man5/slabinfo.5

   1 .\" Copyright (c) 2001 Andreas Dilger (adilger@turbolinux.com)
   2 .\" and Copyright (c) 2017 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH SLABINFO 5 2021-03-22 "" "Linux Programmer's Manual"
   7 .SH NAME
   8 slabinfo \- kernel slab allocator statistics
   9 .SH SYNOPSIS
  10 .nf
  11 .B cat /proc/slabinfo
  12 .fi
  13 .SH DESCRIPTION
  14 Frequently used objects in the Linux kernel
  15 (buffer heads, inodes, dentries, etc.)
  16 have their own cache.
  17 The file
  18 .I /proc/slabinfo
  19 gives statistics on these caches.
  20 The following (edited) output shows an example of the
  21 contents of this file:
  22 .PP
  23 .EX
  24 $ \fBsudo cat /proc/slabinfo\fP
  25 slabinfo \- version: 2.1
  26 # name    <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> ...
  27 sigqueue      100  100  160   25  1 : tunables  0  0  0 : slabdata   4   4  0
  28 sighand_cache 355   405 2112  15  8 : tunables  0  0  0 : slabdata  27  27  0
  29 kmalloc\-8192   96   96  8192   4  8 : tunables  0  0  0 : slabdata  24  24  0
  30 \&...
  31 .EE
  32 .PP
  33 The first line of output includes a version number,
  34 which allows an application that is reading the file to handle changes
  35 in the file format.
  36 (See VERSIONS, below.)
  37 The next line lists the names of the columns in the remaining lines.
  38 .PP
  39 Each of the remaining lines displays information about a specified cache.
  40 Following the cache name,
  41 the output shown in each line shows three components for each cache:
  42 .IP * 3
  43 statistics
  44 .IP *
  45 tunables
  46 .IP *
  47 slabdata
  48 .PP
  49 The statistics are as follows:
  50 .TP
  51 .I active_objs
  52 The number of objects that are currently active (i.e., in use).
  53 .TP
  54 .I num_objs
  55 The total number of allocated objects
  56 (i.e., objects that are both in use and not in use).
  57 .TP
  58 .I objsize
  59 The size of objects in this slab, in bytes.
  60 .TP
  61 .I objperslab
  62 The number of objects stored in each slab.
  63 .TP
  64 .I pagesperslab
  65 The number of pages allocated for each slab.
  66 .PP
  67 The
  68 .I tunables
  69 entries in each line show tunable parameters for the corresponding cache.
  70 When using the default SLUB allocator, there are no tunables, the
  71 .I /proc/slabinfo
  72 file is not writable, and the value 0 is shown in these fields.
  73 When using the older SLAB allocator,
  74 the tunables for a particular cache can be set by writing
  75 lines of the following form to
  76 .IR /proc/slabinfo :
  77 .PP
  78 .in +4n
  79 .EX
  80 # \fBecho \(aqname limit batchcount sharedfactor\(aq > /proc/slabinfo\fP
  81 .EE
  82 .in
  83 .PP
  84 Here,
  85 .I name
  86 is the cache name, and
  87 .IR limit ,
  88 .IR batchcount ,
  89 and
  90 .IR sharedfactor
  91 are integers defining new values for the corresponding tunables.
  92 The
  93 .I limit
  94 value should be a positive value,
  95 .I batchcount
  96 should be a positive value that is less than or equal to
  97 .IR limit ,
  98 and
  99 .I sharedfactor
 100 should be nonnegative.
 101 If any of the specified values is invalid,
 102 the cache settings are left unchanged.
 103 .PP
 104 The
 105 .I tunables
 106 entries in each line contain the following fields:
 107 .TP
 108 .I limit
 109 The maximum number of objects that will be cached.
 110 .\" https://lwn.net/Articles/56360/
 111 .\"     This is the limit on the number of free objects that can be stored
 112 .\"     in the per-CPU free list for this slab cache.
 113 .TP
 114 .I batchcount
 115 On SMP systems, this specifies the number of objects to transfer at one time
 116 when refilling the available object list.
