Hello list, I would greatly appreciate your feedback on some assumptions and findings about the latency of USB-connected input devices. a) are there any known downsides to always polling USB-connected input devices at 1000 Hz instead of polling only every bInterval ms? b) IF there are no known downsides: why isn't polling at 1000 Hz the default on Linux? c) While the USB 3.0 standard actually seems to allow specifying polling intervals of 0.5 ms and 0.25 ms, it seems that the Linux USB stack does not support polling at these rates. Is this correct? Background: For an upcoming research paper on input latency, we have characterized the latency of 35 USB-connected input devices (mice, gamepads, keyboards) by electrically triggering device buttons and measuring the time until an input event arrived in the application. To this end, we used a Raspberry Pi 2 running Raspbian (iirc). A major, somewhat unsurprising, finding is that device latency varies a lot between devices but also depends to a large degree on the USB polling rate. Both average latency and variance in latency increase with longer polling intervals. We also tested each device with a slightly patched kernel that activated the equivalent of usbhid.mousepoll=1 for all input devices (when we started with this project, jspoll and kbpoll params were not yet supported by the kernel we used). Polling at 1 ms resulted in significant improvements for all devices which did not already report a bInterval of 1 (most reported 10). The average latency dropped by 5-10 ms for every device. The variance of the latency (max latency - min latency of 1000 measurements) dropped consistently from ~15 ms to ~5 ms. In our tests, no detrimental effects of 1000 Hz polling could be observed. In an earlier experiment, a few years ago, a cheapo gamepad became unresponsive during testing. We do not know whether this was caused by polling at a too high rate or due to mechanical stress and do not have it anymore. Feel free to check out https://hci.ur.de/projects/latency The page does not yet contain the new results, however. We will update it in early 2019. Cheers, Raphael Wimmer
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