114 lines
4.0 KiB
Plaintext
114 lines
4.0 KiB
Plaintext
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
|