sys_attrs_vm(5)sys_attrs_vm(5)NAMEsys_attrs_vm - system attributes for the vm kernel subsystem
DESCRIPTION
This reference page describes system attributes for the Virtual Memory
(vm) kernel subsystem. See sys_attrs(5) for general guidelines about
changing system attributes.
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.
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 restric‐
tions. If the demand for memory exceeds the number of free
pages, processes with large resident set sizes are likely candi‐
dates for swapping.
The anon_rss_enforce attribute enables different levels of con‐
trol 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 sys‐
tem-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.
This attribute supports diskless systems and enables the pager
to be more responsive. It functions under the following condi‐
tions: The diskless driver is loaded and configured. Diskless
system services are part of the Dataless Management Services
(DMS). DMS enables systems to run the operating system from a
server without requiring a local hard disk on each client sys‐
tem. The server is serving a realtime pre-emptive kernel.
Default value: 0 (off)
Maximum value: 1 (on)
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.
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. On NUMA systems, the gh_chunks attribute affects only the
first Resource Affinity Domain (RAD). See the entry for
rad_gh_regions for more information.
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 data‐
base servers. Using segmented shared memory (SSM) is the alter‐
native 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.
The gh_* attributes, which includes gh_chunks, are automatically
disabled if the vm_bigpg_enabled attribute is set to 1. The
vm_bigpg_enabled attribute turns on “big pages” memory alloca‐
tion mode, which provides the advantages of using extended vir‐
tual page sizes without hard wiring a specific amount of physi‐
cal memory at boot time for this purpose.
See your database product documentation and the System Configu‐
ration and Tuning manual for more information about using granu‐
larity hints or SSM.
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() func‐
tion 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 sat‐
isfy the request.
Default value: 1 (enabled)
A value that specifies whether the memory reserved for granular‐
ity hints is (1) or is not (0) allocated from low physical mem‐
ory 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)
Specifies whether the memory reserved for granularity hints is
(1) or is not (0) sorted.
Default value: 0 (not sorted)
Size, in bytes, of the segment in which shared memory is allo‐
cated 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
Number of pages per thread that are used for stack space in ker‐
nel mode.
Default value: 2 (pages per thread)
Minimum value: 2
Maximum value: 3
It is strongly recommended that you not modify ker‐
nel_stack_pages unless directed to do so by your support repre‐
sentative. 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 ker‐
nel stack not valid halt error is always an unexpected error
that should be reported to your support representative for fur‐
ther investigation.
Number of freed kernel stack pages that are saved for reuse.
Above this limit, freed kernel stack pages are immediately deal‐
located.
Default value: 5 (pages)
Minimum value: 0
Maximum value: 2,147,483,647
Deallocation of freed kernel stack pages ensures that memory is
available for other operations. However, the processor time
required for deallocating freed kernel stack pages has a nega‐
tive performance impact that might be more noticeable on NUMA
systems than on other systems. You can use the kstack_free_tar‐
get value to make the most appropriate tradeoff between
increased memory consumption and time spent by CPUs in a purge
operation.
You can change the value of the kstack_free_target attribute
while the system is running.
A value that enables (1) or disables (0) caching of malloc mem‐
ory 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 docu‐
mentation.
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 docu‐
mentation.
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
For NUMA systems, the granularity hints chunk size (in
megabytes) for the Resource Affinity Domain (RAD) identified by
n. There are 64 elements in the attribute array,
rad_gh_regions[0] to rad_gh_regions[63]. Although all elements
in the array are visible on all systems, the kernel uses only
the element values corresponding to RADs that exist on the sys‐
tem. See the entry for the gh_chunks attribute for general
information about granularity hints memory allocation.
Default value: 0 (MB) (Granularity hints is disabled.)
The array of rad_gh_regions[n] attributes replace the gh_chunks
attribute, which affects only the first or (for non-NUMA sys‐
tems) only RAD (rad_gh_regions[0]) supported by the system.
Although gh_chunks and the set of rad_gh_regions attributes both
specify how much memory is manipulated through granularity hints
memory allocation, the unit of measurement for the former is
4-megabyte units whereas the unit of measurement for the latter
is megabytes. Therefore:
rad_gh_regions[0] = gh_chunks * 4
Setting the gh_chunks attribute, not the rad_gh_regions[0]
attribute, is recommended if you want to use granularity hints
memory allocation on non-NUMA systems.
