io_uring is faster than mmap

The io_uring code looks like it is doing all the fetch work in the background (with 6 threads), then just handing the completed buffers to the counter.

Do the same with 6 threads that would first read the first byte on each page and then hand that page section to the counter, you'll find similar performance.

And you can use both madvice / huge pages to control the mmap behavior

Even if you had a million SSDs and somehow were able to connect them to a single machine somehow, you would not outperform memory, because the data needs to be read into memory first, and can only then be processed by the CPU.

Basic `perf stat` and minor/major faults should be a first-line diagnostic.

This is an oversimplification. It depends what you mean by memory. It may be true when using NVMe on modern architectures in a consumer use case, but it's not true about computer architecture in general.

External devices can have their memory mapped to virtual memory addresses. There are some network cards that do this for example. The CPU can load from these virtual addresses directly into registers, without needing to make a copy to the general purpose fast-but-volatile memory. In theory a storage device could also be implemented in this way.

Which external device has memory access as fast or faster than generic RAM?

If a NIC can do that over PCI, probably other PCI devices could do the same, at least in theory.