mirror of https://github.com/mkerrisk/man-pages
slabinfo.5: Rewrite to try to bring the content close to current reality
There's still gaps to fill in, but the existing page was in any case hugely out of date. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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man5/slabinfo.5
234
man5/slabinfo.5
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@ -1,4 +1,5 @@
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.\" Copyright (c) 2001 Andreas Dilger (adilger@turbolinux.com)
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.\" and Copyright (c) 2017 Michael kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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@ -22,15 +23,6 @@
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" FIXME Over time, the slabinfo format has gone through
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.\" version changes. These should be documented:
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.\"
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.\" slabinfo 1.0 - Linux 2.2 (precisely: 2.1.23)
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.\" slabinfo 1.1 - Linux 2.4 (precisely: 2.4.0-test3)
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.\" slabinfo 1.2 - Linux 2.5.45
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.\" slabinfo 2.0 - Linux 2.6 (precisely: 2.5.71)
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.\" slabinfo 2.1 - Linux 2.6.10
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.\"
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.TH SLABINFO 5 2017-03-13 "" "Linux Programmer's Manual"
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.SH NAME
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slabinfo \- kernel slab allocator statistics
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@ -42,95 +34,181 @@ Frequently used objects in the Linux kernel
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have their own cache.
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The file
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.I /proc/slabinfo
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gives statistics.
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For example:
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gives statistics on these caches.
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The following (edited) output shows an example of the
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contents of this file:
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.LP
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.in +4n
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.in 0
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.nf
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% cat /proc/slabinfo
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slabinfo \- version: 1.1
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kmem_cache 60 78 100 2 2 1
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blkdev_requests 5120 5120 96 128 128 1
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mnt_cache 20 40 96 1 1 1
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inode_cache 7005 14792 480 1598 1849 1
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dentry_cache 5469 5880 128 183 196 1
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filp 726 760 96 19 19 1
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buffer_head 67131 71240 96 1776 1781 1
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vm_area_struct 1204 1652 64 23 28 1
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\&...
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size-8192 1 17 8192 1 17 2
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size-4096 41 73 4096 41 73 1
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$ \fBsudo cat /proc/slabinfo\fP
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slabinfo - version: 2.1
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# name <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> ...
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sigqueue 100 100 160 25 1 : tunables 0 0 0 : slabdata 4 4 0
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sighand_cache 355 405 2112 15 8 : tunables 0 0 0 : slabdata 27 27 0
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kmalloc-8192 96 96 8192 4 8 : tunables 0 0 0 : slabdata 24 24 0
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\&...
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.fi
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.in
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.LP
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For each slab cache, the cache name, the number of currently
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active objects, the total number of available objects, the
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size of each object in bytes, the number of pages with at
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least one active object, the total number of allocated pages,
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and the number of pages per slab are given.
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The first line of output includes a version number,
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which allows an application that is reading the file to handle changes
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in the file format.
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(See VERSIONS, below.)
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The next line lists the names of the columns in the remaining lines.
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Each of the remaining lines displays information about a specified cache.
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Following the cache name,
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the output shown in each line shows three components for each cache:
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.IP * 3
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statistics
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.IP *
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tunables
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.IP *
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slabdata
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.PP
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The statistics are as follows:
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.TP
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.I active_objs
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The number of objects that are currently active (i.e., in use).
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.TP
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.I num_objs
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The total number of allocated objects
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(i.e., objects that are both in use and not in use).
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.TP
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.I objsize
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The size of objects in this slab, in bytes.
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.TP
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.I objperslab
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The number of objects stored in each slab.
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.TP
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.I pagesperslab
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The number of pages allocated for each slab.
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.PP
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The
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.I tunables
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entries in each line show tunable parameters for the corresponding cache.
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When using the default SLUB allocator, there are no tunables, the
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.I /proc/slabinfo
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is not writable, and the value 0 is shown in these fields.
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When using the older SLAB allocator,
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the tunables for a particular cache can be set by writing
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lines of the following form to
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.IR /proc/slabinfo :
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# \fBecho 'name limit batchcount sharedfactor' > /proc/slabinfo\fP
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Here,
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.I name
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is the cache name, and
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.IR limit ,
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.IR batchcount ,
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and
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.IR sharedfactor
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are integers defining new values for the corresponding tunables.
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The
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.I tunables
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entries in each line contain the following fields:
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.TP
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.I limit
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The maximum number of objects that will be cached.
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.\" https://lwn.net/Articles/56360/
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.\" This is the limit on the number of free objects that can be stored
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.\" in the per-CPU free list for this slab cache.
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.TP
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.I batchcount
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[To be documented]
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.\" https://lwn.net/Articles/56360/
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.\" On SMP systems, when we refill the available object list, instead
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.\" of doing one object at a time, we do batch-count objects at a time.
