Release 8.1.5
A67775-01
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| Oracle8i Tuning Release 8.1.5 A67775-01 |
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This chapter provides an overview of factors affecting performance in properly designed systems. Following the guidelines in this chapter cannot, however, compensate for poor design!
Later chapters discuss each of these factors in depth.
Figure 17-1 illustrates the factors involved in Oracle system performance for well designed applications.
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Note: Tuning these factors is effective only after you have tuned the business process and the application, as described in Chapter 2, "Performance Tuning Methods". |
Performance problems tend to be interconnected rather than isolated and unrelated. Table 17-1 identifies the key performance factors in existing systems as well as the areas in which symptoms may appear. For example, buffer cache problems may show up as CPU, memory, or I/O problems. Therefore, tuning the buffer cache CPU may improve I/O.
In a CPU-bound system, CPU resources might be completely allocated and service time could be excessive too. In this situation, you must improve your system's processing ability. Alternatively, you could have too much idle time and the CPU might not be completely used up. In either case, you need to determine why so much time is spent waiting.
To determine why there is insufficient CPU, identify how your entire system is using CPU. Do not just rely on identifying how CPU is used by Oracle server processes. At the beginning of a workday, for example, the mail system may consume a large amount of available CPU while employees check their messages. Later in the day, the mail system may be much less of a bottleneck and its CPU use drops accordingly.
Workload is a very important factor when evaluating your system's level of CPU use. During peak workload hours, 90% CPU use with 10% idle and waiting time may be understandable and acceptable; 30% utilization at a time of low workload may also be understandable. However, if your system shows high utilization at normal workloads, there is no more room for a "peak workload". You have a CPU problem if idle time and time waiting for I/O are both close to zero, or less than 5%, at a normal or low workload.
Sometimes a memory problem may be detected as an I/O problem. There are two types of memory requirements: Oracle and system. Oracle memory requirements affect the system requirements. Memory problems may be the cause of paging and swapping that occurs in the machine. So make sure your system does not start paging and swapping. The system should be able to run within the limitations set by internal memory.
System memory requirements for non-Oracle processes plus Oracle memory requirements should be equal to or less than the total available internal memory. To achieve this, reduce the size of some of the Oracle memory structures, such as the buffer cache, shared pool, or the redo log buffer. On the system level, you can reduce the number of processes and/or the amount of memory each process uses. You can also identify which processes are using the most memory. One way to reduce memory use is by sharing SQL.
Be sure to distribute I/O evenly across disks and channels. I/O resources include:
I/O problems may result from hardware limitations. Your system needs enough disks and SCSI busses to support the transaction throughput you need. You can evaluate the configuration by calculating the quantity of messages all your disks and busses can potentially support, and comparing that to the number of messages required by your peak workload.
If the response time of an I/O becomes excessive, the most common problem is that wait time has increased (response time = service time + wait time). If wait time increases, it means there are too many I/O requests for this device. If service time increases, this normally means that the I/O requests are larger, so you write more bytes to disk.
The different background processes, such as DBWR, ARCH, and so on, perform different types of I/O, and each process has different I/O characteristics. Some processes read and write in the block size of the database, some read and write in larger chunks. If service time is too high, stripe the file across different devices.
Mirroring can also be a cause of I/O bottlenecks unless the data is mirrored to a destination database that has the same number of disks as the source database.
Network constraints are similar to I/O constraints. You need to consider:
As the number of users increases and demand rises, the network can quietly become the bottleneck in an application. You may spend a lot of time waiting for network availability. Use available operating system tools to see how busy your network is.
Operating system software determines:
Before you can tune Oracle effectively, ensure the operating system is performing optimally. Work closely with the hardware/software system administrators to ensure that Oracle is allocated the proper operating system resources.
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See Also:
Operating system tuning is different for every platform. Refer to your operating system hardware/software documentation as well as your Oracle operating system-specific documentation for more information. In addition, see Chapter 24, "Tuning the Operating System". |
