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Oracle Performance Tuning and Optimization
(Publisher: Macmillan Computer Publishing)
Author(s): Edward Whalen
ISBN: 067230886x
Publication Date: 04/01/96

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Part VI
Tuning the Network

Chapter 37 What Affects Network Performance?
38 Tuning the Network Components

In Part V of this book, you saw how the application can be designed and modified to take advantage of SQL. You also looked at how to best tune the client operating system to take maximum advantage of the resources available to it.

Part VI, “Tuning the Network,” covers everything that is left over after you have tuned the server and the application. The two chapters in this part explain how you know whether you have network problems and what to do to solve the problems.

Chapter 37
What Affects Network Performance?

This chapter looks at the network itself: how the physical networking hardware operates and communicates with the device drivers and OS. Once you understand how the network operates, the chapter describes some of the things that affect performance and how to know when your network requires an upgrade to accommodate more capacity.

With the increasing move to client/server applications, the network itself has become more important. For your application to function properly, you may have to move large amounts of data between the client and server machines. If there is any delay in the movement of this data, caused by network contention or inefficiency, your overall performance suffers.

By understanding how the network hardware and software operate, you can understand what affects the performance of the network and why. The chapter starts by looking at the different types of network hardware and software available.

Network Architecture

The network itself is made up of many components, both hardware and software, and may vary in function. Several different types of networks are available on the market today, including Ethernet, Token Ring, fiber optics, and some new technologies such as ATM. On top of these different hardware layers lies the OS device driver, the network protocol layer, and finally SQL*Net itself (see Figure 37.1).


Figure 37.1  Network layers.

For the SQL request to be sent from the client to the server, the request must go through each of these layers—on both the client and the server. The following sections describe these components in more detail.

Hardware Components

At the lowest layer of the network subsystem lies the network hardware itself. This hardware can be of a variety of different types that provide different functionalities and characteristics. Probably the most popular of these network types are Ethernet, Token Ring, and fiber optics networks (FDDI).

Ethernet

Ethernet is probably the most popular of the network protocols. Ethernet has been around since the early days of computer networking. The Ethernet standard includes various types of wiring (such as twisted pair and coaxial) and speeds (a standard speed of 10 megabits/second for most Ethernet and 100 megabits/second for the newer 100Base-T and 100Base-VG).

Ethernet is standards-based, which means that multiple vendors can build Ethernet NICs, hubs, routers, and so on that all work together, regardless of the brand. An Ethernet network is built around the idea that all machines on an Ethernet segment have equal access to the network. As long as the network is available, the NIC can use it (see Figure 37.2).


Figure 37.2  An Ethernet network.

The NIC sends the data in a structure called an Ethernet packet or frame. These packets contain not only the data network protocol information but also information the NIC has sent concerning the packet and the receiving NICs address. When a packet is sent, all NICs or routers on the network receive the packet. Each NIC performs a quick check; if the packet is not addressed for that NIC, the packet is quickly discarded. If the packet is intended for the NIC, further processing is done.

The Ethernet packet contains a network packet generated by the network protocol driver. Additional Ethernet information is placed around this network packet to create an Ethernet packet. The structure of the packet is determined by the network protocol being used.

When the device driver sends a network packet to the NIC to be sent across the network, the NIC checks to see whether there is any traffic currently on the network. If the network is busy, the NIC waits a while and then tries again. The packet is stalled waiting for the network and is referred to as a deferred packet.

When the NIC believes that the network is available, it sends the packet out. The NIC checks during the sending of the packet to make sure that no other NIC initiates a transmission at the same time. If another NIC had sent a packet out at the same time, a collision occurs. When a collision occurs, both NICs must wait a small amount of time and try to resend their packets. The contents of any sent packets that have experienced a collision are discarded.

Whenever a collision occurs and the packet has to be re-sent, performance is degraded a little. Although the effect of a single collision is very small and hardly noticed, the effect of thousands of collisions is noticeable. Collisions prevent the true bandwidth potential of an Ethernet network from being reached.

As the network is driven closer to its maximum bandwidth, the number of collisions increase exponentially. The more traffic there is on the network, the more likely it is that your packet will collide with another packet. Therefore, even though your network may be able to theoretically handle 10 megabits/second, it is unlikely that you will ever achieve this rate. These things make collisions more likely:

  High numbers of connections. If there are many NICs on the LAN segment, you are more likely to see collisions because it is more likely that a NIC will be transmitting.
  Small packet sizes. If the packet size is small, there will more likely be more packets, and the collision rate will increase.

With smaller packets, there is likely to be more wasted time between packets (that is, time during which there is no network activity). With larger packets, there is less inactivity on the network. Larger packets are more efficient and perform better.

As you see in Chapter 38, “Tuning the Network Components,” there are ways to avoid collisions and increase the performance of an Ethernet network by segmenting the network and reducing overhead.


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