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Paging-Related Parameters
The Solaris OS uses a demand paged virtual memory system. As the
system runs, pages are brought into memory as needed. When memory becomes occupied above
a certain threshold and demand for memory continues, paging begins. Paging goes through
several levels that are controlled by certain parameters. The general paging algorithm is as follows:
A memory deficit is noticed. The page scanner thread runs and begins to walk through memory. A two-step algorithm is employed:
A page is marked as unused. If still unused after a time interval, the page is viewed as a subject for reclaim.
If the page has been modified, a request is made to the pageout thread to schedule the page for I/O. Also, the page scanner continues looking at memory. Pageout causes the page to be written to the page's backing store and placed on the free list. When the page scanner scans memory, no distinction is made as to the origin of the page. The page might have come from a data file, or it might represent a page from an executable's text, data, or stack. As memory pressure on the system increases, the algorithm becomes more aggressive in the pages it will consider as candidates for reclamation and in how frequently the paging algorithm runs. (For more information, see fastscan and slowscan.) As available memory falls between the range lotsfree and minfree, the system linearly increases the amount of memory scanned in each invocation of the pageout thread from the value specified by slowscan to the value specified by fastscan. The system uses the desfree parameter to control a number of decisions about resource usage and behavior.
The system initially constrains itself to use no more than 4 percent of
one CPU for pageout operations. As memory pressure increases, the amount of CPU
time consumed in support of pageout operations linearly increases until a maximum of
80 percent of one CPU is consumed. The algorithm looks through some amount
of memory between slowscan and fastscan, then stops when one of the
following occurs:
Enough pages have been found to satisfy the memory shortfall. The planned number of pages have been looked at. Too much time has elapsed.
If a memory shortfall is still present when pageout finishes its scan, another
scan is scheduled for 1/4 second in the future. The configuration mechanism of the paging subsystem was changed, starting in the Solaris
9 release. Instead of depending on a set of predefined values for fastscan,
slowscan, and handspreadpages, the system determines the appropriate settings for these parameters at
boot time. Setting any of these parameters in the /etc/system file can cause
the system to use less than optimal values.
Caution - Remove all tuning of the VM system from the /etc/system file. Run with
the default settings and determine if it is necessary to adjust any of
these parameters. Do not set either cachefree or priority_paging. They have been
removed, starting in the Solaris 9 release.
Beginning in the Solaris 7 5/99 release, dynamic reconfiguration (DR) for CPU and
memory is supported. A system in a DR operation that involves the addition
or deletion of memory recalculates values for the relevant parameters, unless the parameter
has been explicitly set in /etc/system. In that case, the value specified in
/etc/system is used, unless a constraint on the value of the variable has
been violated. In this case, the value is reset.
lotsfree
- Description
Serves as the initial trigger for system paging to begin. When this threshold is crossed, the page scanner wakes up to begin looking for memory pages to reclaim. - Data Type
Unsigned long - Default
The greater of 1/64th of physical memory or 512 Kbytes - Range
The minimum value is 512 Kbytes or 1/64th of physical memory, whichever is greater, expressed as pages using the page size returned by getpagesize. For more information, seegetpagesize(3C). The maximum value is the number of physical memory pages. The maximum value should be no more than 30 percent of physical memory. The system does not enforce this range, other than that described in the Validation section. - Units
Pages - Dynamic?
Yes, but dynamic changes are lost if a memory-based DR operation occurs. - Validation
If lotsfree is greater than the amount of physical memory, the value is reset to the default. - Implicit
The relationship of lotsfree being greater than desfree, which is greater than minfree, should be maintained at all times. - When to Change
When demand for pages is subject to sudden sharp spikes, the memory algorithm might be unable to keep up with demand. One workaround is to start reclaiming memory at an earlier time. This solution gives the paging system some additional margin. A rule of thumb is to set this parameter to 2 times what the system needs to allocate in a few seconds. This parameter is workload dependent. A DBMS server can probably work fine with the default settings. However, you might need to adjust this parameter for a system doing heavy file system I/O. For systems with relatively static workloads and large amounts of memory, lower this value. The minimum acceptable value is 512 Kbytes, expressed as pages using the page size returned by getpagesize. - Commitment Level
Unstable
desfree
- Description
Specifies the preferred amount of memory to be free at all times on the system. - Data Type
Unsigned integer - Default
lotsfree / 2 - Range
The minimum value is 256 Kbytes or 1/128th of physical memory, whichever is greater, expressed as pages using the page size returned by getpagesize. The maximum value is the number of physical memory pages. The maximum value should be no more than 15 percent of physical memory. The system does not enforce this range other than that described in the Validation section. - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or calculated from the new physical memory value. - Validation
If desfree is greater than lotsfree, desfree is set to lotsfree / 2. No message is displayed. - Implicit
The relationship of lotsfree being greater than desfree, which is greater than minfree, should be maintained at all times. - Side Effects
Several side effects can arise from increasing the value of this parameter. When the new value nears or exceeds the amount of available memory on the system, the following can occur:
Asynchronous I/O requests are not processed, unless available memory exceeds desfree. Increasing the value of desfree can result in rejection of requests that otherwise would succeed. NFS asynchronous writes are executed as synchronous writes. The swapper is awakened earlier, and the behavior of the swapper is biased towards more aggressive actions. The system might not prefault as many executable pages into the system. This side effect results in applications potentially running slower than they otherwise would.
