static void blk_queue_congestion_threshold(struct request_queue *q)

Figure 17-9 displays the congestion thresholds that are computed for a request queue with a given length. The values for congestion_on and congestion_off differ slightly. This minor difference (known as hysteresis in the kernel sources, a term borrowed from physics) prevents queues from switching constantly between both states when the number of requests is close to the congestion threshold.

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Continue reading here: Setting and Clearing the Congested State

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