Kernel dynamic memory analysis

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This page has notes and results from the project Kernel dynamic memory allocation tracking and reduction

[This page is fairly random at the moment...]

Instrumentation overview

  • Slab_accounting patches
    • uses __builtin_return_address(0) to record the address of the caller, the same mechanism used by kmem events
    • starts from very first allocation
    • does not use kernel allocators (instead it uses fixed-size static buffers)
      • This increases overhead, but avoids interspersing tracing allocations in the rest of the system allocations
  • Ftrace kmem events
    • does not start until ftrace system is initialized, after some allocations are already performed
    • supported in mainline - no need to add our own instrumentation

These two instrumentation methods are basically the same: trap each kmalloc, kfree, etc. event and produce relevant information with them. The difference between them is that the first post-processes the events in-kernel and create a /proc/slab_account file to access the results. This output is more or less like this:

total bytes allocated:  1256052
total bytes requested:  1077112
slack bytes allocated:   178940
number of allocs:          7414
number of frees:           5022
number of callers:          234

  total    slack      req alloc/free  caller
   2436      232     2204    29/0     bio_kmalloc+0x33
    268        8      260     1/0     pci_alloc_host_bridge+0x1f
     32        8       24     1/0     tracepoint_entry_add_probe.isra.2+0x86
     44        8       36     1/0     cpuid4_cache_sysfs_init+0x30
      0        0        0     0/3     platform_device_add_data+0x33

On the other hand, analysing ftrace kmem events will defer post-processing to be done at user space, thus achieving much more flexibility.

The disadvantage of the ftrace method is that it needs to be initialized before capturing events. Currently, this initialization is done at fs_initcall and we're working on enabling them earlier. For more information, checkout this upstreamed patch:

trace: Move trace event enable from fs_initcall to core_initcall

This patch allows to enable events at core_initcall. It's also possible to enable it at early_initcall. Another posibility is to create a static ring buffer and then copy the captured events into the real ring buffer.

Also, we must find out if early allocations account for significant memory usage. If not, it may not be that important to capture them. Yet another possibility is to use a printk brute-force approach for very early allocations, and somehow coalesce the data into the final report.

Using debugfs and ftrace

For more information, please refer to the canonical trace documentation at the linux tree:

  • Documentation/trace/ftrace.txt
  • Documentation/trace/tracepoint-analysis.txt
  • and everything else inside Documentation/trace/

(Actually, some of this information has been copied from there.)


The debug filesystem it's a ram-based filesystem that can be used to output a lot of different debugging information. This filesystem is called debugfs and can be enabled with CONFIG_DEBUG_FS:

Kernel hacking
     [*] Debug filesystem

After you enable this option and boot the built kernel, it creates the directory /sys/kernel/debug as a location for the user to mount the debugfs filesystem. Do this manually:

$  mount -t debugfs none /sys/kernel/debug

You can add a link to type less and get less tired:

$ ln -s /debug /sys/kernel/debug


Once we have enabled debugfs, you need to enable tracing support, selecting CONFIG_TRACING, CONFIG_FUNCTION_TRACER and CONFIG_DYNAMIC_FTRACE options.

Kernel hacking
          [*] Kernel Function Tracer
          [*] enable/disable ftrace tracepoints dynamically

This option will add a /sys/kernel/debug/tracing directory on your mounted debugfs filesystem. Traced events can be read through /sys/kernel/debug/tracing/trace. You can list available events by listing file /sys/kernel/debug/tracing/available_events.

To enable events on bootup you can add them to kernel parameters, for instance to enable kmem events:


Warning: if you use SLOB on non-NUMA systems, where you might expect kmalloc_node not get called, actually it is the only one called. This is due to SLOB implementing only kmalloc_node and having kmalloc call it without a node. Same goes to kem_cache_alloc_node.

Obtaining accurate call sites (or The painstaking task of wrestling against gcc)

The compiler inlines a lot automatically and without warning. In this scenario, it's impossible to get the real call site name based on just calling address.

When some function is inlined, it gets collapsed and it won't get listed as a symbol if you use tools like readelf, objdump, etc.

