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Diffstat (limited to 'Documentation/vm/slub.txt')
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diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt new file mode 100644 index 000000000000..727c8d81aeaf --- /dev/null +++ b/Documentation/vm/slub.txt @@ -0,0 +1,113 @@ +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 |