- When to Change
For systems with relatively static workloads and large amounts of memory, lower this value. The minimum acceptable value is 256 Kbytes, expressed as pages using the page size returned by getpagesize. - Commitment Level
Unstable
minfree
- Description
Specifies the minimum acceptable memory level. When memory drops below this number, the system biases allocations toward allocations necessary to successfully complete pageout operations or to swap processes completely out of memory. Either allocation denies or blocks other allocation requests. - Data Type
Unsigned integer - Default
desfree / 2 - Range
The minimum value is 128 Kbytes or 1/256th of physical memory, whichever is greater, expressed as pages using the page size returned by getpagesize. The maximum value is the number of physical memory pages. The maximum value should be no more than 7.5 percent of physical memory. The system does not enforce this range other than that described in the Validation section. - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or calculated from the new physical memory value. - Validation
If minfree is greater than desfree, minfree is set to desfree / 2. No message is displayed. - Implicit
The relationship of lotsfree being greater than desfree, which is greater than minfree, should be maintained at all times. - When to Change
The default value is generally adequate. For systems with relatively static workloads and large amounts of memory, lower this value. The minimum acceptable value is 128 Kbytes, expressed as pages using the page size returned by getpagesize. - Commitment Level
Unstable
throttlefree
- Description
Specifies the memory level at which blocking memory allocation requests are put to sleep, even if the memory is sufficient to satisfy the request. - Data Type
Unsigned integer - Default
minfree - Range
The minimum value is 128 Kbytes or 1/256th of physical memory, whichever is greater, expressed as pages using the page size returned by getpagesize. The maximum value is the number of physical memory pages. The maximum value should be no more than 4 percent of physical memory. The system does not enforce this range other than that described in the Validation section. - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or calculated from the new physical memory value. - Validation
If throttlefree is greater than desfree, throttlefree is set to minfree. No message is displayed. - Implicit
The relationship of lotsfree is greater than desfree, which is greater than minfree, should be maintained at all times. - When to Change
The default value is generally adequate. For systems with relatively static workloads and large amounts of memory, lower this value. The minimum acceptable value is 128 Kbytes, expressed as pages using the page size returned by getpagesize. For more information, seegetpagesize(3C). - Commitment Level
Unstable
pageout_reserve
- Description
Specifies the number of pages reserved for the exclusive use of the pageout or scheduler threads. When available memory is less than this value, nonblocking allocations are denied for any processes other than pageout or the scheduler. Pageout needs to have a small pool of memory for its use so it can allocate the data structures necessary to do the I/O for writing a page to its backing store. This variable was introduced in the Solaris 2.6 release to ensure that the system would be able to perform a pageout operation in the face of the most severe memory shortage. - Data Type
Unsigned integer - Default
throttlefree / 2 - Range
The minimum value is 64 Kbytes or 1/512th of physical memory, whichever is greater, expressed as pages using the page size returned by getpagesize(3C). The maximum is the number of physical memory pages. The maximum value should be no more than 2 percent of physical memory. The system does not enforce this range, other than that described in the Validation section. - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or calculated from the new physical memory value. - Validation
If pageout_reserve is greater than throttlefree / 2, pageout_reserve is set to throttlefree / 2. No message is displayed. - Implicit
The relationship of lotsfree being greater than desfree, which is greater than minfree, should be maintained at all times. - When to Change
The default value is generally adequate. For systems with relatively static workloads and large amounts of memory, lower this value. The minimum acceptable value is 64 Kbytes, expressed as pages using the page size returned by getpagesize. - Commitment Level
Unstable
pages_pp_maximum
- Description
Defines the number of pages that must be unlocked. If a request to lock pages would force available memory below this value, that request is refused. - Data Type
Unsigned long - Default
The greater of (tune_t_minarmem + 100 and [4% of memory available at boot time + 4 Mbytes]) - Range
Minimum value enforced by the system is tune_t_minarmem + 100. The system does not enforce a maximum value. - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or was calculated from the new physical memory value. - Validation
If the value specified in the /etc/system file or the calculated default is less than tune_t_minarmem + 100, the value is reset to tune_t_minarmem + 100. No message is displayed if the value from the /etc/system file is increased. Validation is done only at boot time and during dynamic reconfiguration operations that involve adding or deleting memory. - When to Change
When memory-locking requests fail or when attaching to a shared memory segment with the SHARE_MMU flag fails, yet the amount of memory available seems to be sufficient. Excessively large values can cause memory locking requests (mlock, mlockall, and memcntl) to fail unnecessarily. For more information, see mlock(3C), mlockall(3C), and memcntl(2). - Commitment Level
Unstable - Change History
For information, see pages_pp_maximum (Solaris Releases Prior to Solaris 9 Releases).
tune_t_minarmem
- Description
Defines the minimum available resident (not swappable) memory to maintain necessary to avoid deadlock. Used to reserve a portion of memory for use by the core of the OS. Pages restricted in this way are not seen when the OS determines the maximum amount of memory available. - Data Type
Signed integer - Default
25 - Range
1 to physical memory - Units
Pages - Dynamic?
