Kernel Size Tuning Guide Config Option Impact

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This page describes examination result which tries to make clear how much impact each config item has for kernel size and RAM usage. Examined config items are basically ones listed in the table in "4.3 Kernel Configuration Options" of Kernel Size Tuning Guide. This result was presented at Japan Technical Jamboree 11] , titled "Difference of the Kernel size between "default" and "small" ". Presentation slides are available on the Japan Technical Jamboree 11 page.

Examination Result

Impact of config items

The table below shows kernel size and RAM usage difference between when config item is set as "from" and "to". Most of items are same as listed in "4.3 Kernel Configuration Options" of Kernel Size Tuning Guide, but some were not examined, and several items are added. Please see "note" cell about the reason to be omitted or added. The data were taken from kernel for i386. More detail of measurement conditions are described in "Examination Detail" later.

Column "avail on" means whether the config item is available for vanilla kernel, or for only Linux-Tiny applied kernel.

config option avail on setting difference[byte] note
from to vmlinux bzImage used mem
CONFIG_BASE_FULL tiny Y N -4096 -32 -16384 changed from CONFIG_CORE_SMALL(which was for 2.6.10 in core-small.patch)
CONFIG_NET_SMALL tiny N Y 0 32 16384
CONFIG_KMALLOC_ACCOUNTING tiny N N - - - compilation failure when enabled(mm/slab.c : __do_kmalloc() kmem_cache has no member of "cs_size")
CONFIG_AUDIT_BOOTMEM tiny Y N 0 -128 -135168
CONFIG_PRINTK vanilla Y N -365530 -142176 - mem measurement uses printk
CONFIG_BUG vanilla Y N -43287 -14848 12288
CONFIG_ELF_CORE vanilla Y N -4424 -2880 -8192
CONFIG_PROC_KCORE vanilla Y N -4467 -3328 20480
CONFIG_AIO tiny Y N -9329 -7328 -16384
CONFIG_XATTR tiny Y N -21517 -10816 77824 see "Tips"
CONFIG_FILE_LOCKING tiny Y Y - - - compilation failure (see "Tips")
/\ CONFIG_DIRECTIO - - - - - - obsolete (was 2.6.10 in direct-io-core.patch)
CONFIG_MAX_SWAPFILES_SHIFT tiny 5 0 0 -96 -4096
CONFIG_NR_LDISCS tiny 16 2 0 -64 -163840
CONFIG_MAX_USER_RT_PRIO tiny 100 5 0 -96 -143360
CONFIG_KALLSYMS vanilla Y N -235189 -130592 -4096
CONFIG_SHMEM vanilla Y N -14233 -7360 -12288
CONFIG_SWAP vanilla Y N -19266 -11136 8192
CONFIG_SYSVIPC vanilla Y N -27365 -12640 -4096 changed from CONFIG_SYSV_IPC (typo?)
CONFIG_POSIX_MQUEUE vanilla Y N -9681 -3936 -4096
CONFIG_SYSCTL vanilla Y N -84593 -28000 -57344
CONFIG_LOG_BUF_SHIFT vanilla 15 12 0 32 20480 see "Tips"
CONFIG_UID16 vanilla Y N -4408 -3040 -163840
CONFIG_CC_OPTIMIZE_FOR_SIZE vanilla N Y -487424 -167904 12288
CONFIG_MODULES vanilla Y N -369445 -44608 -147456
CONFIG_KMOD vanilla Y N -4959 -4128 -159744
CONFIG_PCI vanilla Y N -1217753 -463200 -249856
/\ CONFIG_XIP_KERNEL - - - - - - not supported for i386 (arch depend)
/\ CONFIG_MAX_RESERVE_AREA - - - - - - ?? (arch depend?)
CONFIG_BLK_DEV_LOOP vanilla Y N -14292 -7488 -163840
CONFIG_BLK_DEV_RAM vanilla Y N -10237 -4512 -40960
CONFIG_BLK_DEV_RAM_COUNT vanilla 16 2 0 -32 -28672
CONFIG_BLK_DEV_RAM_SIZE vanilla 4096 1024 0 0 -16384
CONFIG_IOSCHED_AS vanilla Y N -10982 -7680 -8192
CONFIG_IOSCHED_DEADLINE vanilla Y N -6107 -3008 -147456
CONFIG_IOSCHED_CFQ vanilla Y N -12393 -6880 -28672
CONFIG_IP_PNP vanilla Y N -14156 -7168 -16384
CONFIG_IP_PNP_DHCP vanilla Y N -101 -224 -4096
CONFIG_IP_PNP_BOOTP vanilla Y N 0 0 -20480 Added to contrast with IP_PNP_DHCP
CONFIG_IP_PNP_RARP vanilla Y N -4190 -2784 -135168 Added to contrast with IP_PNP_DHCP
CONFIG_IDE vanilla Y N -175576 -69568 - mem measurement environment uses IDE as a root device
CONFIG_SCSI vanilla Y N -284965 -114720 -315392
CONFIG_SMP vanilla Y N -177845 -64064 -61440 Added 'cause probably effective
CONFIG_NFS_FS vanilla Y N -63186 -28384 -86016 Added 'cause probably effective
CONFIG_NFSD vanilla Y N -56816 -24256 -454656 Added 'cause probably effective

Kernel Overall Size and RAM Usage

The table below shows static size of kernel configured as "default" and "small". The difference of them means overall impact of config items listed in the table "Impact of config items" above. Please see "Examination detail" about detail of "default" and "small" configurations.

