linux-vybrid_public/Documentation/vm/slub.txt

Short users guide for SLUB
--------------------------

First of all slub should transparently replace SLAB. If you enable
SLUB then everything should work the same (Note the word "should".
There is likely not much value in that word at this point).

The basic philosophy of SLUB is very different from SLAB. SLAB
requires rebuilding the kernel to activate debug options for all
SLABS. SLUB always includes full debugging but its off by default.
SLUB can enable debugging only for selected slabs in order to avoid
an impact on overall system performance which may make a bug more
difficult to find.

In order to switch debugging on one can add a 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
slabs that have data in them. See "slabinfo -h" for more options when
running the command. slabinfo can be compiled with

gcc -o slabinfo Documentation/vm/slabinfo.c

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
debugging is enabled. Format:

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
	F		Sanity checks on (enables SLAB_DEBUG_FREE. Sorry
			SLAB legacy issues)
	Z		Red zoning
	P		Poisoning (object and padding)
	U		User tracking (free and alloc)
	T		Trace (please only use on single slabs)

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

	slub_debug=,dentry_cache

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

	slub_debug=F,dentry_cache

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:

/sys/slab/<slab name>/

Look at the writable files. Writing 1 to them will enable the
corresponding debug option. All options can be set on a slab that does
not contain objects. If the slab already contains objects then sanity checks
and tracing may only be enabled. The other options may cause the realignment
of objects.

Careful with tracing: It may spew out lots of information and never stop if
used on the wrong slab.

SLAB Merging
------------

If no debugging is 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.

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 8)
slub_min_order=x		(default 0)
slub_max_order=x		(default 4)

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.


Christoph Lameter, <clameter@sgi.com>, April 10, 2007