Memory access on the Apple M1 processor

Re:Memory access on the Apple M1 processor by celrod on Thursday January 07, 2021 @05:45AM (#25669553)

I just ran it again, and got more or less the same results:

  N = 1000000000, 953.7 MB
  starting experiments.
  two  : 29.7 ns
  two+  : 36.5 ns
  three: 43.8 ns

This surprises me. Normally, it does very well in most benchmarks I run.

Looking a little closer at the script, it loads numbers from "random", a vector of 3 million `Int` (this is hard coded, separate from `N`). This vector is about 11.4 MiB.

The Tiger Lake CPU has 12 MiB of L3 cache (same as your i7-9750H), so it barely fits. Meanwhile, the L1 cache is 48 KiB and the L2 cache is 1.5 MiB -- huge compared to most recent CPUS, and a lot of benefit in most benchmarks, but at the cost of higher latency. https://www.anandtech.com/show/16084/intel-tiger-lake-review...

Skylake's L3 latency was 26-37 cycles, and in Willow Cove's (Tiger Lake), it is 39-45 cycles. That difference by itself isn't big enough to account for the difference we're seeing, so something else must be going on.

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Re:Memory access on the Apple M1 processor by nkurz on Thursday January 07, 2021 @09:18AM (#25671254)

The caching of the random[] array (almost) shouldn't matter, as the access is sequential.

I'm wondering if the difference is the the number of active memory channels. How many channels does your respective computers support? Do you have enough RAM installed so all channels are in use? Are you able to do a RAM bandwidth test by some other means to verify?

Another possibility is that for some reason the base latency is just different between your machines. A commenter added a pointer-chasing variation of Daniel's test on his blog. Maybe run this to find the full latency and see how the times differ?

Finally, there was one more commenter on the blog who reported anomalously fast times on a Windows laptop. It's possible there is a bug with Daniel's time measurements on windows.

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Re:Memory access on the Apple M1 processor by gspr on Friday January 08, 2021 @02:40AM (#25682517)

I'll run the pointer-chasing version later. Thanks

Re timing issues: I am on Linux.

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