Does this matter? Well, it matters if you want to obtain an accurate call site report when tracing kernel memory events (which will see later).

However, there is one solution! You can turn off gcc inlining using some gcc options on kernel Makefile. You need to add these to your CFLAGS:

KBUILD_CFLAGS   += -fno-default-inline \
                  -fno-inline \
                  -fno-inline-small-functions \
                  -fno-indirect-inlining \

It's not yet clear if all of these are needed, or just some of them.

Using ./scripts/bloat-o-meter it's possible to determine the effect of using this gcc flags:

$ ./scripts/bloat-o-meter vmlinux-def vmlinux-no-inline

add/remove: 1574/33 grow/shrink: 154/1099 up/down: 218552/-199352 (19200)
function                                     old     new   delta
hidinput_configure_usage                       -    8672   +8672
do_con_trol                                    -    2955   +2955
do_con_write                                   -    2223   +2223
copy_process                                   -    2078   +2078

The effect of disable inlining is now very clear:

  • There are 1574 new functions, and 33 less functions.
  • Some functions have grown, but many more have shrunk.
  • Overall, the kernel symbols's size is 19k bigger.

We must keep in mind that no matter what internal mechanisms we use to record call_site, if they're based on __builtin_address, then their accuracy will depend entirely on gcc *not* inlining automatically.

The emphasis is in the automatic part. There will be lots of functions we will need to get inlined in order to determine the caller correctly. These will be marked as __always_inline.

See upstreamed patch:


See for some patch feedback details

Memory accounting

Types of memory

The kernel, being a computer program, can consume memory in two different ways: statically and dynamically.

Static memory can be measured offline, therefore accounted before actually running the kernel using standard binary inspection utilites (readelf, objdump, size, etc). We will explore this utilities in detail.

Dynamic memory cannot be measured offline, and it's not only necessary to probe a running kernel but also to enable aditional probe code to trace each allocation. Fortunately for us, the linux kernel has ftrace which is a tracing framework that allows to trace general events, and in particular memory allocation events. We will explore this framework in detail.

Static memory

A compiled kernel will allocate static memory to store two kinds of symbols: code symbols (a.k.a text) and data symbols (a.k.a data).

For instance, let's look at this piece of C code:

long curr;
int max = 10;
int foo(int var)
    int i = var + max;
    if (i < max)
        curr = i;

We have three different symbols:

  • curr : a variable (data zero initialized)
  • max  : an initialized variable (data non-zero initialized)
  • foo  : a function (text)

Once this code is running each of these symbols will need memory for its own storage. However, the zero initialized variable will not use space in the compiled binary. This is due to a special section inside the binary (called bss for no good reason) where all the zero initialized variables are placed. Since they carry no information, they need no space. Static variables have the same life as the executing program.

On the other side, var and i variables are dynamically allocated, since they live in the stack. They are called automatic variables, meaning that they have a life cycle that's not under our control.

Note that when we talk about static memory, the word static has nothing to do with the C-language keyword. This keyword references a visibility class, where static means local, as opposed to global.

The size command

The most simple command to get a binary static size, is the wonderfully called size command. Let's start by seeing it in action:

$ size ./fs/ext2/ext2.o 
  text	   data	    bss	    dec	    hex	filename
 51273	     68	      8	  51349	   c895	./fs/ext2/ext2.o

According to this output, this object file has roughly 50k bytes of text and 68 bytes of data. Now, size comes in two flavors: berkeley and sysv. Each of this shows a different output. The default is berkeley, so the previous example was a berkeley output.

However, if we use the same command with sysv output format, we'll find very different results:

$ size --format=sysv ./fs/ext2/ext2.o 
./fs/ext2/ext2.o  :
section             size   addr
.text              43005      0
.init.text           138      0
.exit.text            25      0
.rodata             2304      0
.rodata.str1.1      1656      0
.rodata.str1.4      3485      0
.data                 60      0
.exitcall.exit         4      0
.initcall6.init        4      0
.bss                   8      0
.note.GNU-stack        0      0
.comment             675      0

Here we see a more detailed description about each section size. Note the appearence of a .rodata (read-only data) section, of 2k byte large. This section is composed of read-only variables (e.g. marked const) that are not accounted under the data column, but rather as text by the standard size format.