The rad_gh_regions[n] attribute are automatically disabled if
the vm_bigpg_enabled attribute is set to 1. The vm_bigpg_enabled
attribute turns on “big pages” memory allocation mode, which
provides the advantages of using extended virtual page sizes
without hard wiring a specific amount of physical memory at boot
time for this purpose.
A value that controls whether user text can or cannot be repli‐
cated on multiple CPUs of a NUMA system. When the value is 1,
replication of user text is enabled. When the value is 0,
replication of user text is disabled. This attribute is some‐
times used by kernel developers when debugging software for NUMA
systems; however, the attribute is not for general use. (The
value is ignored on non-NUMA systems and changing it to 0 on
NUMA systems might degrade performance.)
Default value: 1
Do not change the value of this attribute unless instructed to
do so by support personnel or patch kit instructions.
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.
Percentage of memory above which the UBC is only borrowing mem‐
ory 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.
Obsolete; currently ignored by the software.
Specifies the number of pages to consolidate before initiating
an I/O operation.
Default value: 32 (pages)
Minimum value: 0
Maximum value: 512
The default value is appropriate for the vast majority of sys‐
tems. Raising this value may improve I/O efficiency if rela‐
tively few users and applications write to only a few very large
files, and there is high probability that write operations
affect contiguous pages. However, the cost is increased time
spent in memory (and holding locks for a longer length of time)
while the system determines what state pages are in and which
ones can be clustered.
A threshold value that forces cleanup of AdvFS metadata that is
being stored in the UBC. The default setting forces return of
pages containing AdvFS metadata when they reach 70 percent of
the UBC.
This is not a tuning parameter. Do not modify the default set‐
ting unless directed to do so by support personnel or patch kit
instructions.
Default value: 70 (percent)
Minimum value: 0
Maximum value: 100
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
Maximum percentage of physical memory that the UBC can use at
one time.
Default value: 100 (percent)
Minimum value: 0
Maximum value: 100
It is recommended that this value be set to a value in the range
of 70 to 80 percent. On an overloaded system, values higher than
80 can delay return of excess UBC pages to vm and adversely
affect performance.
Minimum percentage of physical memory that the UBC can use.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
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.
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
The minimum amount of anonymous memory (in Kbytes) that a user
process must request before the kernel will map a virtual page
in the process address space to more than one physical page.
Anonymous memory is requested by calls to mmap(), nmmap(), mal‐
loc(), and amalloc().
Default value: 64 (Kbytes)
Minimum value: 0 (big pages allocation mode disabled for anony‐
mous memory)
Consult with your support representative before changing
vm_bigpg_anon to a value other than the 64 Kbyte default.
The vm_bigpg_anon attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1.
Currently, big pages allocation of anonymous memory is not sup‐
ported for memory-mapped files.
If the anon_rss_enforce attribute (which sets a limit on the
resident set size of a process) is set to 1 or 2, it overrides
and disables big pages memory allocation mode for anonymous and
stack memory. Make sure that anon_rss_enforce is set to 0 if you
want big pages memory allocation to be applied to anonymous and
stack memory.
The master switch that enables (1) or disables (0) memory allo‐
cation for user processes in “big pages” mode.
Default value: 0 (disabled)
Big pages memory allocation allows a virtual page in the process
address space to be mapped to multiple pages in the system's
physical memory. This mapping can be to 8-pages, 64-pages, or
512-pages (64, 512, or 4096 Kbytes, respectively) of physical
memory.
Big pages uses threshold values set on a per memory-type basis
to determine whether a memory allocation request is eligible to
use the extended page sizes. The attributes that set these
thresholds are vm_bigpg_anon, vm_bigpg_shm, vm_bigpg_ssm,
vm_bigpg_seg, and vm_bigpg_stack.
Enabling the vm_bigpg_enabled attribute also enables level three
granularity hints. Level three granularity hints are controlled
by the vm_l3gh_anon, vm_l3gh_shm, and vm_l3gh_ssm attributes.
If big pages memory allocation is disabled, the kernel maps each
virtual page in the user address space to 8 Kbytes of memory.