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.TP
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.I sharedfactor
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[To be documented]
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.\"
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.PP
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The
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.I slabdata
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entries in each line contain the following fields:
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.TP
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.I active_slabs
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The number of active slabs.
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.TP
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.I nums_slabs
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The total number of slabs.
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.TP
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.I sharedavail
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[To be documented]
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.PP
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Note that because of object alignment and slab cache overhead,
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objects are not normally packed tightly into pages.
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Pages with even one in-use object are considered in-use and cannot be
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freed.
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Kernels compiled with slab cache statistics will also have
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"(statistics)" in the first line of output, and will have 5
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additional columns, namely: the high water mark of active
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Kernels configured with
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.B CONFIG_DEBUG_SLAB
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will also have additional statistics fields in each line,
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and the first line of the file will contain the string "(statistics)".
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The statistics field include : the high water mark of active
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objects; the number of times objects have been allocated;
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the number of times the cache has grown (new pages added
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to this cache); the number of times the cache has been
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reaped (unused pages removed from this cache); and the
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number of times there was an error allocating new pages
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to this cache.
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If slab cache statistics are not enabled
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for this kernel, these columns will not be shown.
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SMP systems will also have "(SMP)" in the first line of
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output, and will have two additional columns for each slab,
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reporting the slab allocation policy for the CPU-local
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cache (to reduce the need for inter-CPU synchronization
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when allocating objects from the cache).
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The first column is the per-CPU limit: the maximum number of objects that
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will be cached for each CPU.
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The second column is the
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batchcount: the maximum number of free objects in the
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global cache that will be transferred to the per-CPU cache
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if it is empty, or the number of objects to be returned
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to the global cache if the per-CPU cache is full.
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If both slab cache statistics and SMP are defined, there
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will be four additional columns, reporting the per-CPU
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cache statistics.
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The first two are the per-CPU cache
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allocation hit and miss counts: the number of times an
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object was or was not available in the per-CPU cache
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for allocation.
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The next two are the per-CPU cache free
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hit and miss counts: the number of times a freed object
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could or could not fit within the per-CPU cache limit,
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before flushing objects to the global cache.
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It is possible to tune the SMP per-CPU slab cache limit
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and batchcount via:
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.in +4n
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.nf
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echo "\fIcache_name limit batchcount\fP" > /proc/slabinfo
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.fi
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.in
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.SH FILES
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.I <linux/slab.h>
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.\"
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.\" SMP systems will also have "(SMP)" in the first line of
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.\" output, and will have two additional columns for each slab,
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.\" reporting the slab allocation policy for the CPU-local
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.\" cache (to reduce the need for inter-CPU synchronization
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.\" when allocating objects from the cache).
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.\" The first column is the per-CPU limit: the maximum number of objects that
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.\" will be cached for each CPU.
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.\" The second column is the
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.\" batchcount: the maximum number of free objects in the
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.\" global cache that will be transferred to the per-CPU cache
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.\" if it is empty, or the number of objects to be returned
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.\" to the global cache if the per-CPU cache is full.
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.\"
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.\" If both slab cache statistics and SMP are defined, there
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.\" will be four additional columns, reporting the per-CPU
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.\" cache statistics.
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.\" The first two are the per-CPU cache
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.\" allocation hit and miss counts: the number of times an
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.\" object was or was not available in the per-CPU cache
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.\" for allocation.
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.\" The next two are the per-CPU cache free
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.\" hit and miss counts: the number of times a freed object
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.\" could or could not fit within the per-CPU cache limit,
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.\" before flushing objects to the global cache.
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.SH VERSIONS
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The
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.I /proc/slabinfo
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exists since Linux 2.1.23.
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SMP per-CPU caches exist since Linux 2.4.0-test3.
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file first appeared in Linux 2.1.23.
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The file is versioned,
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and over time there have been a number of versions with different layouts:
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.TP
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1.0
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Present throughout the Linux 2.2.x kernel series.
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.TP
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1.1
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Present in the Linux 2.4.x kernel series.
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.\" First appeared in 2.4.0-test3
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.TP
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1.2
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A format that was briefly present in the Linux 2.5 development series.
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.\" from 2.5.45 to 2.5.70
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.TP
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2.0
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Present in Linux 2.6.x kernels up to and including Linux 2.6.9.
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.\" First appeared in 2.5.71
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.TP
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2.1
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The current format, which first appeared in Linux 2.6.10.
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.SH NOTES
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Since Linux 2.6.16 the file
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.I /proc/slabinfo
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is present only if the
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.B CONFIG_SLAB
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kernel configuration option is enabled.
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Only root can read and (if the kernel was configured with
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.BR CONFIG_SLAB )
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write the
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.IR /proc/slabinfo
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file.
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.SH SEE ALSO
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.BR slabtop (1)
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