slabinfo - kernel slab allocator statistics
cat /proc/slabinfo
Frequently used objects in the Linux kernel (buffer heads, inodes, dentries,
etc.) have their own cache. The file
/proc/slabinfo gives statistics on
these caches. The following (edited) output shows an example of the contents
of this file:
$ sudo cat /proc/slabinfo
slabinfo - version: 2.1
# name <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> ...
sigqueue 100 100 160 25 1 : tunables 0 0 0 : slabdata 4 4 0
sighand_cache 355 405 2112 15 8 : tunables 0 0 0 : slabdata 27 27 0
kmalloc-8192 96 96 8192 4 8 : tunables 0 0 0 : slabdata 24 24 0
...
The first line of output includes a version number, which allows an application
that is reading the file to handle changes in the file format. (See VERSIONS,
below.) The next line lists the names of the columns in the remaining lines.
Each of the remaining lines displays information about a specified cache.
Following the cache name, the output shown in each line shows three components
for each cache:
- *
- statistics
- *
- tunables
- *
- slabdata
The statistics are as follows:
- active_objs
- The number of objects that are currently active (i.e., in use).
- num_objs
- The total number of allocated objects (i.e., objects that are both in use
and not in use).
- objsize
- The size of objects in this slab, in bytes.
- objperslab
- The number of objects stored in each slab.
- pagesperslab
- The number of pages allocated for each slab.
The
tunables entries in each line show tunable parameters for the
corresponding cache. When using the default SLUB allocator, there are no
tunables, the
/proc/slabinfo file is not writable, and the value 0 is
shown in these fields. When using the older SLAB allocator, the tunables for a
particular cache can be set by writing lines of the following form to
/proc/slabinfo:
# echo 'name limit batchcount sharedfactor' > /proc/slabinfo
Here,
name is the cache name, and
limit,
batchcount, and
sharedfactor are integers defining new values for the corresponding
tunables. The
limit value should be a positive value,
batchcount
should be a positive value that is less than or equal to
limit, and
sharedfactor should be nonnegative. If any of the specified values is
invalid, the cache settings are left unchanged.
The
tunables entries in each line contain the following fields:
- limit
- The maximum number of objects that will be cached.
- batchcount
- On SMP systems, this specifies the number of objects to transfer at one
time when refilling the available object list.
- sharedfactor
- [To be documented]
The
slabdata entries in each line contain the following fields:
- active_slabs
- The number of active slabs.
- nums_slabs
- The total number of slabs.
- sharedavail
- [To be documented]
Note that because of object alignment and slab cache overhead, objects are not
normally packed tightly into pages. Pages with even one in-use object are
considered in-use and cannot be freed.
Kernels configured with
CONFIG_DEBUG_SLAB will also have additional
statistics fields in each line, and the first line of the file will contain
the string "(statistics)". The statistics field include : the high
water mark of active objects; the number of times objects have been allocated;
the number of times the cache has grown (new pages added to this cache); the
number of times the cache has been reaped (unused pages removed from this
cache); and the number of times there was an error allocating new pages to
this cache.
The
/proc/slabinfo file first appeared in Linux 2.1.23. The file is
versioned, and over time there have been a number of versions with different
layouts:
- 1.0
- Present throughout the Linux 2.2.x kernel series.
- 1.1
- Present in the Linux 2.4.x kernel series.
- 1.2
- A format that was briefly present in the Linux 2.5 development
series.
- 2.0
- Present in Linux 2.6.x kernels up to and including Linux 2.6.9.
- 2.1
- The current format, which first appeared in Linux 2.6.10.
Only root can read and (if the kernel was configured with
CONFIG_SLAB)
write the
/proc/slabinfo file.
The total amount of memory allocated to the SLAB/SLUB cache is shown in the
Slab field of
/proc/meminfo.
slabtop(1)
The kernel source file
Documentation/vm/slub.txt and
tools/vm/slabinfo.c.