Solaris Tunable Parameters Reference Manual
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Kernel Parameters

Process-Sizing Tunables

max_nprocs (Solaris 9 Releases)

The Solaris 10 description section was updated by removing the text “sun4m.”

General I/O Tunable Parameters

maxphys (Solaris 10 Releases)

The default value is updated to include sun4v systems. For more information, see maxphys.

maxphys (Solaris 9 Releases)

The text “126,976 (sun4m)” was removed from the Solaris 10 maxphys default section.

rlim_fd_max (Solaris 8 Release)

In the Solaris 8 version, the default is 1024. In later Solaris releases, the default is 65,536.

General Kernel and Memory Parameters

noexec_user_stack (Solaris 9 Releases)

The Solaris 10 description section was updated by removing the text “and sun4m” and adding the text “64–bit SPARC and AMD64.”

lwp_default_stksize (Solaris 9 Releases)

The Solaris 10 description section was updated by adding default and maximum values for AMD64.

The Solaris 10 default value for SPARC platforms was changed to 24,576.

noexec_user_stack (Solaris 2.6, 7, and 8 Releases)

The Solaris 9 description section was updated by removing the text “and sun4d.”

segkpsize (Solaris 9 12/02 Release)

In previous Solaris 9 releases, units were incorrectly identified as Mbytes instead of 8-Kbyte pages. In addition, the following text is removed from the range and default descriptions in the Solaris 10 release because this parameter is only available on systems running 64-bit kernels:

32-bit kernels, 512 Mbytes

Paging-Related Tunable Parameters

maxpgio (Solaris 10 Releases)

In the Solaris 10 versions, the range value was incorrectly documented as 1 to 1024. The actual range depends on system architecture and I/O subsystems. For more information, see maxpgio.

pages_pp_maximum (Solaris Releases Prior to Solaris 9 Releases)

In the Solaris 8 versions, the default description is as follows:

Maximum of the triplet (200, tune_t_minarmem + 100, [10% of memory available at boot time])

General File System Parameters

ncsize (Solaris 9 and Previous Releases)

In the Solaris 9 release and previous releases, the nfs:nrnode parameter was incorrectly identified as nfs:nfs_rnode in the when to change description.

ncsize (Solaris 10 Release)

In the Solaris 10 release, the default value of the ncsize parameter was incorrectly described as follows:

4 x (v.v_proc + maxusers) + 320 / 100

The correct default value is as follows:

(4 x (v.v_proc + maxusers) + 320) + (4 x (v.v_proc + maxusers) + 320 / 100

For more information, see ncsize.

UFS Tunable Parameters

bufhwm (Solaris 9 Releases)

This parameter information changed significantly in the Solaris 10 release. Please see bufhwm and bufhwm_pct for more information.


Maximum amount of memory for caching I/O buffers. The buffers are used for writing file system metadata (superblocks, inodes, indirect blocks, and directories). Buffers are allocated as needed until the amount to be allocated would exceed bufhwm. At this point, enough buffers are reclaimed to satisfy the request.

For historical reasons, this parameter does not require the ufs: prefix.

Data Type

Signed integer


2% of physical memory


80 Kbytes to 20% of physical memory




No. Value is used to compute hash bucket sizes and is then stored into a data structure that adjusts the value in the field as buffers are allocated and deallocated. Attempting to adjust this value without following the locking protocol on a running system can lead to incorrect operation.


If bufhwm is less than 80 Kbytes or greater than the lesser of 20% of physical memory or twice the current amount of kernel heap, it is reset to the lesser of 20% of physical memory or twice the current amount of kernel heap. The following message appears on the system console and in the /var/adm/messages file.

"binit: bufhwm out of range (value attempted). Using N."

Value attempted refers to the value entered in /etc/system or by using the kadb -d command. N is the value computed by the system based on available system memory.

When to Change

Since buffers are only allocated as they are needed, the overhead from the default setting is the allocation of a number of control structures to handle the maximum possible number of buffers. These structures consume 52 bytes per potential buffer on a 32–bit kernel and 104 bytes per potential buffer on a 64–bit kernel. On a 512 Mbyte 64–bit kernel this consumes 104*10144 bytes, or 1 Mbyte. The header allocations assumes buffers are 1 Kbyte in size, although in most cases, the buffer size is larger.

The amount of memory, which has not been allocated in the buffer pool, can be found by looking at the bfreelist structure in the kernel with a kernel debugger. The field of interest in the structure is bufsize, which is the possible remaining memory in bytes. Looking at it with the buf macro by using mdb:

# mdb -kLoading modules: [ unix krtld genunix ip nfs ipc ]
> bfreelist$<buf
[ elided ]
bfreelist + 0x78:    bufsize            [ elided ]

bufhwm on this system, with 6 Gbytes of memory, is 122277. It is not directly possible to determine the number of header structures used since the actual buffer size requested is usually larger than 1 Kbyte. However, some space might be profitably reclaimed from control structure allocation for this system.

The same structure on the 512 Mbyte system shows that only 4 Kbytes of 10144 Kbytes has not been allocated. When the biostats kstat is examined with kstat -n biostats, it is seen that the system had a reasonable ratio of buffer_cache_hits to buffer_cache_lookups as well. This indicates that the default setting is reasonable for that system.

Commitment Level


TMPFS Parameters

tmpfs:tmpfs_maxkmem (Solaris 10 Releases)

The range description is updated to include sun4v systems. For more information, see tmpfs:tmpfs_maxkmem.

tmpfs:tmpfs_minfree (Solaris 8 Releases)

In the Solaris 8 versions, the units was incorrectly described as “Bytes,” instead of “Pages.”

sun4u or sun4v Specific Parameters (Solaris 10 Releases)

The title of the Sun-4u Specific Parameters section was revised in the Solaris 10 8/07 release to include sun4v systems.

Solaris Volume Manager Parameters


In the Solaris Express release, the default value was increased to 1024 blocks from 128 blocks.

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