To enable big pages, you must set the vm_bigpg_enabled attribute
at system boot time. Allow big pages to distribute memory
across RADs as a priority over getting the largest page size
possible
Default value: 1 (Use smp)
Setting the value to 0 enables this feature.
The minimum amount of memory (in Kbytes) that a user process
must request for a program text object before the kernel will
map a virtual page in the process address space to more than one
physical page. Allocations for program text objects are gener‐
ated when the process executes a program or loads a shared
library. See also the descriptions of vm_segment_cache_max and
vm_segmentation.
Default value: 64 (Kbytes)
Minimum value: 0 (big pages memory allocation disabled for pro‐
gram text objects)
Consult with your support representative before changing
vm_bigpg_seg to a value other than the 64 Kbyte default.
The vm_bigpg_seg attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1.
The minimum amount of System V shared memory (in Kbytes) that a
user process must request before the kernel will map a virtual
page in the process address space to more than one physical
page. Requests for System V shared memory are generated by calls
to shmget(), shmctl(), and nshmget().
Default value: 64 (Kbytes)
Minimum value: 0 (big pages allocation disabled for System V
shared memory)
Consult with your support representative before changing
vm_bigpg_shm to a value other than the 64 Kbyte default.
The vm_bigpg_shm attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1.
The minimum amount (in Kbytes) of segmented shared memory (Sys‐
tem V shared memory with shared page tables) that a user process
must request before the kernel will map a virtual page in the
process address space to more than one physical page. Requests
for this type of memory are generated by calls to shmget(), shm‐
ctl(), and nshmget().
Default value: 64 (Kbytes)
Minimum value: 0 (big pages allocation disabled for segmented
shared memory)
Consult with your support representative before changing
vm_bigpg_ssm to a value other than the 64 Kbyte default.
The vm_bigpg_ssm attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1.
The vm_bigpg_ssm attribute is disabled if the ssm_threshold
attribute is set to 0 (zero). If you want to use big pages mem‐
ory allocation for segmented shared memory, make sure that the
ssm_threshold is set to a value that is at least equal to the
value of SSM_SIZE. This value is defined in the <machine/pmap.h>
file. See sys_attrs_ipc(5) for more information.
The minimum amount of memory (in Kbytes) needed for the user
process stack before the kernel will map a virtual page in the
process address space to more than one physical page. Stack mem‐
ory is automatically allocated by the kernel on the user's
behalf.
Default value: 64 (Kbytes)
Minimum value: 0 (big pages allocation disabled for the user
process stack)
Consult with your support representative before changing
vm_bigpg_stack to a value other than the 64 Kbyte default.
The vm_bigpg_stack attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1.
If the anon_rss_enforce attribute (which sets a limit on the
resident set size of a process) is set to 1 or 2, it overrides
and disables big pages memory allocation of anonymous and stack
memory. Make sure that anon_rss_enforce is set to 0 if you want
big pages memory allocation to be applied to anonymous and stack
memory.
The percentage of physical memory that should be maintained on
the free page list for each of the four possible page sizes (8,
64, 512, and 4096 Kbytes).
When a page of memory is freed, an attempt is made to coalesce
the page with adjacent pages to form a bigger page. The
vm_bigpg_thresh attribute sets the threshold at which coalescing
begins. With smaller values, more pages are coalesced, hence
there are fewer pages available at the smaller sizes. This may
result in a performance degradation as a larger page will then
have to be broken into smaller pieces to satisfy an allocation
request for one of the smaller page sizes. If vm_bigpg_thresh
is too large, fewer large size pages will be available and
applications may not be able to take full advantage of big
pages. Generally, the default value will suffice, but this value
can be increased if the system work load requires more small
pages than large pages.
Default value: 6%
Minimum value: 0%
Maximum value: 25%
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
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
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
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
Obsolete; currently ignored by the software.
Minimum amount of time, in seconds, that a task remains in the
inswapped state before it is considered a candidate for outswap‐
ping.
Default value: 1 (second)
Minimum value: 0
Maximum value: 60 Enable level 3 granularity hints in anonymous
memory. This attribute has no effect unless the vm_bigpg_enabled
attribute is set to 1. Enable level 3 granularity hints in Sys‐
tem V shared memory. This attribute has no effect unless the
vm_bigpg_enabled attribute is set to 1. Enable level 3 granu‐
larity hints in segmented shared memory (System V shared memory
with shared page tables). This attribute has no effect unless
the vm_bigpg_enabled attribute is set to 1.
