Capacity Planning and Architecture - Expert Oracle RAC Performance Diagnostics and Tuning - page 21

Analyzing the Stack
Typically, the computer system stack consists of the layers illustrated in Figure 2-1 . The application communicates
with the software libraries, which in turn communicate with the operating system (O/S), and the O/S depends on
system resources. Layers 1 to 4 in Figure 2-1 are primarily pass-through layers, and most of the activity happens when
the application or user session tries to get the result or compute the end results requested by the operation. Such
computations require resources, and obviously resources are not in abundance. Because there are limited resources,
this can cause several types of delays based on what resources are currently not available, causing processing delays,
transmission delays, propagation delays, and retransmission delays, to name a few. When processes are not able to
complete operations in time or there are delays in any of the layers illustrated in Figure 2-1 , the requests are queued.
When these processes don't release the resources on time, queuing delays are formed. When multiple requests for
resources are sent, over and above what is available, to obtain the right resource, large queues are formed (illustrated
in step 5), causing significant delays in response time.

Application

1

Layer of Software

2

Operating System

3

System Resources

4

5

Figure 2-1. System stack

Queuing is primarily due to lack of resources, or overutilization, or processes holding on to resources for long
periods of time.
To better understand this, we look at a simple metaphor of a restaurant where a customer spends a fair amount of
time inside to obtain service. The restaurant service time depends on how many customers come into the restaurant and
how soon a customer obtains the required service and leaves the restaurant. If the number of customers coming into
the restaurant increases or doubles, but the time required to service a customer remains the same, the customer spends
the same amount or an increased amount of time at the restaurant. This can best be understood using Little's theorem.
Little's theorem states that the average number of customers ( N ) can be determined from the following equation:

N = l T