docs/vm: slub.txt: convert to ReST format
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mike Rapoport
Jonathan Corbet
parent
acc9f3a35c
commit
0c14398bf2
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@ -1,5 +1,8 @@
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.. _slub:
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==========================
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Short users guide for SLUB
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--------------------------
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==========================
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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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@ -8,18 +11,19 @@ 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 an option "slub_debug"
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In order to switch debugging on one can add an 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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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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running the command. ``slabinfo`` can be compiled with
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::
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gcc -o slabinfo tools/vm/slabinfo.c
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gcc -o slabinfo tools/vm/slabinfo.c
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Some of the modes of operation of slabinfo require that slub debugging
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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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@ -27,14 +31,17 @@ 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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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>
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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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Enable options only for select slabs
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Possible debug options are::
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Possible debug options are
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F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
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Sorry SLAB legacy issues)
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Z Red zoning
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@ -47,18 +54,18 @@ Possible debug options are
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- Switch all debugging off (useful if the kernel is
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configured with CONFIG_SLUB_DEBUG_ON)
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F.e. in order to boot just with sanity checks and red zoning one would specify:
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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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Trying to find an issue in the dentry cache? Try::
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slub_debug=,dentry
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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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to the dentry cache with::
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slub_debug=F,dentry
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@ -66,15 +73,15 @@ Debugging options may require the minimum possible slab order to increase as
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a result of storing the metadata (for example, caches with PAGE_SIZE object
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sizes). This has a higher liklihood of resulting in slab allocation errors
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in low memory situations or if there's high fragmentation of memory. To
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switch off debugging for such caches by default, use
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switch off debugging for such caches by default, use::
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slub_debug=O
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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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contents of::
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/sys/kernel/slab/<slab name>/
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/sys/kernel/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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@ -86,98 +93,103 @@ 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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============
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If no debug options are 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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``slabinfo -a`` displays which slabs were merged together.
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Slab validation
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---------------
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===============
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SLUB can validate all object if the kernel was booted with slub_debug. In
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order to do so you must have the slabinfo tool. Then you can do
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order to do so you must have the ``slabinfo`` tool. Then you can do
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::
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slabinfo -v
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slabinfo -v
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which will test all objects. Output will be generated to the syslog.
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This also works in a more limited way if boot was without slab debug.
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In that case slabinfo -v simply tests all reachable objects. Usually
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In that case ``slabinfo -v`` simply tests all reachable objects. Usually
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these are in the cpu slabs and the partial slabs. Full slabs are not
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tracked by SLUB in a non debug situation.
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Getting more performance
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------------------------
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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 4)
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slub_min_order=x (default 0)
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slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER))
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.. slub_min_objects=x (default 4)
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.. slub_min_order=x (default 0)
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.. slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER))
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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_objects``
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allows to specify how many objects must at least fit into one
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slab in order for the allocation order to be acceptable. In
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general slub will be able to perform this number of
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allocations on a slab without consulting centralized resources
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(list_lock) where 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_min_order``
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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. Setting command line
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parameter debug_guardpage_minorder=N (N > 0), forces setting
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slub_max_order to 0, what cause minimum possible order of slabs
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allocation.
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``slub_max_order``
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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
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generate super large order pages to fit ``slub_min_objects``
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of a slab cache with large object sizes into one high order
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page. Setting command line parameter
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``debug_guardpage_minorder=N`` (N > 0), forces setting
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``slub_max_order`` to 0, what cause minimum possible order of
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slabs allocation.
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SLUB Debug output
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-----------------
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=================
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Here is a sample of slub debug output:
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Here is a sample of slub debug output::
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====================================================================
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BUG kmalloc-8: Redzone overwritten
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--------------------------------------------------------------------
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====================================================================
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BUG kmalloc-8: Redzone overwritten
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--------------------------------------------------------------------
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INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
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INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
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INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
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INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
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INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
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INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
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INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
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INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
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Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
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Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
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Redzone 0xc90f6d28: 00 cc cc cc .
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Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
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Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
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Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
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Redzone 0xc90f6d28: 00 cc cc cc .
