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sys_attrs_vm(5)
NAME
sys_attrs_vm - system attributes for the vm kernel subsystem
DESCRIPTION
This reference page describes system attributes for the Virtual Memory (vm)
kernel subsystem.
Do not directly edit the system configuration file to directly change the
value of the system parameter; use the dxkerneltuner application, the
/sbin/sysconfig -r command, or the sysconfigdb command to change the value
of the attribute. See dxkerneltuner(8), sysconfig(8), and sysconfigdb(8)
for more information about your options for configuring kernel subsystems.
The System Administration and System Configuration and Tuning books also
discuss this topic.
In the following list, an asterisk (*) precedes the names of attributes
whose values you can change while the system is running. Changes to values
of attributes whose names are not preceded by an asterisk take effect only
when the system is rebooted.
anon_rss_enforce
A value that sets no limit (0), a soft limit (1), or a hard limit (2)
on the resident set size of a process.
Default value: 0 (no limit)
By default, applications can set a process-specific limit on the number
of pages resident in memory by specifying the RLIMIT_RSS resource value
in a setrlimit() call. However, applications are not required to limit
the resident set size of a process and there is no system-wide default
limit. Therefore, the resident set size for a process is limited only
by system memory restrictions. If the demand for memory exceeds the
number of free pages, processes with large resident set sizes are
likely candidates for swapping.
The anon_rss_enforce attribute enables different levels of control over
process set sizes and when the pages that a process is using in
anonymous memory are swapped out (blocking the process) during times of
contention for free pages. Setting anon_rss_enforce to either 1 or 2,
allows you to enforce a system-wide limit on resident set size for a
process through the vm_rss_max_percent attribute. Setting
anon_rss_enforce to 1 (a soft limit), enables finer control over
process blocking and paging of anonymous memory by allowing you to set
the vm_rss_block_target and vm_rss_wakeup_target attributes.
When anon_rss_enforce is set to 2, the resident set size for a process
cannot exceed the system-wide limit set by the vm_rss_max_percent
attribute or a process-specific limit, if any, that is set by an
application's setrlimit() call. When the resident set size exceeds
either of these limits, the system starts to swap out pages of
anonymous memory that the process is already using to keep the resident
set size within the specified limit.
When anon_rss_enforce is set to 1, any system-default and process-
specific limits on resident set size still apply and will cause
swapping to occur when exceeded. Otherwise, a process's pages are
swapped out when the number of free pages is less than the value of the
vm_rss_block_target attribute. The process remains blocked until the
number of free pages reaches the value of the vm_rss_wakeup_target.
delayed_swapon
A value that enables or disables delayed swap when vm_swap_eager is
set. The kernel software manipulates this attribute internally to
resolve timing issues related to swap device availability.
Do not modify the default value of this attribute unless instructed to
do so by support personnel or patch kit instructions.
Default value: 0 (disallowed)
* dump_user_pte_pages
A value that enables (1) or disables (0) writing pages from the user
page table to a crash dump.
Default value: 0 (disabled)
It is recommended that you start to use the dump_user_pte_pages
attribute in the generic subsystem rather than the vm subsystem.
Attributes in both subsystems change the same address value; however,
dump_user_pte_pages will not be visible as a vm subsystem attribute in
a future release of the operating system.
* enable_yellow_zone
A value that enables (1) or disables (0) a soft guard page on the
program stack. This allows an application to enter a signal handler on
stack overflows, which otherwise would cause a core dump.
Default value: 0 (disabled)
The enable_yellow_zone attribute is intended for use by systems
programmers who are debugging kernel applications, such as device
drivers.
gh_chunks
Number of 4-MB chunks of memory reserved at boot time for shared memory
use. This memory cannot be used for any other purpose, nor can it be
returned to the system or reclaimed when not being used.
Default value: 0 (chunks) (The zero value means that use of granularity
hints is disabled.)
