docs/vm: slub.txt: convert to ReST format

.. _slub:

==========================

Short users guide for SLUB

--------------------------

==========================

The basic philosophy of SLUB is very different from SLAB. SLAB

requires rebuilding the kernel to activate debug options for all

an impact on overall system performance which may make a bug more

difficult to find.

In order to switch debugging on one can add an option "slub_debug"

In order to switch debugging on one can add an option ``slub_debug``

to the kernel command line. That will enable full debugging for

all slabs.

Typically one would then use the "slabinfo" command to get statistical

data and perform operation on the slabs. By default slabinfo only lists

Typically one would then use the ``slabinfo`` command to get statistical

data and perform operation on the slabs. By default ``slabinfo`` only lists

slabs that have data in them. See "slabinfo -h" for more options when

running the command. slabinfo can be compiled with

running the command. ``slabinfo`` can be compiled with

::

	gcc -o slabinfo tools/vm/slabinfo.c

Some of the modes of operation of slabinfo require that slub debugging

Some of the modes of operation of ``slabinfo`` require that slub debugging

be enabled on the command line. F.e. no tracking information will be

available without debugging on and validation can only partially

be performed if debugging was not switched on.

Some more sophisticated uses of slub_debug:

-------------------------------------------

Parameters may be given to slub_debug. If none is specified then full

Parameters may be given to ``slub_debug``. If none is specified then full

debugging is enabled. Format:

slub_debug=<Debug-Options>       Enable options for all slabs

slub_debug=<Debug-Options>

	Enable options for all slabs

slub_debug=<Debug-Options>,<slab name>

	Enable options only for select slabs

Possible debug options are

Possible debug options are::

	F		Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS

			Sorry SLAB legacy issues)

	Z		Red zoning

	-		Switch all debugging off (useful if the kernel is

			configured with CONFIG_SLUB_DEBUG_ON)

F.e. in order to boot just with sanity checks and red zoning one would specify:

F.e. in order to boot just with sanity checks and red zoning one would specify::

	slub_debug=FZ

Trying to find an issue in the dentry cache? Try

Trying to find an issue in the dentry cache? Try::

	slub_debug=,dentry

to only enable debugging on the dentry cache.

Red zoning and tracking may realign the slab.  We can just apply sanity checks

to the dentry cache with

to the dentry cache with::

	slub_debug=F,dentry

a result of storing the metadata (for example, caches with PAGE_SIZE object

sizes).  This has a higher liklihood of resulting in slab allocation errors

in low memory situations or if there's high fragmentation of memory.  To

switch off debugging for such caches by default, use

switch off debugging for such caches by default, use::

	slub_debug=O

In case you forgot to enable debugging on the kernel command line: It is

possible to enable debugging manually when the kernel is up. Look at the

contents of:

contents of::

	/sys/kernel/slab/<slab name>/

used on the wrong slab.

Slab merging

------------

============

If no debug options are specified then SLUB may merge similar slabs together

in order to reduce overhead and increase cache hotness of objects.

slabinfo -a displays which slabs were merged together.

``slabinfo -a`` displays which slabs were merged together.

Slab validation

---------------

===============

SLUB can validate all object if the kernel was booted with slub_debug. In

order to do so you must have the slabinfo tool. Then you can do

order to do so you must have the ``slabinfo`` tool. Then you can do

::

	slabinfo -v

which will test all objects. Output will be generated to the syslog.

This also works in a more limited way if boot was without slab debug.

In that case slabinfo -v simply tests all reachable objects. Usually

In that case ``slabinfo -v`` simply tests all reachable objects. Usually

these are in the cpu slabs and the partial slabs. Full slabs are not

tracked by SLUB in a non debug situation.

Getting more performance

------------------------

========================

To some degree SLUB's performance is limited by the need to take the

list_lock once in a while to deal with partial slabs. That overhead is

governed by the order of the allocation for each slab. The allocations

can be influenced by kernel parameters:

slub_min_objects=x		(default 4)

slub_min_order=x		(default 0)

slub_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))

slub_min_objects allows to specify how many objects must at least fit

into one slab in order for the allocation order to be acceptable.

In general slub will be able to perform this number of allocations

on a slab without consulting centralized resources (list_lock) where

contention may occur.

slub_min_order specifies a minim order of slabs. A similar effect like

slub_min_objects.

slub_max_order specified the order at which slub_min_objects should no

longer be checked. This is useful to avoid SLUB trying to generate

super large order pages to fit slub_min_objects of a slab cache with

large object sizes into one high order page. Setting command line

parameter debug_guardpage_minorder=N (N > 0), forces setting

slub_max_order to 0, what cause minimum possible order of slabs

allocation.

.. slub_min_objects=x		(default 4)

.. slub_min_order=x		(default 0)

.. slub_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))

``slub_min_objects``

	allows to specify how many objects must at least fit into one

	slab in order for the allocation order to be acceptable.  In

	general slub will be able to perform this number of

	allocations on a slab without consulting centralized resources

	(list_lock) where contention may occur.