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
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
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 sev‐
eral gigabytes of physical memory and several thousand large
processes). The vm_overflow attribute changes how large memory
applications “borrow” memory from other resource affinity
domains (RADs) when their local memory resources are exhausted.
This can be used on all NUMA class systems.
When memory resources are depleted on a RAD in a NUMA system,
the vm subsystem automatically overflows to another RAD to ful‐
fill the memory allocation request. The following list illus‐
trates the default overflow behavior. If a step fails, the
process moves to the next step: Attempt an allocation from the
local RAD Page out a page of memory and “steal” it Attempt an
allocation from the next RAD Page out a page of memory and
“steal” it Continue until allocation/stealing has been attempted
on all RADs
Setting the vm_overflow attribute to 1 changes the order of page
allocations and page stealing as follows: Attempt an allocation
from the local RAD Attempt an allocation from the next RAD Con‐
tinue until allocation has been attempted on all RADs Revert to
the default behavior.
Setting vm_overflow to 1 may result in less paging activity for
some applications, thereby improving performance.
The threshold value that stops paging. 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.
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
The threshold value that begins hard swapping. When the number
of pages on the free list falls below this value for five sec‐
onds, 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
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 mem‐
ory.
Default value: 10 (pages)
Minimum value: 1
Maximum value: 2,147,483,647
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
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
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
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 pag‐
ing 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.
A percentage of the total pages of anonymous memory on the sys‐
tem 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.
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).
Number of text segments that can be cached in the segment cache.
(Applies only if you enable segmentation.)
Default value: 50 (segments)
Minimum value: 0
Maximum value: 8192
The vm subsystem uses the segment cache to cache inactive exe‐
cutables 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.)
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.
Specifies the swap allocation mode, which can be immediate mode
(1) or deferred mode (0). Immediate mode is commonly referred
to as “eager” mode and deferred mode is commonly referred to as
“lazy” mode.
Default value: 1 (eager swap mode)
In eager swap mode, the kernel will block a memory allocation
when it cannot reserve in advance a matching amount of swap
space. Eager swap mode is recommended for systems with variable
workloads, particularly for those with unpredictably high peaks
of memory consumption. For eager swap mode, swap space should
not be less than 111 percent of system memory. A swap space con‐
figuration of 150 percent of memory is recommended for most sys‐
tems, and small memory systems are likely to require swap space
in excess of 150 percent of memory. In eager swap mode, if swap
space is not configured to exceed the amount of memory by a
large enough percentage, the likelihood that system memory will
be underutilized during times of peak demand is increased. In
fact, configuring swap space that is less than the amount of
memory on the system, even if swapping does not occur, prevents
the kernel from using memory that represents the difference
between memory and swap space amounts. When swap space is
unavailable in eager swap mode, processes start blocking one
another and, worst case, cause the system to hang.
In lazy swap mode, the kernel does not require a matching amount
of swap space to be available in advance of a memory allocation.
However, in lazy swap mode, the kernel kills processes to
reclaim memory if an attempt to swap out a process fails because
of insufficient swap space. Because key kernel processes can be
killed, this condition increases the likelihood of a system
crash. Lazy swap mode is appropriate on very large memory sys‐
tems for which it is impractical to configure swap space that is
half again as large as memory. Lazy swap mode is also appropri‐
ate for smaller systems with a relatively constant and pre‐
dictable workload or for systems on which peak memory consump‐
tion is always well below the amount of memory that is avail‐
able. In all cases where lazy swap mode is used, enough swap
space must be configured to accommodate times of peak memory
consumption, plus an extra amount of swap space to provide a
margin of safety. To determine the amount of swap space that is
needed, monitor memory and swap space consumption over time to
determine consumption peaks and then factor in a generous margin
of safety.
The number of synchronous I/O requests that can be outstanding
to the swap partitions at one time. Synchronous swap requests
are used for page-in operations and task swapping.
