Efficiency of Network Tables

[Jaci]

Originally Posted by Thad House

If you actually look at the code, Joystick get button and get axes calls are cached, however calling GetJoystickName or GetJoystickIsXbox are not cached, and are slow. The Robot State methods are not cached either, and directly call into the FPGA and are slow. And finally, in CANTalon, whenever some parameters are requested, there is an explicit Thread.sleep for about 1ms called, which also causes a context switch, and could be delaying even longer. So even though these methods should be non blocking, in the current implementation they are not exactly, however they are known block times, rather then indefinite waits like actual blocking calls would be.

GetJoystickName and GetJoystickIsXbox both retrieve values from the HALJoystickDescriptor, which stores values in memory (source 1 source 2). The Joystick Descriptor is updated each control packet as defined by DriverStation::GetData(). This code appears to be common between Java and C++ variants of the library.

A state retrieval doesn't affect the inner loop at all, since state ticking is done by WPILib before user code, and therefore should not cause a loop to slow.

I will, however, admit that CAN Status Signals are delayed, and this was a mistake in my reading. My apologies for that.