 117 .\" https://lwn.net/Articles/56360/
 118 .\"     On SMP systems, when we refill the available object list, instead
 119 .\"     of doing one object at a time, we do batch-count objects at a time.
 120 .TP
 121 .I sharedfactor
 122 [To be documented]
 123 .\"
 124 .PP
 125 The
 126 .I slabdata
 127 entries in each line contain the following fields:
 128 .TP
 129 .I active_slabs
 130 The number of active slabs.
 131 .TP
 132 .I nums_slabs
 133 The total number of slabs.
 134 .TP
 135 .I sharedavail
 136 [To be documented]
 137 .PP
 138 Note that because of object alignment and slab cache overhead,
 139 objects are not normally packed tightly into pages.
 140 Pages with even one in-use object are considered in-use and cannot be
 141 freed.
 142 .PP
 143 Kernels configured with
 144 .B CONFIG_DEBUG_SLAB
 145 will also have additional statistics fields in each line,
 146 and the first line of the file will contain the string "(statistics)".
 147 The statistics field include : the high water mark of active
 148 objects; the number of times objects have been allocated;
 149 the number of times the cache has grown (new pages added
 150 to this cache); the number of times the cache has been
 151 reaped (unused pages removed from this cache); and the
 152 number of times there was an error allocating new pages
 153 to this cache.
 154 .\"
 155 .\" SMP systems will also have "(SMP)" in the first line of
 156 .\" output, and will have two additional columns for each slab,
 157 .\" reporting the slab allocation policy for the CPU-local
 158 .\" cache (to reduce the need for inter-CPU synchronization
 159 .\" when allocating objects from the cache).
 160 .\" The first column is the per-CPU limit: the maximum number of objects that
 161 .\" will be cached for each CPU.
 162 .\" The second column is the
 163 .\" batchcount: the maximum number of free objects in the
 164 .\" global cache that will be transferred to the per-CPU cache
 165 .\" if it is empty, or the number of objects to be returned
 166 .\" to the global cache if the per-CPU cache is full.
 167 .\"
 168 .\" If both slab cache statistics and SMP are defined, there
 169 .\" will be four additional columns, reporting the per-CPU
 170 .\" cache statistics.
 171 .\" The first two are the per-CPU cache
 172 .\" allocation hit and miss counts: the number of times an
 173 .\" object was or was not available in the per-CPU cache
 174 .\" for allocation.
 175 .\" The next two are the per-CPU cache free
 176 .\" hit and miss counts: the number of times a freed object
 177 .\" could or could not fit within the per-CPU cache limit,
 178 .\" before flushing objects to the global cache.
 179 .SH VERSIONS
 180 The
 181 .I /proc/slabinfo
 182 file first appeared in Linux 2.1.23.
 183 The file is versioned,
 184 and over time there have been a number of versions with different layouts:
 185 .TP
 186 1.0
 187 Present throughout the Linux 2.2.x kernel series.
 188 .TP
 189 1.1
 190 Present in the Linux 2.4.x kernel series.
 191 .\" First appeared in 2.4.0-test3
 192 .TP
 193 1.2
 194 A format that was briefly present in the Linux 2.5 development series.
 195 .\" from 2.5.45 to 2.5.70
 196 .TP
 197 2.0
 198 Present in Linux 2.6.x kernels up to and including Linux 2.6.9.
 199 .\" First appeared in 2.5.71
 200 .TP
 201 2.1
 202 The current format, which first appeared in Linux 2.6.10.
 203 .SH NOTES
 204 Only root can read and (if the kernel was configured with
 205 .BR CONFIG_SLAB )
 206 write the
 207 .IR /proc/slabinfo
 208 file.
 209 .PP
 210 The total amount of memory allocated to the SLAB/SLUB cache is shown in the
 211 .I Slab
 212 field of
 213 .IR /proc/meminfo .
 214 .SH SEE ALSO
 215 .BR slabtop (1)
 216 .PP
 217 The kernel source file
 218 .IR Documentation/vm/slub.txt
 219 and
 220 .IR tools/vm/slabinfo.c .