| I mean at 2GHz, and 2/3c, the signal travels about 10cm in 1 clock cycle. So it's not negligible, but I suspect it has much more to do with signal integrity and the transmission line characteristics of the data bus. I think since on mobile CPUs, the RAM sits right on top of the SoC, very likely the CPUs are designed with a low RAM latency in mind. |
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| ▲ | formerly_proven 2 days ago | parent | next [-] | | > I mean at 2GHz, and 2/3c, the signal travels about 10cm in 1 clock cycle. So it's not negligible That's 0.5ns - if you look at end-to-end memory latencies, which are usually around 100ns for mobile systems, that actually is negligible, and M series chips do not have particularly low memory latency (they trend higher in comparison). | |
| ▲ | christkv 3 days ago | parent | prev [-] | | I think the m chips have much wider databus so bandwith is much higher as well as lower latency? | | |
| ▲ | VHRanger 3 days ago | parent [-] | | huh, it seems like the M4 pro can hit >400GB/s of RAM bandwidth whereas even a 9950x hits only 100GB/s. I'm curious how that is; in practice it "feels" like my 9950x is much more efficient at "move tons of RAM" tasks like a duckDB workload above a M4. But then again a 9950x has other advantages going on like AVX512 I guess? | | |
| ▲ | hnuser123456 3 days ago | parent [-] | | Yes, the M-series chips effectively use several "channels" of RAM (depending on the tier/size of chip) while most desktop parts, including the 9950x, are dual-channel. You get 51.2 GB/s of bandwidth per channel of DDR5-6400. You can get 8-RAM-channel motherboards and CPUs and have 400 GB/s of DDR5 too, but you pay a price for the modularity and capacity over it all being integrated and soldered. DIMMs will also have worse latency than soldered chips and have a max clock speed penalty due to signal degradation at the copper contacts. A Threadripper Pro 9955WX is $1649, a WRX90 motherboard is around $1200, and 8x16GB sticks of DDR5 RDIMMS is around $1200, $2300 for 8x32GB, $3700 for 8x64GB sticks, $6000 for 8x96GB. | | |
| ▲ | christkv 2 days ago | parent [-] | | Or you can get a strix halo 395+ that has 8 memory channels with a max of 128gb of ram. I think it does around 400 GB/s | | |
| ▲ | hnuser123456 2 days ago | parent [-] | | From what I see Strix Halo has a 256 bit memory bus, which would be like quad channel ddr5, but it's soldered so can run at 8000mt/s, which comes out to 256 GB/s. | | |
| ▲ | christkv a day ago | parent [-] | | Yeah you are right still up from the other consumer platforms |
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