To add even more confusion to this picture, gcc can decide (at his own will) to put inside .rodata section symbols not marked as const. These symbols are not written by anyone, and gcc considers them as read-only (pretty smart, uh?). This means you can have a .rodata section bigger than what you expected to have.

This happens since gcc v4.7 (???)


This two commands can give us any information we need about a binary. In particular, they can output the complete list of symbols with detailed information about each one. Let's see an example of readelf on the same file we used for size. The output is stripped for clarity.

$ readelf -s fs/ext2/ext2.o
Symbol table '.symtab' contains 413 entries:
  Num:    Value  Size Type    Bind   Vis      Ndx Name
    0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
    1: 00000000     0 SECTION LOCAL  DEFAULT    1 
  339: 00004da0   286 FUNC    GLOBAL DEFAULT    1 ext2_evict_inode
  340: 000003e0    76 OBJECT  GLOBAL DEFAULT    7 ext2_nobh_aops
  341: 00000780   128 OBJECT  GLOBAL DEFAULT    7 ext2_symlink_inode_operat
  342: 000008d8    20 OBJECT  GLOBAL DEFAULT    7 ext2_xattr_acl_default_ha
  343: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND generic_file_aio_write
  344: 00000280   128 OBJECT  GLOBAL DEFAULT    7 ext2_file_inode_operation
  345: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __dquot_alloc_space
  346: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND generic_setxattr
  347: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND unlock_buffer
  348: 000014c0    36 FUNC    GLOBAL DEFAULT    1 ext2_bg_num_gdb
  349: 00005240   684 FUNC    GLOBAL DEFAULT    1 ext2_setattr

For instance, ext2_nobh_aops is an OBJECT symbol (data) of 76 bytes and ext2_evict_inode is a FUNC symbol (text) of 286 bytes. Notice there are some UND symbols. They are undefined symbols for this file, that are defined elsewhere and therefore not of interest for us when inspecting a file size.

Of course, this output can be combined with grep to get fantastic results. Let's count the numbers of defined functions:

$ readelf -s fs/ext2/ext2.o | grep -v UND | grep FUNC

With a little awk magic we could even sum these sizes and get the size of the .text section. TODO!




Dynamic memory in kernel land is a little different from user land.

In user land, all one needs to do to get a chunk of memory is call malloc(). In kernel land, we have a similar function: kmalloc(). But we also have lots of other functions to alloc memory, and we must have some special considerations.

The first thing it's important to understand is that kernel obtains memory (well, in most architectures) on a fixed-size chunk, that we call a 'page' of memory. This page of memory tipically is 4096 bytes large, but this depends on the architecture.

In order to delivery smaller pieces of memory, the kernel have a few couple of layers that ultimately lets you do kmalloc(100) and get 100 bytes. These layers are called: buddy allocator and slab allocator.

We will focus on the latter. Slab allocator comes in three different flavors: SLAB, SLOB and SLUB. These funny names are historically, but the meaning is:

  • SLAB is the traditional (or default in some platforms ?)
  • SLOB is aimed at tiny embedded systesm (e.g. without mmu)
  • SLUB is the default (on desktop?)

Each of these implement the allocation in a different way, but they all share a common property: internal fragmentation.

Internal fragmentation

For different reasons (alignment, overhead, etc) when we request 100 bytes with kmalloc(100) the slab allocator may really allocate 128 bytes (or 140 bytes, we can't really know). These extra 28 bytes can't be used, and therefore you are wasting them. This is called internal fragmentation, and one of the main goals of the slab allocator is to minimize it. In other words, trying to match as nearly as possible the requested size with the truly allocated size.

Accounting with kmem events trace

Ftrace kmem events are a great source of information. By using them you can trace each kmalloc, getting the requested bytes, the allocated bytes, the caller address and the returned pointer. You can also trace kfree, getting the caller address and the freed pointer.

Once you have the caller address you can use file to get the caller function name. Also, by using the returned pointer and correlating with kfree traces you can keep track of currently used dynamic memory by each kernel function / subsystem.

