The convoy effect in process scheduling
FCFS (First-Come, First-Served) scheduling can also cause blocking in a busy dynamic system in another way, known as the convoy effect. 

When one CPU intensive process blocks the CPU, a number of I/O intensive processes can get backed up behind it, leaving the I/O devices idle. When the CPU hog finally relinquishes the CPU, then the I/O processes pass through the CPU quickly, leaving the CPU idle while everyone queues up for I/O, and then the cycle repeats itself when the CPU intensive process gets back to the ready queue.