No - Validation
None. Large values result in wasted physical memory. - When to Change
The default value is generally adequate. Consider increasing the default value if the system locks up and debugging information indicates that no memory was available. - Commitment Level
Unstable
fastscan
- Description
Defines the maximum number of pages per second that the system looks at when memory pressure is highest. - Data Type
Signed integer - Default
The lesser of 64 Mbytes and 1/2 of physical memory. - Range
1 to one-half of physical memory - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided by /etc/system or calculated from the new physical memory value. - Validation
The maximum value is the lesser of 64 Mbytes and 1/2 of physical memory. - When to Change
When more aggressive scanning of memory is preferred during periods of memory shortfall, especially when the system is subject to periods of intense memory demand or when performing heavy file I/O. - Commitment Level
Unstable
slowscan
- Description
Defines the minimum number of pages per second that the system looks at when attempting to reclaim memory. - Data Type
Signed integer - Default
The smaller of 1/20th of physical memory in pages and 100. - Range
1 to fastscan / 2 - Units
Pages - Dynamic?
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to the value provided in the /etc/system file or calculated from the new physical memory value. - Validation
If slowscan is larger than fastscan / 2, slowscan is reset to fastscan / 2. No message is displayed. - When to Change
When more aggressive scanning of memory is preferred during periods of memory shortfall, especially when the system is subject to periods of intense memory demand. - Commitment Level
Unstable
min_percent_cpu
- Description
Defines the minimum percentage of CPU that pageout can consume. This parameter is used as the starting point for determining the maximum amount of time that can be consumed by the page scanner. - Data Type
Signed integer - Default
4 - Range
1 to 80 - Units
Percentage - Dynamic?
Yes - Validation
None - When to Change
Increasing this value on systems with multiple CPUs and lots of memory, which are subject to intense periods of memory demand, enables the pager to spend more time attempting to find memory. - Commitment Level
Unstable
handspreadpages
- Description
The Solaris OS uses a two-handed clock algorithm to look for pages that are candidates for reclaiming when memory is low. The first hand of the clock walks through memory marking pages as unused. The second hand walks through memory some distance after the first hand, checking to see if the page is still marked as unused. If so, the page is subject to being reclaimed. The distance between the first hand and the second hand is handspreadpages. - Data Type
Unsigned long - Default
fastscan - Range
1 to maximum number of physical memory pages on the system - Units
Pages - Dynamic?
Yes. This parameter requires that the kernel reset_hands parameter also be set to a non-zero value. Once the new value of handspreadpages has been recognized, reset_hands is set to zero. - Validation
The value is set to the lesser of either the amount of physical memory and the handspreadpages value. - When to Change
When you want to increase the amount of time that pages are potentially resident before being reclaimed. Increasing this value increases the separation between the hands, and therefore, the amount of time before a page can be reclaimed. - Commitment Level
Unstable
pages_before_pager
- Description
Defines part of a system threshold that immediately frees pages after an I/O completes instead of storing the pages for possible reuse. The threshold is lotsfree + pages_before_pager. The NFS environment also uses this threshold to curtail its asynchronous activities as memory pressure mounts. - Data Type
Signed integer - Default
200 - Range
1 to amount of physical memory - Units
Pages - Dynamic?
No - Validation
None - When to Change
You might change this parameter when the majority of I/O is done for pages that are truly read or written once and never referenced again. Setting this variable to a larger amount of memory keeps adding pages to the free list. You might also change this parameter when the system is subject to bursts of severe memory pressure. A larger value here helps maintain a larger cushion against the pressure. - Commitment Level
Unstable
maxpgio
- Description
Defines the maximum number of page I/O requests that can be queued by the paging system. This number is divided by 4 to get the actual maximum number used by the paging system. This parameter is used to throttle the number of requests as well as to control process swapping. - Data Type
Signed integer - Default
40 - Range
1 to a variable maximum that depends on the system architecture, but mainly by the I/O subsystem, such as the number of controllers, disks, and disk swap size - Units
I/0s - Dynamic?
No - Validation
None - Implicit
The maximum number of I/O requests from the pager is limited by the size of a list of request buffers, which is currently sized at 256. - When to Change
Increase this parameter to page out memory faster. A larger value might help to recover faster from memory pressure if more than one swap device is configured or if the swap device is a striped device. Note that the existing I/O subsystem should be able to handle the additional I/O load. Also, increased swap I/O could degrade application I/O performance if the swap partition and application files are on the same disk. - Commitment Level
Unstable - Change History
For information, see maxpgio (Solaris 10 Releases).
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