FIFO Dynamics

FIFO Dynamics
As you recall, the FIFO passes the data from the producer to the consumer. In general, the rates at which data are produced and consumed can vary dynamically. Humans do not enter data into a keyboard at a constant rate.
Even printers require more time to print color graphics versus black and white text. Let tp be the time (in sec) between calls to PutFifo, and rp be the arrival rate (producer rate in bytes/sec) into the system. Similarly, let tg be the time (in sec) between calls to GetFifo, and rg be the service rate (consumerrate in bytes/sec) out of the system.

rg=1/tg
rp=1/tp
If the minimum time between calls to PutFifo is greater than the maximum time between calls to GetFifo,
min tp > max tg


the other hand, if the time between calls to PutFifo becomes less than the time between calls to GetFifo because either

• the arrival rate temporarily increases
• the service rate temporarily decreases


then information will be collected in the FIFO. For example, a person might type very fast for a while followed by long pause. The FIFO could be used to capture without loss all the data as it comes in very fast. Clearly on average the system must be able to process the data (the consumer thread) at least as fast as the average rate at which the data arrives (producer thread). If the average producer rate is larger than the average consumer rate


rp > rg




then the FIFO will eventually overflow no matter how large the FIFO. If the producer rate is temporarily high, and that causes the FIFO to become full, then this problem can be solved by increasing the FIFO size.

There is fundamental difference between an empty error and a full error. Consider the application of using a FIFO between your computer and its printer. This is a good idea because the computer can temporarily generate data to be printed at a very high rate followed by long pauses. The printer is like a
turtle. It can print at a slow but steady rate (e.g., 10 characters/sec.) The computer will put a byte into the FIFO that it wants printed. The printer will get a byte out of the FIFO when it is ready to print another character. A full error occurs when the computer calls PutFifo at too fast a rate. A full error is serious,
because if ignored data will be lost. On the other hand, an empty error occurs when the printer is ready to print but the computer has nothing in mind. An empty error is not serious, because in this case the printer just sits there doing nothing.