Database and charts for the latencies of input devices

1 - Latency charts 2 - 8000 Hz Polling rate - real benefit or voodoo? 3 - Test system and methods

Advantages and disadvantages of a polling rate of 8000 Hz

A polling rate of 8000 Hz for USB mice can no longer be technically implemented directly within the standardized USB protocol. USB Full Speed (USB 1.1), which is used by most input devices, is nominally designed for a maximum of 1000 queries per second (1 ms interval). However, in order to achieve a sampling rate of 8000 Hz – i.e. one query every 0.125 milliseconds – the mouse firmware, together with the driver and any special chipsets on the receiver side, must take a detour. This is usually done via so-called interval emulation or by using interrupt endpoints with greatly shortened time windows, in which the host is actively queried several times per USB frame.

Such solutions are technically complex, do not function stably system-wide and require both the mouse and the associated driver to be coordinated with each other. In addition, USB communication is always a pull process controlled by the host (PC), in which the devices do not send data independently, but only deliver data when the host explicitly requests it. This limits efficiency with very short polling intervals.A significant disadvantage of such “high-polling” configurations is the greatly increased USB bus load. As a mouse operating at 8000 Hz generates eight times as many interrupts as a mouse operating at 1000 Hz, it takes up a disproportionately large amount of the available bandwidth and host capacity – especially on USB controllers that manage several devices together (e.g. keyboard, audio interface, webcam or other mice). This bus load can lead to prioritization problems, such as other input devices being queried with a delay or at a reduced polling rate. This becomes particularly critical when using several high-frequency devices at the same time or with older chipsets with limited USB host performance.

In practice, this can lead to the following effects:

• Inputs from keyboards or controllers are accepted with a slight delay or irregularly.

• Audio interfaces with USB transmission can react unstably or generate short interruptions.

• There may be spikes in the CPU load, as each USB interrupt triggers a context switch in the operating system, which has a considerable impact at 8000 events per second.

For these reasons, 8000 Hz mice should preferably be operated alone or at least not on a shared USB hub. It is also advisable to connect them directly to a dedicated USB port on the mainboard, ideally one that is managed by the chipset separately from the rest of the I/O subsystem (e.g. AMD via separate USB root hubs or Intel via dedicated xHCI controllers). It is also a fact that 8000 Hz polling under USB can currently only be achieved through specific technical workarounds. Although this solution can bring advantages in certain cases, it requires a stable and coordinated system environment and is associated with non-negligible systemic risks and restrictions for other USB-based peripherals.