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I invite you to search the ARM reference manual (the so called "ARM ARM") for the terms UNKNOWN and UNPREDICTABLE. There is also actually UNDEFINED, but in the context of ARM it means that a specific exception happens, so it's pretty much the most PREDICTABLE of the bunch.

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The old VLSI Technology Inc (later Philips, now NXP) reference manuals for one of the first ARM processors (the one used in the Acorn Archimedes 305, 310) mentioned that the algorithm used for cache eviction was not something relatively predictable like LIFO or FIFO but essentially random. Which in theory means that if you were 'unlucky' then your program would run slower than for a luckier person etc.

Ha. Though if you're seeing a persistent difference over time, thousands and millions of evictions, your luck is already so immaculately bad that you should expect the power to go out every time you try to run a program.

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