Let's see this in detail.

Enabling and reading kmem trace

We can activate this on boot up with kernel parameter trace_event. For instance,


or you can activate on-the-fly with:


Once you have enabled events, you can run your favourite program (in order to trigger some allocations). When you're done you may disable events (or not) and read them:

$ cat /sys/kernel/debug/tracing/trace > kmem.log

Let's see a piece of this log:

# tracer: nop
# entries-in-buffer/entries-written: 10500/10500   #P:1
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
           init-1     [000] .N..     0.170000: kmalloc: call_site=c104deff ptr=c147dd20 bytes_req=29 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.170000: kmalloc: call_site=c104e2ac ptr=c147dd00 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.170000: kmalloc: call_site=c104deff ptr=c147dce0 bytes_req=22 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.170000: kmalloc: call_site=c104e2ac ptr=c147dcc0 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.170000: kmalloc: call_site=c10c0185 ptr=c14a4ba0 bytes_req=36 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
           init-1     [000] .N..     0.170000: kmalloc: call_site=c10bfa28 ptr=c147dca0 bytes_req=6 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.170000: kfree: call_site=c10bfa4a ptr=  (null)
           init-1     [000] .N..     0.170000: kmalloc: call_site=c10b9619 ptr=c147dc80 bytes_req=6 bytes_alloc=32 gfp_flags=GFP_KERNEL
           init-1     [000] .N..     0.180000: kmem_cache_alloc: call_site=c10b963e ptr=c1610c08 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO

This log can be post-processed. For your convenience we have a script that does exactly this (see below).

About kmem trace events

As we have seen there are a few more events than kmalloc and kfree. Let's see them all:

  • kmalloc
  • kfree
  • kmalloc_node
  • kmem_cache_alloc
  • kmem_cache_alloc_node
  • kmem_cache_free

Pitfall: ringbuffer overrun

Ftrace events (in particular kmem trace events) are stored internally in a ringbuffer. If the kernel keeps writing events to it, eventually it will overrun, replacing the oldest events with the most recent.

If you're doing a bootup analysis, an overrun will trash your test.

There are few ways to avoid this.

  • Increase ringbuffer size

By using trace_buf_size kernel parameter it's possible to control the buffer size. Keep in mind that a larger buffer will take longer to get allocated and will need more memory.

  • Use a minimal setup and try to stop tracing as soon as possible

When doing bootup analysis, it may make sense to try to minimize the scenario setup, in order to reduce events. Also, you probably want to disable events as soon as possible.

  • Perform smaller tests

Besides doing a bootup analysis, it's possible to run small tests, focused on some specific workload. Something like this:

 $ echo "1" > tracing_on # enable events
 $ modprobe ext2
 $ mount ...
 $ fsck
 $ umount ...
 $ rmmod ext2
 $ echo "0" > tracing_on # disable_events

You can test if the buffer is overrunning by looking at per-cpu tracing stat:

$ cat /sys/kernel/debug/tracing/per_cpu/cpu0/stats

entries: 0
overrun: 0
commit overrun: 0
bytes: 0
oldest event ts:     0.000000
now ts:   997.471882

Using the post-processing script

Getting the script

You can get script last version from this project's github repository

 git clone
 cd kmem-probe-framework
 ./post-process/ -h

Using for static analysis typically needs access to: a built kernel tree and an ftrace kmem log.

However, if one lacks the latter but can provide a built kernel tree, the script will fallback to 'static' analysis.

Using it

Usage is fairly simple

 $ ./ -k /usr/src/linux -r rings.png
 $ ./ --kernel /usr/src/linux -r rings.png

This should produce a ringchart png file in the current directory.

Of course, you can use absolute and relative paths in that parameter

 $ ./ -k ./torvalds -r rings.png

If you're interested in a specific subsystem you can use a parameter to specify the directory tree branch to take as root

 $ ./trace_analyze -k ./torvalds -b fs -r fs.png
 $ ./trace_analyze -k ./torvalds -b drivers/base -r base.png
 $ ./trace_analyze -k ./torvalds -b mm -r mm.png

Each of this commands will produce a ringchart png file in the curent directory, named as specified.

