|Oracle9i Database Performance Tuning Guide and Reference
Release 2 (9.2)
Part Number A96533-02
This chapter explains how to tune the operating system for optimal performance of the Oracle database server.
This chapter contains the following sections:
Operating system performance issues commonly involve process management, memory management, and scheduling. If you have tuned the Oracle instance and you still need better performance, then verify your work or try to reduce system time. Make sure that there is enough I/O bandwidth, CPU power, and swap space. Do not expect, however, that further tuning of the operating system will have a significant effect on application performance. Changes in the Oracle configuration or in the application are likely to make a more significant difference in operating system efficiency than simply tuning the operating system.
For example, if an application experiences excessive buffer busy waits, then the number of system calls increases. If you reduce the buffer busy waits by tuning the application, then the number of system calls decreases.
Your Oracle platform-specific documentation and your operating system vendor's documentation
Operating systems and device controllers provide data caches that do not directly conflict with Oracle's own cache management. Nonetheless, these structures can consume resources while offering little or no benefit to performance. This is most noticeable on a UNIX system that has the database files in the UNIX file store; by default all database I/O goes through the file system cache. On some UNIX systems, direct I/O is available to the filestore. This arrangement allows the database files to be accessed within the UNIX file system, bypassing the file system cache. It saves CPU resources and allows the file system cache to be dedicated to non-database activity, such as program texts and spool files.
This problem does not occur on NT. All file requests by the database bypass the caches in the file system.
Although the operating system cache is often redundant because the Oracle buffer cache buffers blocks, there are a number of cases where Oracle does not use the Oracle buffer cache. In these cases, using direct I/O which bypasses the Unix or operating system cache or using raw devices which do not use the operating system cache may yield worse performance than using operating system buffering. Some examples of this include the following:
You may want a mix with some files cached at the operating system level and others not.
Some underlying I/O subsystems implement hardware-level caching of disk reads and writes to speed up their response times to I/O requests. It is important to ensure that such subsystems are configured to acknowledge write requests only after the written data is guaranteed to be safe.
For example, consider a subsystem that implements a RAM cache that acknowledges writes as soon as the written data is in the RAM cache. The Oracle database server considers this data to be safe. Then a power failure occurs and the data is lost. This can lead to corruption, because the Oracle server has no way of knowing that the write was lost. In order to overcome this problem, most I/O subsystems have a mechanism for ensuring data in the RAM cache is not lost across power failures. If the subsystem cannot guarantee this, then it is generally best to configure the system in such a way that a write is acknowledged only after the data has been written to disk, rather than only to the RAM cache.
With synchronous I/O, when an I/O request is submitted to the operating system, the writing process blocks until the write is confirmed as complete. It can then continue processing.
Asynchronous I/O allows a process to submit an I/O request but to then continue processing. It may then check on the result of the I/O at a later time. It is also possible to submit several I/O requests and then collect the status of those requests at a later time, thus allowing the operating system to parallelize any of those I/O operations, where possible. Parallel processing can reduce the overall time to complete an operation.
Consider an extreme example: Imagine you want to write out four data blocks to four different files. With synchronous I/O you must submit block 1, wait, submit block 2, wait, submit block 3, wait, submit block 4, and wait. With asynchronous I/O, you can submit blocks 1, 2, 3, and 4 and then wait for all four blocks to complete. Because you gave the operating system all four I/O requests at once, it can act on all the requests in parallel. The total response time is only the duration of the longest I/O of the four, rather than the sum of all four I/O durations.
Some platforms support asynchronous I/O by default, others need special configuration, and some only support asynchronous I/O for certain underlying file system types.
With Oracle9i Release 2 (9.2), you can use the
FILESYSTEMIO_OPTIONS initialization parameter to enable or disable asynchronous I/O or direct I/O on file system files. This parameter is platform-specific and has a default value that is best for a particular platform. It can be dynamically changed to update the default setting.
FILESYTEMIO_OPTIONS can be set to one of the following values:
ASYNCH:enable asynchronous I/O on file system files
DIRECTIO:enable direct I/O on file system files
SETALL:enable both asynchronous and direct I/O on file system files
NONE:disable both asynchronous and direct I/O on file system files
Memory usage is affected by both buffer cache limits and initialization parameters.
The UNIX buffer cache consumes operating system memory resources. Although in some versions of UNIX the UNIX buffer cache may be allocated a set amount of memory, it is common today for more sophisticated memory management mechanisms to be used. Typically these will allow free memory pages to be used to cache I/O. In such systems it is common for operating system reporting tools to show that there is no free memory which is not generally a problem. If processes require more memory, the memory caching I/O data is usually released to allow the process memory to be allocated.
The memory required by any one Oracle session depends on many factors. Typically the major contributing factors are:
In Oracle9i, the
PGA_AGGREGATE_TARGET initialization parameter gives greater control over a session's memory usage.
Many processes, or threads on NT systems, are involved in the operation of Oracle. They all access the shared memory resources in the SGA.