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Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
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[<c010523d>] dump_trace+0x63/0x1eb
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[<c01053df>] show_trace_log_lvl+0x1a/0x2f
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[<c010601d>] show_trace+0x12/0x14
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[<c0106035>] dump_stack+0x16/0x18
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[<c017e0fa>] object_err+0x143/0x14b
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[<c017e2cc>] check_object+0x66/0x234
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[<c017eb43>] __slab_free+0x239/0x384
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[<c017f446>] kfree+0xa6/0xc6
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[<c02e2335>] get_modalias+0xb9/0xf5
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[<c02e23b7>] dmi_dev_uevent+0x27/0x3c
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[<c027866a>] dev_uevent+0x1ad/0x1da
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[<c0205024>] kobject_uevent_env+0x20a/0x45b
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[<c020527f>] kobject_uevent+0xa/0xf
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[<c02779f1>] store_uevent+0x4f/0x58
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[<c027758e>] dev_attr_store+0x29/0x2f
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[<c01bec4f>] sysfs_write_file+0x16e/0x19c
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[<c0183ba7>] vfs_write+0xd1/0x15a
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[<c01841d7>] sys_write+0x3d/0x72
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[<c0104112>] sysenter_past_esp+0x5f/0x99
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[<b7f7b410>] 0xb7f7b410
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=======================
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[<c010523d>] dump_trace+0x63/0x1eb
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[<c01053df>] show_trace_log_lvl+0x1a/0x2f
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[<c010601d>] show_trace+0x12/0x14
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[<c0106035>] dump_stack+0x16/0x18
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[<c017e0fa>] object_err+0x143/0x14b
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[<c017e2cc>] check_object+0x66/0x234
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[<c017eb43>] __slab_free+0x239/0x384
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[<c017f446>] kfree+0xa6/0xc6
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[<c02e2335>] get_modalias+0xb9/0xf5
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[<c02e23b7>] dmi_dev_uevent+0x27/0x3c
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[<c027866a>] dev_uevent+0x1ad/0x1da
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[<c0205024>] kobject_uevent_env+0x20a/0x45b
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[<c020527f>] kobject_uevent+0xa/0xf
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[<c02779f1>] store_uevent+0x4f/0x58
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[<c027758e>] dev_attr_store+0x29/0x2f
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[<c01bec4f>] sysfs_write_file+0x16e/0x19c
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[<c0183ba7>] vfs_write+0xd1/0x15a
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[<c01841d7>] sys_write+0x3d/0x72
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[<c0104112>] sysenter_past_esp+0x5f/0x99
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[<b7f7b410>] 0xb7f7b410
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=======================
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FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
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FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
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If SLUB encounters a corrupted object (full detection requires the kernel
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to be booted with slub_debug) then the following output will be dumped
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@ -185,38 +197,38 @@ into the syslog:
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1. Description of the problem encountered
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This will be a message in the system log starting with
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This will be a message in the system log starting with::
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===============================================
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BUG <slab cache affected>: <What went wrong>
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-----------------------------------------------
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===============================================
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BUG <slab cache affected>: <What went wrong>
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-----------------------------------------------
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INFO: <corruption start>-<corruption_end> <more info>
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INFO: Slab <address> <slab information>
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INFO: Object <address> <object information>
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INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
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INFO: <corruption start>-<corruption_end> <more info>
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INFO: Slab <address> <slab information>
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INFO: Object <address> <object information>
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INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
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cpu> pid=<pid of the process>
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INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
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pid=<pid of the process>
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INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
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pid=<pid of the process>
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(Object allocation / free information is only available if SLAB_STORE_USER is
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set for the slab. slub_debug sets that option)
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(Object allocation / free information is only available if SLAB_STORE_USER is
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set for the slab. slub_debug sets that option)
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2. The object contents if an object was involved.
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Various types of lines can follow the BUG SLUB line:
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Various types of lines can follow the BUG SLUB line:
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Bytes b4 <address> : <bytes>
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Bytes b4 <address> : <bytes>
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Shows a few bytes before the object where the problem was detected.
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Can be useful if the corruption does not stop with the start of the
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object.
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Object <address> : <bytes>
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Object <address> : <bytes>
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The bytes of the object. If the object is inactive then the bytes
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typically contain poison values. Any non-poison value shows a
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corruption by a write after free.
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Redzone <address> : <bytes>
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Redzone <address> : <bytes>
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The Redzone following the object. The Redzone is used to detect
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writes after the object. All bytes should always have the same
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value. If there is any deviation then it is due to a write after
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@ -225,7 +237,7 @@ Redzone <address> : <bytes>
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(Redzone information is only available if SLAB_RED_ZONE is set.
