More into the basics – II

Agreed that was a lot of theory, and since I am no fan of theory myself, here are a few pictorial representations of what we were trying to understand until now.

Throughput:
I was mentioning before that throughput has a maximum, and once it has reached a maximum no additional load can increase the throughput. The following graph illustrates that.

The x-axis represents the load, and the y-axis represents the throughput. We can see that, when the load is 1, the throughput is 1. Relate this to the gift shop we were talking about, when there is one customer, one customer is served by the counter in a minute. Likewise, if there are 3 customers, 3 customers will be served by the shop at the same time (in a minute, in this case). This is the case until we have 5 customers, where the throughput also increases to 5 customers at the same time, as we have 5 counters. Now, if we have more than 5 customers, the throughput stays at 5 – logical, isn’t it? We have only 5 counters, and any customers more than 5 will have to queue up as the maximum number of customers served will be only 5. So here, you can see that once the throughput reaches 5, it stays constant there.

Something worth noting here: throughput basically represents a bottleneck in your system, so to say that something is preventing your system from yielding more results. In this case, we can say that our store’s number of customers processed is limited by the number of counters present in the store. If we need more than 5 customers to be attended to at the same time, we need to take the necessary steps to have that done. But let’s talk about this part later, when we have covered the necessary basics.

Response Time vs Throughput:

Let us now look at the relation between response time, throughput and load (we have covered this theoretically earlier).

You can see what I meant with respect to response times, that the response time is the sum of the waiting time and the processing time. For the first 5 users, the response time is 1 minute, the for the second set of 5 users, the response time is 2 minutes and so on. This illustrates that, as the load increases, the response time increases linearly. Although the throughput stays constant after a certain level, the response times will increase as the load increases.

We have been talking about load for quite some time now, let’s discuss how we decide on the load when we have to performance test an application in the next section.