Deconstructing Response Time

The overall response time is what most people care about. It is the average amount of time it takes for a job (a.k.a. request, transaction, etc.) to get processed.  The two big contributors to response time (ignoring transmission time for the moment) are the service time: the time to do the work and the wait time: the time you waited for your turn to be serviced.  Here is the formula: ResponseTime = WaitTime + ServiceTime

If you know the wait time, you can show how much faster things will flow if your company spends the money to fix the problem(s) you’ve discovered. If you know the service time then you know the max throughput as MaxThroughput ≤ 1 / AverageServiceTime 

For example: A key process in your transaction path with an average service time of 0.1 seconds has a maximum throughput of: 1 / 0.1 = 10 per second.

Sadly, response time is the only number that most meters are likely to give you. So how do you find the wait and the service time if there are no meters for them? The service time can be determined by metering the response time under a very light load when there are plenty of resources available. Specifically, when:

• Transactions are coming in slowly with no overlap
• There have been a few minutes of warm-up transactions
• The machines are almost idle

Under these conditions, the response time will equal the service time, as the wait time is approximately zero.

ServiceTime + WaitTime = ResponseTime
ServiceTime + 0 = ResponseTime
ServiceTime = ResponseTime          

The wait time can be calculated under any load by simply subtracting the average service time from the average response time.

WaitTime = ResponseTime – ServiceTime

Performance work is all about time and money. When you’ve found a problem, a question like: “How much better will things be when you fix this?” it is a very reasonable thing for the managers to ask. These simple calculations can help you answer that question.

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Other helpful hints can be found in: The Every Computer Performance Book which is available at Amazon, B&N, or Powell’s Books. The e-book is on iTunes.

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