Default value: 128 (requests)
Minimum value: 1
Maximum value: 2,147,483,647
Maximum percentage of physical memory that can be dynamically
wired. The kernel and user processes use this memory for dynam‐
ically allocated data structures and address space, respec‐
tively.
Default value: 80 (percent)
Minimum value: 1
Maximum value: 100
Enables, disables, and tunes the trolling rate for the memory
troller on systems supported by the memory troller.
When enabled, the memory troller continually reads the system's
memory to proactively discover and handle memory errors. The
troll rate is expressed as a percentage of the system's total
memory trolled per hour and you can change it at any time. Valid
troll rate settings are: Default value: 4 percent per hour
This default value applies if you do not specify any value for
vm_troll_percent in the /etc/sysconfigtab. At the default troll
rate, each 8 kilobyte memory page is trolled once every 24
hours. Disable value: 0 (zero)
Specify a value of 0 (zero) to disable memory trolling. Range:
1 - 100 percent
Specify a value in the range 1 to 100 to set the troll rate to a
percentage of memory to troll per hour. For example, a troll
rate of 50 reads half the total memory in one hour. After all
memory is read, the troller starts a new pass at the beginning
of memory. Accelerated trolling: 101 percent
Specify a value greater than 100 percent to invoke one pass
accelerated trolling. At this rate, all system memory is trolled
at a rate of approximately 6000 pages per second, where one page
equals 8 kilobytes. Trolling is then automatically disabled
after a single pass. This mode is intended for trolling all mem‐
ory quickly during off peak hours.
Low troll rates, such as the 4 percent default, have a negligi‐
ble impact on system performance. Processor usage for memory
trolling increases as the troll rate is increased. Refer to mem‐
ory_trolling(5) for additional performance information and mem‐
ory troller usage instructions.
Specifies the number of I/O operations that can be outstanding
while purging dirty (modified) pages from the UBC. The dirty
pages are flushed to disk to reclaim memory. The UBC purge dae‐
mon will stop flushing dirty pages when the number of I/Os
reaches the vm_ubcbuffers limit or there are no more dirty pages
in the UBC. AdvFS software does not use this attribute; only UFS
software uses it.
Default value: 256 (I/Os)
Minimum value: 0
Maximum value: 2,147,483,647
For systems running at capacity and on which many interactive
users are performing write operations to UFS file systems, users
might detect slowed response times if many pages are flushed to
disk each time the UBC buffers are purged. Decreasing the value
of vm_ubcbuffers causes shorter but more frequent purge opera‐
tions, thereby smoothing out system response times. Do not, how‐
ever, decrease vm_ubcbuffers to a value that completely disables
purging of dirty pages. One I/O for certain file systems might
be associated with many pages because of write clustering of
dirty pages.
Note
Changes to this attribute only take affect when made at boot
time.
You can also set the smoothsync_age attribute of the vfs kernel
subsystem to address response-time delays that can occur during
periods of intense write activity. The smoothsync_age attribute
uses a different metric (age of dirty pages rather than number
of I/Os) to balance the frequency and duration time of purge
operations and therefore does not support the ability of UFS to
flush all dirty pages for the same write operation at the same
time. However, smoothsync_age can be changed while the system is
running and is used by AdvFS as well as UFS software. See
sys_attrs_vfs(5) for information about the smoothsync_age
attribute.
The percentage of pages that must be dirty (modified) before the
UBC starts writing them to disk.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
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.
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
The maximum percentage of UBC memory that can be used to cache a
single file. See vm_ubcseqstartpercent for information about
controlling when the UBC checks this limit.
Default value: 10 (percent)
Minimum value: 0
Maximum value: 100
A threshold value (a percentage of the UBC in terms of its cur‐
rent size) that determines when the UBC starts to check the per‐
centage of UBC pages cached for each file object. If the cached
page percentage for any file exceeds the value of vm_ubcseqper‐
cent, the UBC returns that file's UBC LRU pages to virtual mem‐
ory.
Default value: 50 (percent)
Minimum value: 0
Maximum value: 100
SEE ALSO
Commands: dxkerneltuner(8), sysconfig(8), and sysconfigdb(8).
Others: memory_trolling(5), sys_attrs_proc(5), and sys_attrs(5).
System Configuration and Tuning
System Administration
sys_attrs_vm(5)
