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The classic Linux I/O model costs at least one system call per operation — and every syscall got materially more expensive after the Spectre/Meltdown mitigations. io_uring flips the model: two ring buffers shared between your app and the kernel, batching hundreds of operations into one syscall or, with a polling thread, zero. Here's the mechanism, why true async file I/O finally exists, and the honest security caveats that keep it off some production boxes.