Under the hood

The script will perform a directory walk, internally creating a tree matching the provided kernel tree. On each object file found (like fs/inode.o) it will perform a 'readelf --syms' to get a list of symbols contained in it.

Nothing fancy.


  • Account for minor differences between running 'size' and using this script

Using for dynamic analysis

Producing a kmem trace log file

The purpose of trace_analyze script is to perform dynamic memory analysis. For this to work you need feed it with a kmem trace log file; of course, you also need to give hime a built kernel tree.

Such log must be produced on the running target kernel, but you can post-process it off-box. For instance, you boot your kernel with kmem parameters to enable ftrace kmem events: (it's recommended to enable all events, despite not running a NUMA machine).


This parameter will have linux to start tracing as soon as possible. Of course some early traces will be lost, see below.

(on your target kernel)

# To stop tracing
$ echo "0" > /sys/kernel/debug/tracing/tracing_on
# Dump 
$ cat /sys/kernel/debug/tracing/trace > kmem.log

Now you need to get this file so you can post-process it using In my case, I use qemu with a file backing serial device, so I simply do:

(on your target kernel)

$ cat /sys/kernel/debug/tracing/trace > /dev/ttyS0

And I get the log on qemu's backing file.

Now you have everything you need to start the analysis. The script will post-process the logfile and will produce two kinds of output:

  • an account file
  • a ringchart png

Let's see how to obtain each of these.

Slab accounting file output

To obtain the account file you need to use --acount-file (-c) parameter, like this:

 ./ -k ../torvalds -f kmem.log --account-file kmem_account.txt

This will produce an account file like this:

current bytes allocated:     669696
current bytes requested:     618823
current wasted bytes:         50873
number of allocs:              7649
number of frees:               2563
number of callers:              115

  total    waste      net alloc/free  caller
 299200        0   298928  1100/1     alloc_inode+0x4fL
 189824        0   140544  1483/385   __d_alloc+0x22L
  51904        0    47552   811/68    sysfs_new_dirent+0x4eL
  16384     8088    16384     1/0     __seq_open_private+0x24L
  15936     1328    15936    83/0     device_create_vargs+0x42L
  14720    10898    14016   460/22    sysfs_new_dirent+0x29L

You can tell the script to read only kmalloc events (notice the option name is *--malloc*):

./ -k ../torvalds -f kmem.log -c kmem_account.txt --malloc

Or you can tell the script to read only kmem_cache events:

./ -k ../torvalds -f kmem.log -c kmem_account.txt --cache

If you want to order the account file you can use --order-by (-o):

./ -k ../torvalds -f kmem.log -c kmem_account.txt --order-by=waste

The possible options for order-by parameter are:

  • total_dynamic: Added allocations size
  • current_dynamic: Currently allocated size
  • alloc_count: Number of allocations
  • free_count: Number of frees
  • waste: Currently wasted size

You can pick a directory to get an account file only for the allocations from that directory. This is done with the --branch (-b) option, just like we've done for the static analysis:

$ ./ -k ../torvalds -f kmem.log -c kmem_account.txt -b drivers

All of these options can be combined. For instance, if you want to get kmalloc events only, coming from fs/ directory and ordered by current size:

$ ./ -k ../torvalds -f kmem.log -b fs -c kmem_account.txt -o current_dynamic --malloc

Producing a pretty ringchart for dynamic allocations

As already explained in the static analysis section, it's possible to produce a ringchart to get 'the big picture' of dynamic allocations. You will need to have matplotlib installed, which should be as easy as:

$ {your_pkg_manager} install matplotlib

The script usage is very simple, just pass the parameter --rings-file (-r)

$ ./ -k ../torvalds -f kmem.log --rings-file=foo.png

This command will produce a png file named as specified. The plot will show current dynamic allocations by default. You can control the used attrbute used for the ringchar plot using --rings-attr (-a) parameter. The available options are:

  • current: static + current dynamic size
  • static: static size
  • waste: wasted size
  • current_dynamic: current dynamic size
  • total_dyamic: added dynamic size
$ ./ -k ../torvalds -f kmem.log -r foo.png -a waste

