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Comment by deckar01 | Hacker News Reader

I once had to troubleshoot the math department director’s PC misbehaving. It turned out that he let prime95 have every spare cycle on a core 2 duo for a decade and the machine would only boot if it had cooled to room temperature.

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It took me way too long to remember that Prime95 is useful for something other than stress testing.

Looks like the project averaged about one new Mersenne Prime per year for 1996-2009, and then only 4 hits for 2010-2018 with none since 2018.

Obviously the tflops::hit difficulty ratio is ramping up as the numbers get larger, but I can't help wondering if the cryptocurrency craze dampened their work rate.

They're reporting 78,012 tflops of work done today, but my five minutes of investigation wasn't enough to find a historical chart of tflops/day and five minutes is about the limit of my curiosity on this matter for now.

When the project started, CPU frequency scaling wasn't a thing, so CPUs would run at full speed (and full power draw) 100% of the time. If you weren't making maximal use of the CPU, any remaining capacity would go to waste. Distributed computing projects could make use of that remaining capacity.

Today, CPUs are built with power efficiency in mind, and will attempt to scale down rapidly when not fully in use. Thus there is no longer such a thing as "spare CPU time". Any time spent on distributed computing projects is paid for in electricity costs. Some choose to continue anyway, but many have been disincentivized.

The hlt instruction has been around for a while.

I don't think that saved any power on a CPU without frequency scaling.

Hmmm.... there was some windows program called "waterfall" that kept your CPU cool when it was idle. Very useful for overclockers.

Was it snake oil like memory compressors?

Are some instructions cooler than others?

For a while I had a Home Assistant automation that would spin up Prime95 on a machine in my homelab when the closet it was in (in an unheated garage) got too cold. The closet also has the water meter, so it has to be kept above freezing. There's also a resistive heater, but I figured I'd rather get a bit of productive use out of those watts.

Then I realized that the computers heated the closet plenty without artificially pegging CPUs, so I didn't bother reimplementing it when I did a migration.

I don't think that's true. Variable frequency certainly helps efficiency, but like the other commenter said, HLT did exist. The CPU would use less power when told by the OS to do nothing for a short while.

It may have been replaced by a cryptocurrency indeed, for there was PrimeCoin, one of the very few that actually did something that was both productive and unprofitable (critically important for the economics of mining) with its mining cycles, and that is look for prime numbers. Although I don't remember if these were Mersenne Primes. It was one of the very earliest altcoins and by its nature was CPU bound which made it unpopular with large scale mining farms, but extremely popular with CPU cycle thieves working in clueless corporate and educational IT departments.

Unused cycles are wasted cycles. /s

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