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This chapter explains how to tune the operating system for optimal performance of the Oracle database server.This chapter contains the following sections:Understanding Operating System Performance Issues
Solving Operating System Problems Understanding CPU Finding System CPU Utilization
Operating system performance issues commonly involve process management, memory management, and scheduling. If you have tuned the Oracle instance and still need to improve performance, verify your work or try to reduce system time. Ensure 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 result in 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.This section covers the following topics related to operating system performance issues:Operating systems and device controllers provide data caches that do not directly conflict with Oracle 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 Windows. 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 include:
Reads or writes to the TEMPORARY tablespace.Data stored in NOCACHE LOB Parallel Query slaves reading data You may want a mix with some files cached at the operating system level and others not.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. With asynchronous I/O, processing continues while the I/O request is submitted and processed. Use asynchronous I/O when possible to avoid bottlenecks.
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.
9.1.1.2 FILESYSTEMIO_OPTIONS Initialization ParameterYou 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, which has no timing requirement for transmission.
DIRECTIO: enable direct I/O on file system files, which bypasses the buffer cache.
SETALL: enable both asynchronous and direct I/O on file system files.
NONE: disable both asynchronous and direct I/O on file system files see Also:Your platform-specific documentation for more details Memory UsageMemory usage is affected by both buffer cache limits and initialization parameters Buffer Cache Limits
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:
Number of open cursors
Memory used by PL/SQL, such as PL/SQL tables
SORT_AREA_SIZE initialization parameter
In Oracle, the PGA_AGGREGATE_TARGET initialization parameter gives greater control over a session's memory usage.
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.
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.
Solving Operating System Problems Understanding CPU Finding System CPU Utilization
Operating system performance issues commonly involve process management, memory management, and scheduling. If you have tuned the Oracle instance and still need to improve performance, verify your work or try to reduce system time. Ensure 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 result in 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.This section covers the following topics related to operating system performance issues:Operating systems and device controllers provide data caches that do not directly conflict with Oracle 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 Windows. 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 include:
Reads or writes to the TEMPORARY tablespace.Data stored in NOCACHE LOB Parallel Query slaves reading data You may want a mix with some files cached at the operating system level and others not.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. With asynchronous I/O, processing continues while the I/O request is submitted and processed. Use asynchronous I/O when possible to avoid bottlenecks.
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.
9.1.1.2 FILESYSTEMIO_OPTIONS Initialization ParameterYou 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, which has no timing requirement for transmission.
DIRECTIO: enable direct I/O on file system files, which bypasses the buffer cache.
SETALL: enable both asynchronous and direct I/O on file system files.
NONE: disable both asynchronous and direct I/O on file system files see Also:Your platform-specific documentation for more details Memory UsageMemory usage is affected by both buffer cache limits and initialization parameters Buffer Cache Limits
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:
Number of open cursors
Memory used by PL/SQL, such as PL/SQL tables
SORT_AREA_SIZE initialization parameter
In Oracle, the PGA_AGGREGATE_TARGET initialization parameter gives greater control over a session's memory usage.
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.
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.
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