The AI Engine architecture uses stream data extensively for DMA-based I/O, for communicating between two AI Engines, and for communicating between the AI Engine and the programmable logic (PL). This raises the potential for a resource deadlock when the data flow graph has reconvergent stream paths. If the pipeline depth of one path is longer than the other, the producer kernel can stall and might not be able to push data into the shorter path because of back pressure. At the same time, the consumer kernel is waiting to receive data on the longer path due to the lack of data. If the order of data production and consumption between two stream paths is different, a deadlock can happen even between two kernels that are directly connected with two stream paths. The following figure illustrates the paths.
If the producer kernel is trying to push data on stream
S1 and runs into back pressure while the consumer kernel is still trying to read data
from stream S2, a deadlock occurs. A general way to fix this situation is to create more
buffering in the paths that have back pressure in the source code by using a fifo_depth constraint on a connection.
p = kernel::create(producer);
c = kernel::create(consumer);
connect<stream> s1(p.out[0], c.in[0]);
connect<stream> s2(p.out[1], c.in[1]);
fifo_depth(s1) = 20;
fifo_depth(s2) = 10;Specifying FIFO Depth on the Producer and Consumer Side
Specifying a single fifo_depth value
assigns the FIFO to the destination side, or consumer kernel. You can also specify
two values for fifo_depth, assigning the first
value to the source side, the producer, and the second value to the consumer for
greater control of the FIFO implementation. Both source and destination can be all
AI Engine kernels, or a combination of
AI Engine kernels and PL kernels. For
example:
fifo_depth(stream_input1) = {10, 6}; // producer fifo depth=10, consumer fifo depth=6
fifo_depth(stream_input2) = {8, 20};