What's the Point of Using Busybox in a Low Ram Embedded System

What's the point of using Busybox in a low ram embedded system

TL-DR; Linux has a huge infrastructure and variety of rootfs or boot file systems available. The choice is due to accommodation of different system constraints and end user functions. Busybox is a good choice for the target system, but any software can be misused if a system engineer doesn't spend time to understand it.

Is my platform an use case for Busybox?

It is if you take time to minimize the kernel size and busybox itself. It is unlikely you need all features in your current busybox.

if not, is there anything to conveniently build linux system utilities each on their own executable?

See klibc information below. You can also build dash with musl, with buildroot and busybox. Many filesystem builders support shared libraries or static binaries. However, there are many goals such as package management, and live updates, that a filesystem builder may target.

More Details

You can configure features out of busybox. The idea is that all of the configured features are needed. Therefore you need them all in memory. With busybox, ls, mkdir, printf, etc are all the same binary. So if you run a shell script the one code load is all code loads. The other way, you have many separate binaries and each will take extra memory. You need to minimize Linux to get more RAM and you can take features out of busybox to make it smaller. Busybox is like a giant shared library; or more accurately a shared process. All code pages are the same.

a custom Cortex-M7 board with 16 Mb of SDRAM and 64 Mb of Flash

...

one and only busybox executable (650 Kb on my system)

Obviously 650KB is far less than 16MB. You don't say what the other RAM is used for. For another good alternative look at the klibc toolsuite. What is not clear is whether the FLASH is NAND/NOR and if you have XIP enabled. Generally, busybox would be better with XIP flash and klibc would be better (and more limited) for SDRAM only, with some filesystem in flash.

See: Memory used by relocatable code, PIC, and static linking in the Busybox FAQ. It is designed to run from Read-only memory which can be a big gain depending on system structure. It probably provides a more rich feature set than klibc as the goal with that project is just to boot some other mount device (a hard drive, SSD etc).

Klibc does not have as much documentation as busybox. It can be either a shared library or statically linked. Each binary will only use the RAM needed for that task with static linking, but this will take more flash space. The binary with klibc are,


 1. dash    2. chroot     3. dd      4.  dmesg  5.  mkdir  6.  mkfifo
 7. mknode  8. pivot_root 9. unmount 10. true   11. false  12. sleep
 13. ln    14. ls        15. mv      16. nuke   17. minips 18. cat
 19. uname 20. halt      21. kill    22. cpio   23. sync   24. readlink
 25. gzip  26. losetup

and that is IT! No networking, no media players, etc. You can write code to use klibc, but it is a very constrained library and may not have features that you require. Generally it would be limit to disk only tasks. It is great to probe USB for external device to boot from for example.

Busybox can do a lot more. Most klibc static binaries will be under 100kB; with 10-30kB typical. Dash and gzip are larger. However, I think you need to remove configuration items from your kernel as 650KB << 16MB and busybox would be a fine choice for this system even without XIP.

I should also be noted that Linux does 'demand page loading' for code with an MMU system. Even if you don't have swap, code can be kicked out of RAM and reloaded later with a page fault. Your system is no-MMU, so busybox will not perform as well in this case. With an mmu and 'demand page loading' it will do much better.

For severe constraints, you can always code a completely library free binary. This avoids libgcc startup and support infrastructure which you might not need. Generally, this is only good to test a kernel vs. initrd issue and for script/binary that must run in many different library environments.

is it recommended to use SPI flash to run code instead internal flash due to memory limitation of internal flash?

Generally speaking it's not recommended, or differently put: the closer to the CPU the better. Both S25LP064A and LPC546xx however support XIP, so it is viable.

This is not a trivial issue as many aspects are affecting. I.e. issue is best avoided and should really have been ironed out in the planning stage. Embedded Systems are more about compromising than anything and making the right/better choices takes skill end experience.

Same question with replies on the NXP forum: link

512K of NVRAM is huge. There are almost certainly room for optimisations even if 3'rd party libraries are used.

On a related note this discussion concerning XIP should give valuable insight: link.

I would strongly encourage use of file-systems if not done already, for which external storage is much better suited. The further from the computational unit, the more relevant. That's not XIP and the penalty is copy-to-RAM either way you do it. I.e. performance will be slower. But in my experience, the need for speed has often-times not been thoroughly considered and at least partially greatly overestimated.

Regarding your mentioning of RTOS and FW-upgrade:

Unless it's a poor RTOS there's file-system awareness built in. Especially for FW upgrading (Note: you'll need room for 3 images, factory reset included), unless already supported by the SoC-vendor by some other means (OTA), it will make life much easier and less risky. If there's no FS-awareness, it can be added.
FW upgrade requires a lot of extra storage. More if simpler. Simpler is however also safer which especially for FW upgrades matters hugely. In the simplest case (binary flat image), you'll need at least twice the amount of memory you're already consuming.

All-in-all: I think the direction you're going is viable and depending on the actual situation perhaps your only choice.

Installation of Busybox on Pandaboard

The precmd error seems to be due to the prompt. Try resetting the prompt, by doing:

$ unset PS1 PROMPT_COMMAND

nWipe Package compiled in Centos not working in Busybox embedded linux

I have managed to fix this by not using busybox. This time around I have used Centos minimal install and used dracut utility to create kernel and initramfs and mounted the root file system as NFS on the server. It works like a charm.

It works like a full blown Linux Centos and it is pretty fast.

Thank you for looking at this post :)

What's the Point of Using Busybox in a Low Ram Embedded System