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2023-12_usb-real-data-rates.md

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    • 2023-12_usb-real-data-rates.md 1.89 KiB

    2023 12 12

    I've been trying to sort out how to get enough data out of the NIST printer to get to the "millisecond benchy" goal - part of that is figuring what real underlying data rates on USB serial links are.

    To start, I spun up ...

    4byte

    64byte

    128byte

    250byte

    So, we do win some speed when we increase packet sizes, but we have a clear trend around ~ 0.6 -> 0.9 Mbits/s ... which is not awesome; our spec for the data printer requires about 4 Mbits/s of real data rate.

    2023 12 20

    So - yeah, I want to keep working through this today, and I'm going to bundle code in here as well.

    I wanted to see about improving with nanocobs and maybe packing more data (up to 512 encoded bytes rather than 255), but I don't think that's the bottleneck - instead I suspect that we can try linux instead of windows... and then anyways move on to i.e. ethernet tests or multiple devices / async patterns etc etc etc, so, let's see about linux, running the same code.

    ... then test to see if we can (1) get some backpressure going on and (2) intelligently control data rates, maybe culminating in some demo of a high-throughput application where we can visualize data flows, and see i.e. how traffic patterns change over time (if new devices are added, for example).

    And then a next step would be to get the COBS thing up to doing flow-control, and testing TS as well, pushing a first-hello-world-for-each... if we can do that today, and show that we can RP2040 UART PIO, that's good shit.

    Results

    The data printer has a spec for 1.4Mbits/s total data, which initial tests look to not be capable of (out of one USB port) (2023-12-12 images).