down to scavenge whatever cycles the foreground left behind. A well-run system was a busy system. If your CPU wasn’t close to flat out, you had the feeling that someone would want to know why they’d bought so much computer for so little work.
Utilisation was the return on investment. You could point at a graph pinned near 100% and say: look, we’re getting our money’s worth.
The quiet inversion
At some point – and I don’t think any of us noticed the exact day – that logic turned completely inside out.
I sensed a shift when my company acquired a Windows NT server. It came with numerous updates containing exciting new features but the VAXs remained steadfastly using tried-and-tested software. There was no appetite for a major overhaul of our existing in-house software.
The silicon got cheap. I moved companies and left the DEC eco system behind and went to Wintel land. Now my always-on server is a small Lenovo that cost less than a VAX service contract’s monthly minimum, and it has more memory than entire machine rooms I once worked in. The expensive resource stopped being the computer. It became me: my time, my attention, the evenings I’d lose if the thing fell over.
And once the scarce resource is the human, “keep the CPU busy” stops being thrift and starts being a liability.
Idle isn’t waste. Idle is the point.
A processor pinned at 100% has no margin. No room for a spike, a runaway process, the sudden bit of work you didn’t plan for. A server loafing at 3% memory use is doing the most important job a home server has: being ready.
I want my server bored. Bored means when I ask it to do something new — a project module for my home CMMS, stand up Home Assistant — it just does it, without me having to go and make room first. That spare capacity I was raised to see as waste is actually a free upgrade path, held in reserve for pennies.
The thrift didn’t disappear. It moved. We’re still being careful with the scarce thing; the scarce thing just changed.
What the greybeards still got right
I’ll give the VAX crowd this, though. They would take one look at a modern operating system idling at three percent and calling itself “efficient,” and they would wince. And they wouldn’t be wrong to. There’s a discipline that came from having to account for every cycle, every process. There was no easy apply patch and reboot. A VAX was a cathedral: expensive, carefully maintained, built to last decades, with minimal changes. However, memory conveniently overlooks the less pleasant aspects like waiting minutes for simple commands, long compile times (since most programs needed compiling), slow disk access, carefully scheduling heavy jobs overnight and battling for CPU time with other users. Then there’s the painful three chevron prompt after a reboot followed by a string of commands to bring the machine back to life.
Both generations are practising thrift. We’re just measuring different kinds ofscarcity. Theirs was cycles, ours is not having to care.
And that, in summary I think, is the real luxury: a machine quiet enough that I get to forget it’s there.