Kernel static size[BYTE]
kernel source configuration vmlinux bzImage $(size vmlinux)
text data bss
vanilla "default" 5267762 2019678 3606842 561724 186972
"small" 2190346 903209 1547095 159976 64276
tiny "default" 5267871 2014684 3603061 562236 185788
"small" 2112179 863484 1478058 159836 57844

And the table below shows RAM usage of kernel configured as "default" and "small(+)". Please see "Examination detail" about detail of "default" and "small(+)" configurations.

RAM usage [BYTE]
kernel source configuration used free total
vanilla "default" 3829760 124260352 128090112
"small(+)" 2920448 127713280 130633728
tiny "default" 3977216 124112896 128090112
"small(+)" 3022848 127692800 130715648

Examination Detail


kernel + Linux-Tiny patches
arch i386
platform for RAM usage measurement qemu 0.6.2 (mem=128MB)(on Pentium IV 3.4GHz, mem=1.0GB)
gcc 3.3.5
binutils 2.15

Examination Method

Impact Examination

The examination was basically done by using Kconfig Size tool (see Kernel Config Weight), but partially by manual manipulation because of some limitation of the tool. The overview of method to measure impact is :

  1. Make a default .config file by "make defconfig".
  2. Change all "=m" in the .config file into "=y".
  3. Enable (which means to set "=y") all boolean or tristate examination target config items.This .config file is now called "base config file".
  4. Make "base kernel" from the base config file.And then, measure size of the base kernel (S0) and RAM usage (M0).
  5. Make examination kernel for each target item.
    1. For boolean or tristate config item, make .config file to disable (set "=n") the item, and make kernel from the .config file.
    2. For item which has numerical value (ie. int), make .config file to set the item as a certain value, and make kernel from the .config file.
  6. Then measure size (Sx) and RAM usage (Mx) of the made kernel.
  7. Impact of the config item is calculated as :Impact for size : Sx - X0 Impact for RAM usage : Mx - M0

The base config file used for this examination is [[[Media:]] here(].

Overall size/RAM Usage

The method to measure overall size and RAM usage is very simple, which means making .config files and measure. We used 2 kinds of kernel-sources, and 3 configurations as below.

Kinds of kernel-sources
name description
vanilla Source code downloaded from
tiny Source code which is applied Linux-Tiny patches on vanilla

Kinds of kernel configurations
config name description .config file
default Mostly same as defconfig, but modified little bit as below. (1) Change all "=m" into "=y" to measure size of vmlinux/bzImage. (2) Enable CONFIG_SERIAL_8250_CONSOLE because our platform for RAM usage measurement uses serial console. Media: Media:
small Based on "default" configuration, but set config items listed in "Impact of config items" to make kernel size smaller.This configuration is used only for size measurement because it is not bootable for our platform for RAM usage measurement. Media: Media:
small(+) Modify "small" configuration as to enable CONFIG_PRINTK and CONFIG_IDE of "small" configuration.The reason why to modify is our RAM measurement method uses PRINTK and our platform for RAM measurement uses IDE as a root device.This configuration is used only for RAM usage measurement. Media: Media:

Note : About RAM Usage Measurement

The definition of "RAM usage" here is amount of RAM which BUDDY system recognizes as in-use just after boot of kernel ( = just before execution of userland ). It should be "just-for-reference" because reproducibility error is known from our experiences.


Appearing below are "Tips" to help one who tries to examine moreover.

  • CONFIG_XATTR When XATTR=n, EXT3_FS_XATTR=y causes compilation failure. ( EXT3_FS_XATTR=y in defconfig )
  • CONFIG_FILE_LOCKING When FILE_LOKING=n, NFSD=y causes compilation failure. ( NFSD=y in defconfig )
  • CONFIG_LOG_BUF_SHIFT LOG_BUF_SHIFT can't be set unless DEBUG_KERNEL=y because "if DEBUG_KERNEL" in lib/Kconfig. Please make sure to disable below items to make kernel compact, which are "depends on DEBUG_KERNEL" and "default y".
  • CONFIG_PCI Disable ACPI in advance if you want to disable PCI because ACPI selects PCI.
  • CONFIG_PRINTK syslogd needs PRINTK=y. You shouldn't launch syslogd on kernel configured as PRINTK=n.