114 lines
4 KiB
Text
114 lines
4 KiB
Text
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Short users guide for SLUB
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--------------------------
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First of all slub should transparently replace SLAB. If you enable
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SLUB then everything should work the same (Note the word "should".
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There is likely not much value in that word at this point).
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The basic philosophy of SLUB is very different from SLAB. SLAB
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requires rebuilding the kernel to activate debug options for all
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SLABS. SLUB always includes full debugging but its off by default.
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SLUB can enable debugging only for selected slabs in order to avoid
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an impact on overall system performance which may make a bug more
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difficult to find.
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In order to switch debugging on one can add a option "slub_debug"
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to the kernel command line. That will enable full debugging for
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all slabs.
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Typically one would then use the "slabinfo" command to get statistical
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data and perform operation on the slabs. By default slabinfo only lists
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slabs that have data in them. See "slabinfo -h" for more options when
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running the command. slabinfo can be compiled with
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gcc -o slabinfo Documentation/vm/slabinfo.c
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Some of the modes of operation of slabinfo require that slub debugging
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be enabled on the command line. F.e. no tracking information will be
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available without debugging on and validation can only partially
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be performed if debugging was not switched on.
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Some more sophisticated uses of slub_debug:
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-------------------------------------------
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Parameters may be given to slub_debug. If none is specified then full
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debugging is enabled. Format:
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slub_debug=<Debug-Options> Enable options for all slabs
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slub_debug=<Debug-Options>,<slab name>
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Enable options only for select slabs
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Possible debug options are
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F Sanity checks on (enables SLAB_DEBUG_FREE. Sorry
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SLAB legacy issues)
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Z Red zoning
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P Poisoning (object and padding)
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U User tracking (free and alloc)
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T Trace (please only use on single slabs)
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F.e. in order to boot just with sanity checks and red zoning one would specify:
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slub_debug=FZ
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Trying to find an issue in the dentry cache? Try
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slub_debug=,dentry_cache
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to only enable debugging on the dentry cache.
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Red zoning and tracking may realign the slab. We can just apply sanity checks
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to the dentry cache with
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slub_debug=F,dentry_cache
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In case you forgot to enable debugging on the kernel command line: It is
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possible to enable debugging manually when the kernel is up. Look at the
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contents of:
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/sys/slab/<slab name>/
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Look at the writable files. Writing 1 to them will enable the
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corresponding debug option. All options can be set on a slab that does
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not contain objects. If the slab already contains objects then sanity checks
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and tracing may only be enabled. The other options may cause the realignment
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of objects.
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Careful with tracing: It may spew out lots of information and never stop if
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used on the wrong slab.
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SLAB Merging
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------------
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If no debugging is specified then SLUB may merge similar slabs together
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in order to reduce overhead and increase cache hotness of objects.
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slabinfo -a displays which slabs were merged together.
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Getting more performance
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------------------------
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To some degree SLUB's performance is limited by the need to take the
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list_lock once in a while to deal with partial slabs. That overhead is
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governed by the order of the allocation for each slab. The allocations
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can be influenced by kernel parameters:
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slub_min_objects=x (default 8)
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slub_min_order=x (default 0)
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slub_max_order=x (default 4)
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slub_min_objects allows to specify how many objects must at least fit
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into one slab in order for the allocation order to be acceptable.
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In general slub will be able to perform this number of allocations
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on a slab without consulting centralized resources (list_lock) where
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contention may occur.
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slub_min_order specifies a minim order of slabs. A similar effect like
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slub_min_objects.
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slub_max_order specified the order at which slub_min_objects should no
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longer be checked. This is useful to avoid SLUB trying to generate
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super large order pages to fit slub_min_objects of a slab cache with
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large object sizes into one high order page.
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Christoph Lameter, <clameter@sgi.com>, April 10, 2007
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