CPU nerdery, M1 Max vs Ryzen 7950x memory bandwidth
"When using all 8 performance cores on my M1 Max, memory bandwidth to CPU is at about 120 GB/s (theoretical max is 200Gb/s).
Yet the Ryzen 7950x can do 81 GB/s memory bandwidth at most as the memory runs at 5200MT/s (* 8 bytes * 2 memory channels)/1024 = 81.25 GB/s (you can stretch this to about 100 GB/s if you heavily overclock). Thus despite the 7950x's 16 faster cores it's as fast as my M1 Max with 10 cores in this task because about 6 Ryzen cores are enough to reach that 81GB/s of bandwidth. The other 10 cores are starved from input and just idling.
This is not new; others have ran similar tasks with similar results. E.g. https://tlkh.dev/benchmarking-the-apple-m1-max who finds that
"... adding more cores on the 5600X does not help (2 cores are enough to maximize memory bandwidth), while 10 cores on the M1 Max is the optimal configuration".
The M1 Ultra has 20 cores and 400GB/s of memory bandwidth and thus runs way faster than the Ryzen 7950X as none of its 20 cores are starved. This is even more so when the Ryzen 7950X is decked out with 128GB of DDR5 RAM instead (4 DIMM slots) and therefore runs at a slower 3600 MT/s instead which is a meager 56.25 GB/s memory bandwidth. 5 Ryzen cores can fully consume that; the other 11 cores will just idle."
re: CPU nerdery, M1 Max vs Ryzen 7950x memory bandwidth
@noiob It does feel sort of inevitable. =:) Probably still different wafers - I vaguely recall CPUs and bulk DRAM use different processes - but interconnects are getting better all the time.
(I wonder how SL runs on an M2.. probably a sight faster than this 2015 MBP =:)