Concurrent programs wait faster

1. 1 Concurrent programswait faster Tony Hoare In the modern world, many programs spend much of their time waiting. Unfortunately, so do their users. To reduce both kinds of waiting time, it is necessary for the programs to wait faster. Wait times can be significantly reduced by waiting for two or more events concurrently – either waiting for both of them, or (more effectively) waiting just for just one of them, whichever happens first. In the modern world, many programs spend much of their time waiting. Unfortunately, so do their users. To reduce both kinds of waiting time, it is necessary for the programs to wait faster. Wait times can be significantly reduced by waiting for two or more events concurrently – either waiting for both of them, or (more effectively) waiting just for just one of them, whichever happens first.

2. 2 Waiting for a bus Let P mean: wait for a bus on route nr 9. The average wait is 28 mins. Let Q mean: wait for a bus on route nr 7. The average wait time is 28 mins (P first Q) means: wait for the first of the two buses The average wait reduces to 11 mins I will explain how this is achieved by an example, taken from real life. Most days I go to work by bus. There are two bus routes that pass between my home and my office: a city route, number 9, and a country route, number 7. Let P stand for a decision to take the more interesting city route; and let Q stand for the decision to take the country route, nice in good weather. If I decide P , I find my average waiting time at the bus stop is twenty eight minutes. If I decide Q , I have to wait on average the same time. The busses arrive independently at random, at widely varying intervals. Since the journey time is only ten minutes, I really have a strong motive to reduce my time spent waiting at the bus stop. There is one remarkable way that I can do this: suppose I do not decide in advance on which bus to take, but just take the first bus that arrives; then I find that my average wait time goes down to just eleven minutes! I get to work much quicker, and I have done it, not by speeding up the bus, but simply by waiting twice as fast, or more! You can tell I am waiting faster, because I’m waiting less than half as long. I will now spend a little time to explain this paradox of waiting times, and show how the same technique applied inside a computer can make concurrent programs wait faster.I will explain how this is achieved by an example, taken from real life. Most days I go to work by bus. There are two bus routes that pass between my home and my office: a city route, number 9, and a country route, number 7. Let P stand for a decision to take the more interesting city route; and let Q stand for the decision to take the country route, nice in good weather. If I decide P , I find my average waiting time at the bus stop is twenty eight minutes. If I decide Q , I have to wait on average the same time. The busses arrive independently at random, at widely varying intervals. Since the journey time is only ten minutes, I really have a strong motive to reduce my time spent waiting at the bus stop. There is one remarkable way that I can do this: suppose I do not decide in advance on which bus to take, but just take the first bus that arrives; then I find that my average wait time goes down to just eleven minutes! I get to work much quicker, and I have done it, not by speeding up the bus, but simply by waiting twice as fast, or more! You can tell I am waiting faster, because I’m waiting less than half as long. I will now spend a little time to explain this paradox of waiting times, and show how the same technique applied inside a computer can make concurrent programs wait faster.

3. 3 Delay probabilities (say)average: .9 + .9 + 1 = 2.8