Re:Apple Silicon M1: Black Magic Fuckery
by kllrnohj on Thursday November 26, 2020 @12:50PM
(#25222108)
> How did you arrive at this number?
It's in the post. Half a cycle for an add or less, and cycles are every 1/3 nanosecond. So upper bound for an add would be around 1/6th a nanosecond. Likely less than that still yet, since the M1 is probably closer to an add in 1/8th a cycle not 1/2. Skylake by comparison is at around 1/4th a cycle for an add, and since M1's IPC is higher it's not going to be worse at basic ALU ops.
6 nanoseconds @ 3ghz is 18 cycles. That's on the slow end of the spectrum for a CPU instruction.
Re:Apple Silicon M1: Black Magic Fuckery
by saagarjha on Friday November 27, 2020 @01:57AM
(#25226152)
Where? 6 nanoseconds is pretty long, that’s about how long it’d take to do an entire retain/release pair, which is a couple dozen instructions I believe.
It's in the post. Half a cycle for an add or less, and cycles are every 1/3 nanosecond. So upper bound for an add would be around 1/6th a nanosecond. Likely less than that still yet, since the M1 is probably closer to an add in 1/8th a cycle not 1/2. Skylake by comparison is at around 1/4th a cycle for an add, and since M1's IPC is higher it's not going to be worse at basic ALU ops.
6 nanoseconds @ 3ghz is 18 cycles. That's on the slow end of the spectrum for a CPU instruction.