How to Handle Synchronization with Different Thread Counts for Producer and Consumer in CUTLASS?

Accelerated Computing CUDA CUDA Programming and Performance
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In scenarios where both producer and consumer threads exist, how can we achieve synchronization using CUTLASS’s barrier.sync/arrive? I understand that in barrier.arrive(a, b), a represents the number of threads required to arrive, and b is the barrier_ID. However, the number of producer and consumer threads is often different.

In FlashAttention3, I saw this example:

Here, the a parameter includes the consumer thread count (256) plus the active threads in the producer (32). However, I don’t understand why it is written this way.