Minimum value: 0
Maximum value: 9,223,372,036,854,775,807
The attributes associated with "granularity hints" (the gh_*attributes)
are sometimes recommended specifically for database servers. Using
segmented shared memory (SSM) is the alternative to using granularity
hints and is recommended for most systems. Therefore, if the gh_chunks
attribute is not set to zero, the ssm_threshold attribute of the ipc
subsystem should be set to zero. If the gh_chunks attribute is set to
zero, the ssm_threshold attribute should not be set to zero.
See your database product documentation and the System Configuration
and Tuning manual for more information about using granularity hints or
SSM.
gh_fail_if_no_mem
A value that enables (1) or disables (0) a failure return by the shmget
function under certain conditions when granularity hints is in use.
When this attribute is set to 1, the shmget() function returns a
failure if the requested segment size is larger than the value of the
gh-min-seg-size attribute and if there is insufficient memory allocated
by the gh-chunks attribute to satisfy the request.
Default value: 1 (enabled)
gh_front_alloc
A value that specifies whether the memory reserved for granularity
hints is (1) or is not (0) allocated from low physical memory
addresses. Allocation from low physical memory addresses is useful if
you have an odd number of memory boards.
Default value: 1 (allocation from low physical memory addresses)
gh_keep_sorted
Specifies whether the memory reserved for granularity hints is (1) or
is not (0) sorted.
Default value: 0 (not sorted)
gh_min_seg_size
Size, in bytes, of the segment in which shared memory is allocated from
the memory reserved for shared memory, according to the value of the
gh-chunks attribute.
Default value: 8,388,608 (bytes, or 8 MB)
Minimum value: 0
Maximum value: 9,223,372,036,854,775,807
kernel_stack_pages
Number of pages per thread that are used for stack space in kernel
mode.
Default value: 2 (pages per thread)
Minimum value: 2
Maximum value: 3
The sysconfig command may display 0 (zero) when the actual setting is
2. This error will be corrected in a release following Tru64 UNIX
Version 5.0.
It is strongly recommended that you not modify kernel_stack_pages
unless directed to do so by your support representative. In the event
of a kernel stack not valid halt error that is caused by a kernel stack
overflow problem, increasing the value of kernel_stack_pages may work
around the problem. This workaround will not be successful if the error
occurred because the stack pointer became corrupted. In any event, a
kernel stack not valid halt error is always an unexpected error that
should be reported to your support representative for further
investigation.
malloc_percpu_cache
A value that enables (1) or disables (0) caching of malloc memory on a
per CPU basis.
Default value: 1
Do not modify the default setting for this attribute unless instructed
to do so by support personnel or by patch kit documentation.
new_wire_method
Default value: 1 (on)
Do not modify the default setting for this attribute unless instructed
to do so by support personnel or by patch kit documentation.
private_cache_percent
Percentage of the secondary cache that is reserved for anonymous
(nonshared) memory. Increasing the cache for anonymous memory reduces
the cache space available for file-backed memory (shared). This
attribute is useful only for benchmarking.
Default value: 0 (percent)
Minimum value: 0
Maximum value: 100
* private_text
Enables (1) or disables (0) mapping of private text pages. When
private_text is set to 1, executable text pages are not shared.
Default value: 0 (disabled)
Mapping of private text pages increases use of physical memory and swap
space, so changing the default setting of this attribute is not
recommended.
swapdevice
The device partitions reserved for swapping. This is a comma-separated
string, for example /dev/disk/dsk0g,/dev/disk/dsk0d that can be up to
256 bytes in length.
* ubc_borrowpercent
Percentage of memory above which the UBC is only borrowing memory from
the virtual memory subsystem. Paging does not occur until the UBC has
returned all its borrowed pages.
Default value: 20 (percent)
Minimum value: 0
Maximum value: 100
Increasing this value may increase UBC cache effectiveness and improve
throughput; however, the cost is a likely degradation of system
response time during a low memory condition.
ubc_ffl
Enables (1) or disables (0) the faulting of Unified Buffer Cache (UBC)
pages off the free list. When ubc_ffl is enabled, a UBC page freed by
the system is cached and can quickly be reclaimed from the free list
before it is allocated for another use.
