Advanced Hardware Features
Contents
Advanced Hardware Features¶
Color Swapping¶
The hardware supports color “swapping,” a feature allowing incoming data on
one color to be sent out on another color. Two colors are swappable if they
differ only in the low bit (i.e. x ^ y == 1).
Color swapping is set per-color East-West (horizontal) or North-South
(vertical).
Consider a color pair consisting of colors 2 and 3. If East-West color swapping is enabled for color 3, but not color 2, then wavelets arriving from either the East or West on color 2 will be received on both color 2 and color 3.
If both color 2 and color 3 have East-West color swapping enabled, then wavelets arriving from either the East or West on color 2 will be received on color 3, and wavelets arriving from either the East or West on color 3 will be received by color 2.
Note that the transmission is still subject to which directions are enabled in the receive and transmit fields for the color. The behavior described above also applies to North-South color swapping.
If both East-West color swap and North-South color swap are enabled for a given color, color swap is also enabled for wavelets arriving from the CE (onramp).
Compute Element (CE) Injection¶
CE inject mode is a per-color setting that can be enabled at compile-time. A CE inject configured color is hard wired to the tile’s output queue corresponding to the color number divided by 4. E.g. color 4 in this mode is connected to output queue 1. This is an integer floor division, so e.g. if color 5 is in this mode it is also connected to output queue 1. Note that this means in practice only one of every four colors can be used in CE inject mode at once.
Either low priority or high priority can be selected when configuring this mode. When either the fabric buffers are empty or the relevant output queue is empty, these modes behave identically. When both are non-empty, low priority mode will preference the fabric buffers, whereas high priority mode will preference the relevant output queue.
To explain this more concretely, in low priority mode, the switch for this color flips to position 1 when the fabric buffers are empty and begins transmitting from the relevant output queue. It switches back to position 0 if the fabric buffers ever become non-empty. In high priority mode, the switch will flip to position 1 whenever the relevant output queue is non-empty and will only flip back to position 0 when the output queue becomes empty. In either mode, control wavelets on this color have no effect; the switch position is controlled entirely by buffer and queue occupancy.