Be sure that all Oracle processes, both background and user processes, have the same process priority. When you install Oracle, all background processes are given the default priority for the operating system. Do not change the priorities of background processes. Verify that all user processes have the default operating system priority.
Assigning different priorities to Oracle processes might exacerbate the effects of contention. The operating system might not grant processing time to a low-priority process if a high-priority process also requests processing time. If a high-priority process needs access to a memory resource held by a low-priority process, then the high-priority process can wait indefinitely for the low-priority process to obtain the CPU, process the request, and release the resource.
Additionally, do not bind Oracle background processes to CPUs. This can cause the bound processes to be CPU-starved. This is especially the case when binding processes that fork off operating system threads. In this case, the parent process and all its threads bind to the CPU.
Some platforms provide operating system resource managers. These are designed to reduce the impact of peak load use patterns by prioritizing access to system resources. They usually implement administrative policies that govern which resources users can access and how much of those resources each user is permitted to consume.
Operating system resource managers are different from domains or other similar facilities. Domains provide one or more completely separated environments within one system. Disk, CPU, memory, and all other resources are dedicated to each domain and cannot be accessed from any other domain. Other similar facilities completely separate just a portion of system resources into different areas, usually separate CPU or memory areas. Like domains, the separate resource areas are dedicated only to the processing assigned to that area; processes cannot migrate across boundaries. Unlike domains, all other resources (usually disk) are accessed by all partitions on a system.
Oracle runs within domains, as well as within these other less complete partitioning constructs, as long as the allocation of partitioned memory (RAM) resources is fixed, not dynamic.
Oracle is not supported in any resource partitioned environment in which memory resources are assigned dynamically.
Operating system resource managers prioritize resource allocation within a global pool of resources, usually a domain or an entire system. Processes are assigned to groups, which are in turn assigned resources anywhere within the resource pool.
Oracle is not supported for use with any operating system resource manager's memory management and allocation facility. Oracle Database Resource Manager, which provides resource allocation capabilities within an Oracle instance, cannot be used with any operating system resource manager.
When running under operating system resource managers, Oracle is supported only when each instance is assigned to a dedicated operating system resource manager group or managed entity. Also, the dedicated entity running all the instance's processes must run at one priority (or resource consumption) level. Management of individual Oracle processes at different priority levels is not supported. Severe consequences, including instance crashes, can result.
This section provides hints for tuning various systems by explaining the following topics:
Familiarize yourself with platform-specific issues so that you know what performance options the operating system provides.
Your Oracle platform-specific documentation and your operating system vendor's documentation
On UNIX systems, try to establish a good ratio between the amount of time the operating system spends fulfilling system calls and doing process scheduling and the amount of time the application runs. The goal should be to run 60% to 75% of the time in application mode (often called user mode) and 25% to 40% of the time in operating system mode. If you find that the system is spending 50% of its time in each mode, then determine what is wrong.
The ratio of time spent in each mode is only a symptom of the underlying problem, which might involve the following:
If such conditions exist, then there is less time available for the application to run. The more time you can release from the operating system side, the more transactions an application can perform.
On NT systems, as with UNIX-based systems, establish an appropriate ratio between time in application mode and time in system mode. On NT you can easily monitor many factors with Performance Monitor: CPU, network, I/O, and memory are all displayed on the same graph to assist you in avoiding bottlenecks in any of these areas.
Consider the paging parameters on a mainframe, and remember that Oracle can exploit a very large working set.
Free memory in VAX or VMS environments is actually memory that is not mapped to any operating system process. On a busy system, free memory likely contains a page belonging to one or more currently active process. When that access occurs, a
soft page fault takes place, and the page is included in the working set for the process. If the process cannot expand its working set, then one of the pages currently mapped by the process must be moved to the free set.
Any number of processes might have pages of shared memory within their working sets. The sum of the sizes of the working sets can thus markedly exceed the available memory. When the Oracle server is running, the SGA, the Oracle kernel code, and the Oracle Forms runtime executable are normally all sharable and account for perhaps 80% or 90% of the pages accessed.
To address CPU problems, first establish appropriate expectations for the amount of CPU resources your system should be using. Then, determine whether sufficient CPU resources are available and recognize when your system is consuming too many resources. Begin by determining the amount of CPU resources the Oracle instance utilizes with your system in the following three cases:
You can capture various workload snapshots using Statspack or the
UTLESTAT utility. Operating system tools, such as
iostat on UNIX and Performance Monitor on NT, should be run during the same time interval as
UTLESTAT to provide a complimentary view of the overall statistics.
Chapter 21, "Using Statspack" for more information on Statspack and
Workload is an important factor when evaluating your system's level of CPU utilization. During peak workload hours, 90% CPU utilization with 10% idle and waiting time can be acceptable. Even 30% utilization at a time of low workload can be understandable. However, if your system shows high utilization at normal workload, then there is no room for a peak workload. For example, Figure 16-1 illustrates workload over time for an application having peak periods at 10:00 AM and 2:00 PM.