You can use --branch (-b) parameter to plot allocations made from just one directory. For instance, if you want to get wasted bytes for ext4 filesystem:

$ ./ -k ../torvalds -f kmem.log -r ext4_waste.png -a waste -b fs/ext4

Or, if you want to see static footprint of arch-dependent mm code:

$ ./ -k ../torvalds -f kmem.log -r foo.png -a static -b arch/x86/mm

Also, you can filter kmalloc or kmem_cache traces using either --malloc, or --cache:

$ ./ -k linux/ -f boot_kmem.log -r malloc.png --malloc

Pitfall: wrongly reported allocation (and how to fix it)

There are a number of functions (kstrdup, kmemdup, krealloc, etc) that do some kind of allocation on behalf of its caller.

Of course, we don't want to get trace reports from these functions, but rather from its caller. To acomplish this, we must use a variant of kmalloc, called kmalloc_track_caller, which does exactly that.

Let's see an example. As of today kvasprintf() implementation looks like this

(see lib/kasprintf.c:14)
char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
       /* code removed */
       p = kmalloc(len+1, gfp); 

And trace_analyze produces the account file

total    waste      net alloc/free  caller
2161     1184     2161   148/0     kvasprintf

The source of this 148 allocations may be a single caller, or it may be multiple callers. We just can't know. However, if we replace kmalloc with kmalloc_track_caller, we're going to find that out.

char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
       /* code removed */
       p = kmalloc_track_caller(len+1, gfp);

After running the re-built kernel, and comparing both current and previous account files, we find this is the real caller:

total    waste      net alloc/free  caller
2161     1184     2161   148/0     kobject_set_name_vargs

So, we've accurately tracked this allocation down to the kobject code.

Internal fragmentation in SLAB allocators: a simple comparison

Using trace_analyze tool and a very minimal kernel + rootfs setup it's possible to evaluate allocators wastage.

This analysis is not complete, as it lacks a comparison of memory overhead imposed by the allocator itself.

Kmalloc objects

Strings, when dynamically allocated, are one of the greatest source of fragmentation. This is intrinsic to the 'string' nature: Strings never have a regular size. String are frequently small, with typical sizes between 4 and 16 (?) bytes. This kind of small and irregular allocations can help evaluate each allocator.

The (primary?) source of strings is directory entries: file names, directory names, and such.

Let's compare each allocator and see how they perform.


SLOB prepends a fixed-size header to every kmalloc allocation. On x86 this header is 8 bytes long (how about other arches?).

So, if requested string size is between 4 and 16 bytes, the fragmentation ratio is between 200% and 50%.

SLOB allocator is different from SLAB and SLUB: it doesn't really implement a 'slab cache' concept. For this reason there is no real difference between kmallocing 5 bytes or 100 bytes. The allocator won't search for a cache of a proper size, but instead just allocate a chunk prepended with a fixed-size header.

Since wastage is independent of requested size, small allocations may waste a lot, but the wastage falls with increasing allocations.


When allocating strings SLUB allocator uses its 8, 16 and 32 byte caches. For requested size between 4 and 16 bytes, the fragmentation ratio is between 100 and 0%, the worst cases being 4 and 9 bytes, with fragmentation of 100% and 78% respectively.

SLUB being a cache-based allocator, the fragmentation will depend on how aligned the requested size is to the available size. For instance, requesting 32 bytes allocates 32 bytes, resulting in zero fragmentation. Requesting 33 bytes allocates 64 bytes, resulting in 94% fragmentation.


SLAB operates in a similar way to SLUB, in the sense that allocations are cache-based and fragmentation depends on fitness between requested size and cache size. However, since the smallest SLAB cache is 32 bytes, and strings are typically much smaller, SLAB is the less efficient of the three.

Just to compare, for requested size between 4 and 16 bytes, the fragmentation ratio is between 700% and 100%.

Cache objects

Cache objects fragmentation occurs when each allocated object needs some extra bytes for alignment, debug or other metadata. Let's analyze each case.


Being a very simple allocator, SLOB only needs to reserve extra bytes for rcu-destroyed objects. Currently, each of these objects wastes 12 bytes to store an rcu related struct.