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slub_debug sets that option)
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Padding <address> : <bytes>
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Padding <address> : <bytes>
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Unused data to fill up the space in order to get the next object
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properly aligned. In the debug case we make sure that there are
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at least 4 bytes of padding. This allows the detection of writes
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@ -233,29 +245,29 @@ Padding <address> : <bytes>
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3. A stackdump
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The stackdump describes the location where the error was detected. The cause
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of the corruption is may be more likely found by looking at the function that
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allocated or freed the object.
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The stackdump describes the location where the error was detected. The cause
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of the corruption is may be more likely found by looking at the function that
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allocated or freed the object.
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4. Report on how the problem was dealt with in order to ensure the continued
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operation of the system.
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operation of the system.
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These are messages in the system log beginning with
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These are messages in the system log beginning with::
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FIX <slab cache affected>: <corrective action taken>
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FIX <slab cache affected>: <corrective action taken>
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In the above sample SLUB found that the Redzone of an active object has
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been overwritten. Here a string of 8 characters was written into a slab that
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has the length of 8 characters. However, a 8 character string needs a
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terminating 0. That zero has overwritten the first byte of the Redzone field.
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After reporting the details of the issue encountered the FIX SLUB message
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tells us that SLUB has restored the Redzone to its proper value and then
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system operations continue.
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In the above sample SLUB found that the Redzone of an active object has
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been overwritten. Here a string of 8 characters was written into a slab that
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has the length of 8 characters. However, a 8 character string needs a
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terminating 0. That zero has overwritten the first byte of the Redzone field.
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After reporting the details of the issue encountered the FIX SLUB message
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tells us that SLUB has restored the Redzone to its proper value and then
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system operations continue.
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Emergency operations:
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---------------------
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Emergency operations
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====================
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Minimal debugging (sanity checks alone) can be enabled by booting with
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Minimal debugging (sanity checks alone) can be enabled by booting with::
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slub_debug=F
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@ -270,73 +282,80 @@ No guarantees. The kernel component still needs to be fixed. Performance
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may be optimized further by locating the slab that experiences corruption
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and enabling debugging only for that cache
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I.e.
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I.e.::
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slub_debug=F,dentry
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If the corruption occurs by writing after the end of the object then it
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may be advisable to enable a Redzone to avoid corrupting the beginning
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of other objects.
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of other objects::
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slub_debug=FZ,dentry
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Extended slabinfo mode and plotting
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-----------------------------------
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===================================
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The slabinfo tool has a special 'extended' ('-X') mode that includes:
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The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes:
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- Slabcache Totals
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- Slabs sorted by size (up to -N <num> slabs, default 1)
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- Slabs sorted by loss (up to -N <num> slabs, default 1)
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Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K)
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and reports everything in bytes (this functionality is also available to
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other slabinfo modes via '-B' option) which makes reporting more precise and
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accurate. Moreover, in some sense the `-X' mode also simplifies the analysis
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of slabs' behaviour, because its output can be plotted using the
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slabinfo-gnuplot.sh script. So it pushes the analysis from looking through
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the numbers (tons of numbers) to something easier -- visual analysis.
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Additionally, in this mode ``slabinfo`` does not dynamically scale
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sizes (G/M/K) and reports everything in bytes (this functionality is
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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:
|
||||
|
||||
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
|
||||
a) collect slabinfo extended records, for example::
|
||||
|
||||
b) pass stats file(-s) to slabinfo-gnuplot.sh script:
|
||||
slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
|
||||
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
|
||||
|
||||
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
|
||||
b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::
|
||||
|
||||
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:
|
||||
slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
|
||||
|
||||
a) Collect as many STATS1, STATS2, .. STATSN files as you need
|
||||
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
|
||||
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
|
||||
|
||||
b) Pre-process those STATS files
|
||||
slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
|
||||
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:
|
||||
|
||||
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
|
||||
a) Collect as many STATS1, STATS2, .. STATSN files as you need::
|
||||
|
||||
This will produce a single plot (png file).
|
||||
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
|
||||
|
||||
Plots, expectedly, can be large so some fluctuations or small spikes
|
||||
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).
|
||||
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::
|
||||
|
||||
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
|
||||
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).
|
||||
|
||||
Christoph Lameter, May 30, 2007
|
||||
Sergey Senozhatsky, October 23, 2015
|
||||
|
|
|
|||
Loading…
Reference in New Issue