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SubjectRe: Limited RAM - how to save it?
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Jan-Benedict Glaw <jbglaw@microdata-pos.de> writes:
> I'm working on a 4MB linux system (for a customer) which has quite
> limited resources [...] If you've got further ideas on getting the
> kernel a bit smaller, would be nice to get a mail dropped...

I started a port of Linux 2.3.99 to a MIPS device which usually has 2M
flash and 4M RAM.

To reduce the size of the text and data sections, I sorted the output
of the "size" command and investigated the files with the largest
sections.

These are the memory-saving changes I arrived at. They are not really
tested, so some of them may break some functionality. Also, I don't
know whether they apply to the current kernels.

fs/dcache.c

Changed HASH_BITS from 14 to 8. This reduces the size of the
cache from 128K to 2K.

fs/inode.c

Changed HASH_BITS from 14 to 8. This reduces the size of the
cache from 128K to 2K.

include/linux/blk.h

Changed NR_REQUEST from 256 to 16. This reduces the number of
requests that can be queued. The size of the queue is reduced
from 16K to 1K.

include/linux/major.h

Changed MAX_BLKDEV and MAX_CHRDEV from 256 to 10 and 20,
respectively. This reduces the number of block and character
devices and saves about 40K.

kernel/printk.c

Changed LOG_BUF_LEN from 16384 bytes to 2048 bytes.

include/linux/tty.h

Changed NR_PTYS and NR_LDISCS from 256 and 16, respectively,
to 16 and 4, respectively. Saved about 12K.

Warning: this change may break the pty driver, in which case
further modifications will have to be done to
drivers/char/pty.c.

kernel/panic.c

Changed a buffer from 1024 bytes to 200 bytes.

include/linux/sched.h

Changed PIDHASH_SZ from 1024 to 16, which saves 1008 bytes.

include/linux/mmzone.h

NR_GPFINDEX from 0x100 to 0x10. Saves 4800 bytes, but I'm not
sure it doesn't break anything.

net/Makefile, net/socket.c, net/nosocket.c

Replacing socket.c with nosocket.c, a file containing dummy
replacement functions for those in socket.c, saves about 24K.

Warning: this disables the socket API entirely, but it is
currently not used in the product.

mm/Makefile, mm/swapfile.c, mm/swap_state.c, mm/noswapfile.c, mm/noswap_state.c

Replacing swapfile.c with noswapfile.c, and swap_state.c with
noswap_state.c saves about 12K. The no*.c files contains
empty replacement functions.

Warning: this disables swapping of anonymous memory, which
isn't used in the product. But note that demand paging of
executables still works.

mm/Makefile, mm/mmap.c

The functions in mmap.c could probably also be replaced by
empty functions. Estimated saving: 9K (not included in the
grand total below).

*, CONFIG_MESSAGES

Applying the CONFIG_MESSAGES patch and disabling all kernel
messages saves about 80K.

The CONFIG_MESSAGES patch was written by Graham Stoney
<greyham@research.canon.com.au>.

With all of the above, and only this enabled in .config:
CONFIG_EXPERIMENTAL=y
CONFIG_CPU_R3000=y
CONFIG_CPU_LITTLE_ENDIAN=y
CONFIG_ELF_KERNEL=y
CONFIG_BINFMT_ELF=y
CONFIG_MODULES=y
CONFIG_MODVERSIONS=y
CONFIG_KMOD=y
CONFIG_CROSSCOMPILE=y
, the kernel is down to about 550K.

Here is the output of "size vmlinux". I think this is without the
CONFIG_MESSAGES patch (it was long since I worked with this).

text data bss dec hex filename
572128 41964 15860 629952 99cc0 vmlinux

--
Lars Brinkhoff http://lars.nocrew.org/ Linux, GCC, PDP-10
Brinkhoff Consulting http://www.brinkhoff.se/ programming
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