Default value: 1 (enabled)
Temporarily setting ubc_ffl to 0 while vm_ffl remains at 1 (or the
reverse) sometimes provides information that is useful to operating
system developers when debugging system problems. However, do not
modify the default setting for ubc_ffl unless instructed to do so by
support personnel.
* ubc_maxdirtywrites
Number of I/O operations (per second) that the virtual memory subsystem
performs when the number of dirty (modified) pages in the UBC exceeds
the value of the vm-ubcdirtypercent attribute.
Default value: 5 (operations per second)
Minimum value: 0
Maximum value: 2,147,483,647
* ubc_maxpercent
Maximum percentage of physical memory that the UBC can use at one time.
Default value: 100 (percent)
Minimum value: 0
Maximum value: 100
* ubc_minpercent
Minimum percentage of physical memory that the UBC can use.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
* vm_aggressive_swap
A value that enables (1) or disables (0) the ability of the task
swapper to aggressively swap out idle tasks.
Default value: 0 (disabled)
Setting this attribute to 1 helps prevent a low-memory condition from
occurring and allows more jobs to be run simultaneously. However,
interactive response times are likely to be longer on a system that is
excessively paging and swapping.
* vm_asyncswapbuffers
The number of asynchronous I/O requests per swap partition that can be
outstanding at one time. Asynchronous swap requests are used for
pageout operations and for prewriting modified pages.
Default value: 4 (requests)
Minimum value: 0
Maximum value: 2,147,483,647
vm_clustermap
Size, in bytes, of the kernel cluster submap, which is used to allocate
the scatter/gather map for clustered file and swap I/O.
Default value: 1,048,576 (bytes, or 1 MB)
Minimum value: 0
Maximum value: 922,337,203,854,775,807
vm_clustersize
Maximum size, in bytes, of a single scatter/gather map for a clustered
I/O request.
Default value: 65,536 (bytes, or 64 KB)
Minimum value: 0
Maximum value: 922,337,203,854,775,807
vm_cowfaults
Number of times that the pages of an anonymous object are copy-on-write
faulted after a fork operation but before they are copied as part of
the fork operation.
Default value: 4 (faults)
Minimum value: 0
Maximum value: 2,147,483,647
vm_csubmapsize
Size, in bytes, of the kernel copy submap.
Default value: 1,048,576 (bytes, or 1 MB)
Minimum value: 0
Maximum value: 922,337,203,854,775,807
vm_ffl
Enables (1) or disables (0) the faulting of Virtual Memory (VM) pages
off the free list. When vm_ffl is enabled, a VM page freed by the
system is cached and can quickly be reclaimed from the free list before
it is allocated for another use.
Default value: 1 (enabled)
Temporarily setting vm_ffl to 0 while ubc_ffl remains at 1 (or the
reverse) sometimes provides information that is useful to operating
system developers when debugging kernel problems. Do not modify the
default setting for vm_ffl unless instructed to do so by support
personnel.
* vm_inswappedmin
Minimum amount of time, in seconds, that a task remains in the
inswapped state before it is considered a candidate for outswapping.
Default value: 1 (second)
Minimum value: 0
Maximum value: 60
vm_max_rdpgio_kluster
Size, in bytes, of the largest pagein (read) cluster that is passed to
the swap device.
Default value: 16,384 (bytes) (16 KB)
Minimum value: 8192
Maximum value: 131,072
vm_max_wrpgio_kluster
Size, in bytes, of the largest pageout (write) cluster that is passed
to the swap device.
Default value: 32,768 (bytes) (32 KB)
Minimum value: 8192
Maximum value: 131,072
vm_min_kernel_address
Base address of the kernel's virtual address space. The value can be
either Oxffffffff80000000 or Oxfffffffe00000000, which sets the size of
the kernel's virtual address space to either 2 GB or 8 GB,
respectively.
Default value: 18,446,744,073,709,551,615 (2 to the power of 64)
You may need to increase the kernel's virtual address space on very
large memory (VLM) systems (for example, systems with several gigabytes
of physical memory and several thousand large processes).
