1. Is the system Response time directly / inversely proportional to the server utilization?
2. Will there be queues(for CPU/Memory/Disk) if Utilization is less than 100% ?
Actually speaking, there is no correlation between system response time and the server utilization. The Response time increase might happen even when the server is less utililised. (To illustrate this, look at the ATM example available in the PEA site - http://www.pea-online.com/resources.htm).
Most of us don’t understand this relationship. Increase in response time (during high load) is caused only because of queuing of the requests. The user request arrival pattern contributes to the high response time. Due to the adhoc user request arrival pattern, long queues are formed in various service centers which leads to high response time.
Response Time - What are the various components of Response time?
Waiting time (Queuing time) + Processing time = Response Time.
For a server, the processing time is always the same irrespective of load. For example, the server might take 2 seconds of processing during 1 user load and during 1000 users load for a transaction. But the waiting time would be high during 1000 users load, which drastically increases the overall response time. So, response time increases, if the waiting time increases which is caused by long queue.
Also, don’t think that there will be queues only if Utilization is more than 100%. There can be queues formed even when the Utilization is less. So, everything boils down to knowing the user arrival pattern of the system. This is going to determine, when the system is going to be loaded and break.
Hence, knowing the user arrival pattern of a system is very important rather than setting mere goals to perform load/stress test. Analyzing the user arrival pattern of an application would help in setting realistic goals which becomes more of organization logistics issue and most organizations bypass and pay for it later.
2. Will there be queues(for CPU/Memory/Disk) if Utilization is less than 100% ?
Actually speaking, there is no correlation between system response time and the server utilization. The Response time increase might happen even when the server is less utililised. (To illustrate this, look at the ATM example available in the PEA site - http://www.pea-online.com/resources.htm).
Most of us don’t understand this relationship. Increase in response time (during high load) is caused only because of queuing of the requests. The user request arrival pattern contributes to the high response time. Due to the adhoc user request arrival pattern, long queues are formed in various service centers which leads to high response time.
Response Time - What are the various components of Response time?
Waiting time (Queuing time) + Processing time = Response Time.
For a server, the processing time is always the same irrespective of load. For example, the server might take 2 seconds of processing during 1 user load and during 1000 users load for a transaction. But the waiting time would be high during 1000 users load, which drastically increases the overall response time. So, response time increases, if the waiting time increases which is caused by long queue.
Also, don’t think that there will be queues only if Utilization is more than 100%. There can be queues formed even when the Utilization is less. So, everything boils down to knowing the user arrival pattern of the system. This is going to determine, when the system is going to be loaded and break.
Hence, knowing the user arrival pattern of a system is very important rather than setting mere goals to perform load/stress test. Analyzing the user arrival pattern of an application would help in setting realistic goals which becomes more of organization logistics issue and most organizations bypass and pay for it later.
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