This example application has 100 users working 8 hours a day. Each user entering one transaction every 5 minutes translates into 9,600 transactions daily. Over an 8-hour period, the system must support 1,200 transactions an hour, which is an average of 20 transactions a minute. If the demand rate were constant, then you could build a system to meet this average workload.
However, usage patterns are not constant and in this context, 20 transactions a minute can be understood as merely a minimum requirement. If the peak rate you need to achieve is 120 transactions a minute, then you must configure a system that can support this peak workload.
For this example, assume that at peak workload, Oracle uses 90% of the CPU resource. For a period of average workload, then, Oracle uses no more than about 15% of the available CPU resource, as illustrated in the following equation:
tpm is transactions a minute.
If the system requires 50% of the CPU resource to achieve 20 tpm, then a problem exists: the system cannot achieve 120 transactions a minute using 90% of the CPU. However, if you tuned this system so that it achieves 20 tpm using only 15% of the CPU, then, assuming linear scalability, the system might achieve 120 transactions a minute using 90% of the CPU resources.
As users are added to an application, the workload can rise to what had previously been peak levels. No further CPU capacity is then available for the new peak rate, which is actually higher than the previous.
CPU capacity issues can be addressed with the following:
Oracle9i Database Performance Planning for information about improving your system architecture
Oracle has the several features for context switching, described in this section.
An Oracle process needs to be able to post another Oracle process (give it a message) and also needs to be able to wait to be posted.
For example, a foreground process may need to post LGWR to tell it to write out all blocks up to a given point so that it can acknowledge a commit.
Often this post-wait mechanism is implemented through UNIX Semaphores, but these can be resource intensive. Therefore, some platforms supply a post-wait driver, typically a kernel device driver that is a lightweight method of implementing a post-wait interface.
Oracle often needs to query the system time for timing information. This can involve an operating system call that incurs a relatively costly context switch. Some platforms implement a memory-mapped timer that uses an address within the processes virtual address space to contain the current time information. Reading the time from this memory-mapped timer is less expensive than the overhead of a context switch for a system call.
List I/O is an application programming interface that allows several asynchronous I/O requests to be submitted in a single system call, rather than submitting several I/O requests through separate system calls. The main benefit of this feature is to reduce the number of context switches required.
Oracle statistics report CPU use by Oracle sessions only, whereas every process running on your system affects the available CPU resources. Therefore, tuning non-Oracle factors can also improve Oracle performance.
Use operating system monitoring tools to determine what processes are running on the system as a whole. If the system is too heavily loaded, check the memory, I/O, and process management areas described later in this section.
Tools such as
-u on many UNIX-based systems let you examine the level of CPU utilization on your entire system. CPU utilization in UNIX is described in statistics that show user time, system time, idle time, and time waiting for I/O. A CPU problem exists if idle time and time waiting for I/O are both close to zero (less than 5%) at a normal or low workload.
On NT, use Performance Monitor to examine CPU utilization. Performance Manager provides statistics on processor time, user time, privileged time, interrupt time, and DPC time. (NT Performance Monitor is not the same as Performance Manager, which is an Oracle Enterprise Manager tool.)
This section describes how to check system CPU utilization on most UNIX-based and NT systems. For other platforms, see your operating system documentation.
Check the following memory management areas:
Use tools such as
vmstat on UNIX or Performance Monitor on NT to investigate the cause of paging and swapping.
On UNIX, if the processing space becomes too large, then it can result in the page tables becoming too large. This is not an issue on NT.
Thrashing is an I/O management issue. Ensure that your workload fits into memory, so the machine is not thrashing (swapping and paging processes in and out of memory). The operating system allocates fixed portions of time during which CPU resources are available to your process. If the process wastes a large portion of each time period checking to be sure that it can run and ensuring that all necessary components are in the machine, then the process might be using only 50% of the time allotted to actually perform work.
Check client/server round trips. There is an overhead in processing messages. When an application generates many messages that need to be sent through the network, the latency of sending a message can result in CPU overload. To alleviate this problem, bundle multiple messages together rather than perform lots of round trips. For example, you can use array inserts, array fetches, and so on.
Several process management issues discussed in this section should be checked.
The operating system can spend excessive time scheduling and switching processes. Examine the way in which you are using the operating system, because you could be using too many processes. On NT systems, do not overload your server with too many non-Oracle processes.
Due to operating system specific characteristics, your system could be spending a lot of time in context switches. Context switching can be expensive, especially with a large SGA. Context switching is not an issue on NT, which has only one process for each instance. All threads share the same page table.
There is a high cost in starting new operating system processes. Programmers often create single-purpose processes, exit the process, and create a new one. Doing this re-creates and destroys the process each time. Such logic uses excessive amounts of CPU, especially with applications that have large SGAs. This is because you need to build the page tables each time. The problem is aggravated when you pin or lock shared memory, because you have to access every page.
For example, if you have a 1 gigabyte SGA, then you might have page table entries for every 4 KB, and a page table entry might be 8 bytes. You could end up with (1G / 4 KB) * 8 byte entries. This becomes expensive, because you need to continually make sure that the page table is loaded.