* vm_page_free_hardswap
The threshold value that stops page swapping. When the number of pages
on the free list reaches this value, paging stops.
Default value: Varies, depending on physical memory size; about 16
times the value of vm_page_free_target
Minimum value: 0
Maximum value: 2,147,483,647
The vm_page_free_hardswap value is computed from the
vm_page_free_target value, which by default scales with physical memory
size. If you change vm_page_free_target, your change affects
vm_page_free_hardswap as well.
* vm_page_free_min
The threshold value that starts page swapping. When the number of pages
on the free page list falls below this value, paging starts.
Default value: 20 (pages, or twice the amount of vm_page_free_reserved)
Minimum value: 0
Maximum value: 2,147,483,647
* vm_page_free_optimal
The threshold value that begins hard swapping. When the number of pages
on the free list falls below this value for five seconds, hard swapping
begins.
Default value: Automatically scaled by using this formula:
vm_page_free_min + ((vm_page_free_target - vm_page_free_min) / 2)
Minimum value: 0 (pages)
Maximum value: 2,147,483,647
* vm_page_free_reserved
The threshold value that determines when memory is limited to
privileged tasks. When the number of pages on the free page list falls
below this value, only privileged tasks can get memory.
Default value: 10 (pages)
Minimum value: 1
Maximum value: 2,147,483,647
* vm_page_free_swap
The threshold value that begins swapping of idle tasks. When the number
of pages on the free page list falls below this value, idle task
swapping begins.
Default value: Automatically scaled by using this formula:
vm_page_free_min + ((vm_page_free_target - vm_page_free_min) / 2)
Minimum value: 0
Maximum value: 2,147,483,647
* vm_page_free_target
The threshold value that stops paging, When the number of pages on the
free page list reaches this value, paging stops.
Default value: Based on the amount of managed memory that is available
on the system, as shown in the following table:
__________________________________________________
Available Memory (M) vm_page_free_target (pages)
__________________________________________________
Less than 512 128
512 to 1023 256
1024 to 2047 512
2048 to 4095 768
4096 and higher 1024
__________________________________________________
Minimum value: 0 (pages)
Maximum value: 2,147,483,647
vm_page_lock_count
Size (in number of locks) of the array that is used to synchronize
access to vm_page kernel structures. Instead of locking each page
structure, the virtual address is used to hash into the lock array.
Default value: 64 (locks)
Minimum value: 0
Maximum value: 2,147,483,647
Adjust the vm_page_lock_count value only if excessive lock contention
occurs.
* vm_page_prewrite_target
Maximum number of modified UBC pages that the vm subsystem will
prewrite to disk if it anticipates running out of memory. The
prewritten pages are the least recently used (LRU) pages.
Default value: vm_page_free_target * 2
Minimum value: 0
Maximum value: 2,147,483,647
* vm_page_private_color
Enables (1) or disables (0) division of the board cache (B-cache) into
two areas: one for private use (program stack, heap, and so forth) and
the other for the Unified Buffer Cache (UBC) and Segmented Shared
Memory (SSM). When vm_page_private_color is set to 1, memory
allocations for private use are limited to one half of the B-cache,
which may prevent B-cache thrashing between text and data pages.
Default value: 0 (disabled)
Setting vm_page_private_color to 1 may adversely affect the performance
of some applications. Therefore, change the default setting for this
attribute only if benchmarks show that the system is experiencing B-
cache thrashing.
* vm_rss_block_target
A threshold number of free pages that will start swapping of anonymous
memory from the resident set of a process. Paging of anonymous memory
starts when the number of free pages meets or exceeds this value. The
process is blocked until the number of free pages reaches the value set
by the vm_rss_wakeup_target attribute.
Default value: Same as vm_page_free_optimal
Minimum value: 0
Maximum value: 2,147,483,647
The default value of the vm_rss_block_target attribute is the same as
the default value of the vm_page_free_optimal attribute that controls
the threshold value for hard swapping. You can increase the value of
vm_rss_block_target to start paging of anonymous memory earlier than
when hard swapping occurs or decrease the value to delay paging of
anonymous memory beyond the point at which hard swapping occurs.