For regular objects the wastage is nil.

It's important to keep in mind that SLOB can leave 'holes' inside slob pages. These holes are unallocated space that is not lost, because it can be used if it fits for some requested allocation.

However this means that, on low memory situations, it's possible to run out of memory for a specific allocation, but still have free space left on these holes.


If compiled without any debug mechanism (poison, redzone, ...) SLAB allocator will only reserve extra bytes to return aligned objects. Of course, the wastage depends on each architecture.

If compiled with debug enabled SLAB will add 24 bytes to each allocated object for red zoning and user store (see comments in mm/slab.c).


If compiled without any debug mechanism (poison, redzone, ...) SLUB allocator will reserve extra bytes for alignment and to store a freelist pointer.

The latter is true only for objects with a defined constructor. In this case, the wastage is fairly small: only 4/8 bytes (depending on the architecture) are needed to store a pointer. However, if many objects are allocated, the wastage can become noticeable.

An example of this is the allocation of inodes. Even on a very minimal system, more than 1k inodes can get allocated, resulting in a total wastage of 8k bytes.

As with SLAB, if compiled with debug enabled, the allocator adds some extra bytes to each object for red zoning and user store.


  • extracting data to host
    • tool for extraction (perf?, cat /debugfs/tracing/<something>?)
  • post-processing the data
    • grouping allocations (assigning to different subsystems, processes, or functional areas)
      • idea to post-process kmem events and correlate with */built-in.o
    • reporting on wasted bytes
    • reporting on memory fragmentation


Matplotlib [[1]]

We will use matplotlib with its Wedge api to create a ring chart (similar to gnome baobab).

We'll get something like this:

Current dynamic footprint

This ring chart shows current dynamic usage, start branch is 'drivers'. Script command line is:

   $ -f kmem.log -k ../linux-mtd/ -a current_dynamic -b drivers/ -r drivers.png

Drivers dynamic.png

Static footprint

This ring chart shows static usage, start branch is 'fs' and the kernel is configured with ext2, ubifs, jffs2. Script command line is:

   $ -f kmem.log -k ../linux-mtd/ -a static -b fs -r fs-static.png

Fs static.png

Mainline status


Here's the list of the mainlined (or waiting at linux-next) patches that were produced for this project.

Patches focuses on:

  • Improving tracing by enabling ftrace earlier in kernel init sequence
  • Improving kmem tracing accuracy by tracking caller correctly
  • Improve or fix slob kmem tracing support
  • Minor slob cleanups
 mm/slob: Use free_page instead of put_page for page-size kmalloc allocations
 commit 8cf9864b1382851d90c7c505f8441c8928f1469e
 mm/sl[aou]b: Move common kmem_cache_size() to slab.h
 commit 242860a47a75b933a79a30f6a40bf4858f4a3ecc
 mm/slob: Use object_size field in kmem_cache_size()
 commit fe74fe2bf293d061826f0d7afc2ca8456bdbb40e
 mm/slob: Drop usage of page->private for storing page-sized allocations
 commit 999d8795d438d396936811b185428d70b7b7de6d
 lib/kasprintf.c: use kmalloc_track_caller() to get accurate traces for kvasprintf
 commit 3e1aa66bd423950aa69c3d50d91818af1d16e0a7
 mm/slab: Fix kmem_cache_alloc_node_trace() declaration
 commit dffa3f985047979ace185e253da89cabda3f7b31
 mm/slab: Fix kmem_cache_alloc_node_trace() declaration
 commit 1e5965bf1f018cc30a4659fa3f1a40146e4276f6
 mm/slab: Fix typo _RET_IP -> _RET_IP_
 commit 592f41450d03f74747edd81779ae206a0647152f
 mm, slub: Rename slab_alloc() -> slab_alloc_node() to match SLAB
 commit 2b847c3cb4f8565911bd8ce59b8428e635c90594
 mm, slab: Rename __cache_alloc() -> slab_alloc()
 commit 48356303ff8cce7036f13a23df9119d3f47461ce
 mm, slab: Match SLAB and SLUB kmem_cache_alloc_xxx_trace() prototype
 commit 4052147c0afa1cf05780ed846f37e87cdde9f628
 mm, slab: Replace 'caller' type, void* -> unsigned long
 commit 7c0cb9c64f83dd9bf18b139ac0de00daba6087c2
 mm, slob: Add support for kmalloc_track_caller()
 commit f3f741019595f1e73564d985f5fe8abcbb98c769
 mm, slab: Remove silly function slab_buffer_size()
 commit ff4fcd01ec86d98d15d2fd96f22f19bb1d341b88
 mm, slob: Use NUMA_NO_NODE instead of -1
 commit 90f2cbbc49a8fe5a49cea1d362d90e377b949d49
 trace: Move trace event enable from fs_initcall to core_initcall
 commit 8781915ad2716adcd8cd5cc52cee791fc8b00fdf
 mm: Use __do_krealloc to do the krealloc job
 commit e21827aadd77e33833eeb393de2e8675e9b399e2