``slub_min_order``

	specifies a minim order of slabs. A similar effect like

	``slub_min_objects``.

``slub_max_order``

	specified the order at which ``slub_min_objects`` should no

	longer be checked. This is useful to avoid SLUB trying to

	generate super large order pages to fit ``slub_min_objects``

	of a slab cache with large object sizes into one high order

	page. Setting command line parameter

	``debug_guardpage_minorder=N`` (N > 0), forces setting

	``slub_max_order`` to 0, what cause minimum possible order of

	slabs allocation.

SLUB Debug output

-----------------

=================

Here is a sample of slub debug output:

Here is a sample of slub debug output::

 ====================================================================

 BUG kmalloc-8: Redzone overwritten

1. Description of the problem encountered

This will be a message in the system log starting with

   This will be a message in the system log starting with::

     ===============================================

     BUG <slab cache affected>: <What went wrong>

4. Report on how the problem was dealt with in order to ensure the continued

   operation of the system.

These are messages in the system log beginning with

   These are messages in the system log beginning with::

	FIX <slab cache affected>: <corrective action taken>

   tells us that SLUB has restored the Redzone to its proper value and then

   system operations continue.

Emergency operations:

---------------------

Emergency operations

====================

Minimal debugging (sanity checks alone) can be enabled by booting with

Minimal debugging (sanity checks alone) can be enabled by booting with::

	slub_debug=F

may be optimized further by locating the slab that experiences corruption

and enabling debugging only for that cache

I.e.

I.e.::

	slub_debug=F,dentry

If the corruption occurs by writing after the end of the object then it

may be advisable to enable a Redzone to avoid corrupting the beginning

of other objects.

of other objects::

	slub_debug=FZ,dentry

Extended slabinfo mode and plotting

-----------------------------------

===================================

The slabinfo tool has a special 'extended' ('-X') mode that includes:

The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes:

 - Slabcache Totals

 - Slabs sorted by size (up to -N <num> slabs, default 1)

 - Slabs sorted by loss (up to -N <num> slabs, default 1)

Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K)

and reports everything in bytes (this functionality is also available to

other slabinfo modes via '-B' option) which makes reporting more precise and

accurate. Moreover, in some sense the `-X' mode also simplifies the analysis

of slabs' behaviour, because its output can be plotted using the

slabinfo-gnuplot.sh script. So it pushes the analysis from looking through

the numbers (tons of numbers) to something easier -- visual analysis.

Additionally, in this mode ``slabinfo`` does not dynamically scale

sizes (G/M/K) and reports everything in bytes (this functionality is

also available to other slabinfo modes via '-B' option) which makes

reporting more precise and accurate. Moreover, in some sense the `-X'

mode also simplifies the analysis of slabs' behaviour, because its

output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it

pushes the analysis from looking through the numbers (tons of numbers)

to something easier -- visual analysis.

To generate plots:

a) collect slabinfo extended records, for example:

a) collect slabinfo extended records, for example::

	while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done

b) pass stats file(-s) to slabinfo-gnuplot.sh script:

b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::

	slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]

The slabinfo-gnuplot.sh script will pre-processes the collected records

   The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records

   and generates 3 png files (and 3 pre-processing cache files) per STATS

   file:

   - Slabcache Totals: FOO_STATS-totals.png

   - Slabs sorted by size: FOO_STATS-slabs-by-size.png

   - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png

Another use case, when slabinfo-gnuplot can be useful, is when you need

to compare slabs' behaviour "prior to" and "after" some code modification.

To help you out there, slabinfo-gnuplot.sh script can 'merge' the

`Slabcache Totals` sections from different measurements. To visually

compare N plots:

Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you

need to compare slabs' behaviour "prior to" and "after" some code

modification.  To help you out there, ``slabinfo-gnuplot.sh`` script

can 'merge' the `Slabcache Totals` sections from different

measurements. To visually compare N plots:

a) Collect as many STATS1, STATS2, .. STATSN files as you need::

a) Collect as many STATS1, STATS2, .. STATSN files as you need

	while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done

b) Pre-process those STATS files

b) Pre-process those STATS files::

	slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN

c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the

generated pre-processed *-totals

c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the

   generated pre-processed \*-totals::

	slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals

   This will produce a single plot (png file).

   Plots, expectedly, can be large so some fluctuations or small spikes

can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two

   can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two

   options to 'zoom-in'/'zoom-out':

 a) -s %d,%d  overwrites the default image width and heigh

 b) -r %d,%d  specifies a range of samples to use (for example,

              in `slabinfo -X >> FOO_STATS; sleep 1;' case, using

              a "-r 40,60" range will plot only samples collected

              between 40th and 60th seconds).

   a) ``-s %d,%d`` -- overwrites the default image width and heigh

   b) ``-r %d,%d`` -- specifies a range of samples to use (for example,

      in ``slabinfo -X >> FOO_STATS; sleep 1;`` case, using a ``-r

      40,60`` range will plot only samples collected between 40th and

      60th seconds).

Christoph Lameter, May 30, 2007

Sergey Senozhatsky, October 23, 2015