* vm_rss_max_percent
A percentage of the total pages of anonymous memory on the system that
is the system-wide limit on the resident set size for any process. The
value of this attribute has an effect only if anon_rss_enforce is set
to 1 or 2.
Default value: 100 (percent)
Minimum value: 1
Maximum value: 100
You can decrease this percentage to enforce a system-wide limit on the
resident set size for any process. Be aware, however, that this limit
applies to privileged, as well as unprivileged, processes and will
override a larger resident set size that may be specified for a process
through the setrlimit() call.
* vm_rss_wakeup_target
A threshold number of free pages that will unblock a process whose
anonymous memory is swapped out. The process is unblocked when the
number of free pages meets this value.
Default value: Same is vm_page_free_optimal
Minimum value: 0
Maximum value: 2,147,483,647
The default value of the vm_rss_wakeup_target attribute is the same as
the default value of the vm_page_free_optimal attribute that controls
the threshold value for hard swapping. You can increase the value of
vm_rss_wakeup_target to free more memory before unblocking a process or
decrease the value to unblock the process sooner (with less freed
memory).
vm_segment_cache_max
Number of text segments that can be cached in the segment cache.
(Applies only if you enable segmentation)
Default value: 50 (segments)
The vm subsystem uses the segment cache to cache inactive executables
and shared libraries. Because objects in the segment cache can be
accessed by mapping a page table entry, this cache eliminates I/O
delays for repeated executions and reloads.
Reducing the number of segments in the segment cache can free memory
and help to reduce paging overhead. (The size of each segment depends
on the text size of the executable or the shared library that is being
cached.)
* vm_segmentation
A value that enables (1) or disables (0) the ability of shared regions
of user address space to also share the page tables that map to those
shared regions.
Default value: 1 (enabled)
In a TruCluster environment, this value must be the same on all cluster
members.
vm_swap_eager
Specifies the swap allocation mode, which can be immediate mode (1) or
deferred mode (0).
Default value: 1 (immediate mode)
* vm_syncswapbuffers
The number of synchronous I/O requests that can be outstanding to the
swap partitions at one time. Synchronous swap requests are used for
pagein operations and task swapping.
Default value: 128 (requests)
Minimum value: 1
Maximum value: 2,147,483,647
vm_syswiredpercent
Maximum percentage of physical memory that can be dynamically wired.
The kernel and user processes use this memory for dynamically allocated
data structures and address space, respectively.
Default value: 80 (percent)
Minimum value: 1
Maximum value: 100
* vm_ubcbuffers
Total number of UBC I/O requests that can be outstanding at one time.
Default value: 256 (requests)
Minimum value: 0
Maximum value: 2,147,483,647
* vm_ubcdirtypercent
The percentage of pages that must be dirty (modified) before the UBC
starts writing them.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
* vm_ubcfilemaxdirtypages
In the context of an application thread, the number of pages that must
be dirty (modified) before the UBC update daemon starts writing them.
This value is for internal use only.
vm_ubcpagesteal
The minimum number of pages to be available for file expansion. When
the number of available pages falls below this number, the UBC steals
additional pages to anticipate the file's expansion demands.
Default value: 24 (file pages)
Minimum value: 0
Maximum value: 2,147,483,647
* vm_ubcseqpercent
The maximum percentage of UBC memory that can be used to cache a single
file.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
* vm_ubcseqstartpercent
A threshold value that determines when the UBC starts to recognize
sequential file access and steal the UBC LRU pages for a file to
satisfy its demand for pages. This value is the size of the UBC in
terms of its percentage of physical memory.
Default value: 50 (percent)
Minimum value: 0
Maximum value: 100
SEE ALSO
Commands: dxkerneltuner(8), sysconfig(8), sysconfigdb(8)
Others: sys_attrs(5)
System Configuration and Tuning
System Administration
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