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Comment by titzer | Hacker News Reader

Sure, and it has a very large out-of-order execution engine, but it is not fundamentally different from what other super scalar processors do. So I am curious what the OP meant by that offhand comment.

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One core of the M1 can drive the memory subsystem to the rails. A single core can copy (load+store) at 60GB/s. This is close to the theoretical design limit for DDR4X. A single core on Tiger Lake can only hit about 34GB/s, and Skylake-SP only gets about 15GB/s. So yes, it is close to 4x faster.

Thanks for clarifying. But this isn't any fundamental difference IMO. There isn't any functional limitation in an Intel core that means it cannot saturate the memory bandwidth from a single core, unless I am missing something.

I agree, it's not fundamental. It is, in particular, not that other popular myth, that it's "because ARM". It's only that 1 core on an Intel chip can have N-many outstanding loads and 1 core of an M1 can have M>N outstanding loads.

One could argue that it's not "fundamental", but it's definitely a functional limitation of the current Intel cores. The memory bandwidth of a single core is hardware limited by the number "Line Fill Buffers". Each buffer keeps track of one outstanding L1 cacheline miss, thus the number of LFB's limits the memory level parallelism (MLP). "Little's Law" gives the relationship between the latency, outstanding requests, and throughput. With 10 LFB's and the current latency of memory, it's physically impossible for a single core to use all available memory bandwidth, especially on machines with more than 2 memory channels.

The M1 chip allows higher MLP, presumably because it has more LFB's per core (or maybe they are using different approach where the LFB's are not per-core?). I apologize for using so many abbreviations. I searched to try to find a better intro, but didn't find anything perfect. I did come across this thread that (apparently) I started several years ago at the point where I was trying to understand what was happening: https://community.intel.com/t5/Software-Tuning-Performance/S....

Came across someone else mentioning the similar bandwidth constraint w.r.to LFB per core a month back.

https://news.ycombinator.com/item?id=25221968

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