A few patches already sent have been nacked and work is needed in order to submit them again:

 [PATCH 05/10] mm, util: Use dup_user to duplicate user memory
 [PATCH v2] Makefile: Add option to disable gcc automatic inlining

Latter's usefulness is still to prove.

The truth is if compiler folds some static, once-called, small function we will report a wrong callsite but we will correlate it with the correct subsystem/file.

Moreover, some parts of the kernel implicitly relies on gcc automatic inlining behavior, and turning this off have shown to break compilation.

We paste the patch here, in case someone wants to use it. It should apply on any recent kernel.

Recommendations for reductions

  • Avoid the use of constructors
  • Avoid to kmalloc / kstrdup small strings
  • Remember to use kmalloc_track_caller on sub-system allocator functions. (see mtdcore.c: mtd_kmalloc_up_to)

Move to a section of its own?

We want to answer thes question: When to replace a kmalloc with a kmem_cache?

We can use trace_analyze on kmalloc events, ordered by alloc_count to find frequent allocers. It may be advantageous to replace this allocations with kmem_cache's.

Do it yourself:

$ trace_analyze -k linux/ -f kmem.log -c kmem_acc.txt -o alloc_count --malloc

And the produced account file looks like this:

current bytes allocated:     113128
current bytes requested:      93232
current wasted bytes:         19896
number of allocs:              2217
number of frees:                145
number of callers:               83

  total    waste      net alloc/free  caller
   5936     1782     5616   523/22    sysfs_new_dirent+0x29L
   4104     1173     4104   326/0     __trace_define_field+0x5dL
  10432        0    10432   326/0     __trace_define_field+0x34L
   3800     1177     3800   326/0     __trace_define_field+0x4aL
   1816      513     1816   177/0     kobject_set_name_vargs+0x18L
   9216     1536     9216    96/0     device_private_init+0x23L         <----------- This one ?
  15936     1328    15936    83/0     device_create_vargs+0x46L         <----------- This one ?
   3040      416     1376    24/16    add_sysfs_param.isra.2+0x46L
   3072        0     3072    24/0     con_insert_unipair+0xb5L
   1408        0        0    22/22    create_unique_id.isra.50+0x2eL
    336       68      200    19/11    add_sysfs_param.isra.2+0x6fL
    512      192      512    16/0     event_subsystem_dir+0xddL
    152       44      152    16/0     event_subsystem_dir+0x9cL
    512       64      512    16/0     event_subsystem_dir+0x64L
    120        0        0    13/13    dir_add+0x48L
    208        0        0    13/13    dir_add+0x20L
    640      160      640    10/0     kset_create+0x31L                 <----------- This one ?

Being a very minimal setup, the allocations that appear frequently are related to sysfs. Before asserting that it's better use kmem_cache in these call sites, it's necessary to estimate to overhead impose by this, considering a new kmem_cache would be created for each case.

Slab account

Slab accounting is another method of obtaining allocation-related information about the kernel. It is an out-of-tree (as of November 2012) patch that uses static buffers to hold information about allocations and frees.

This is the version of the patch for 3.6:

Testing feedback

There is some testing feedback from Tim on the Talk:Kernel_dynamic_memory_analysis page.