Re: ramfs: mounting ramfs as / at boot time

From: Werner Almesberger (
Date: Thu May 25 2000 - 18:32:17 EST

Dave Cinege wrote:
> It happened anyway.... : P DOOOOHHHH! <slamming head on desk>

;-)) Well, you know the alternative: continue with the "pull everything
into the kernel" approach and special-case ramfs handling in mount_root.

> > - you load a small initrd, and boot as usual, with or without /linuxrc
> > - your initrd environment creates and populates the ramfs (*)
> That's not going to work very well in an embedded system with only 8MB of ram
> and a (possibliy) unmountable boot device, OK?

My experience with the Psion S5 suggests that there's actually quite
a lot you can do with 8 MB ...

Maybe you're concerned that you won't have space for initrd+initrd.gz+
ramfs+ramfs.tar.gz. Fortunately, you can free things quite
aggressively, if you wish. E.g. you could free pages from /dev/initrd
as soon as they're read. (*)

And the initrd that populates ramfs doesn't need to contain a regular
execution environment with libc and such. The only significant
redundancy/overhead here is the empty FS structure plus the gunzip

(*) /dev/initrd is mainly designed for debugging purposes (and most
    people probably don't even know it exists ;-), but it may still
    be a good idea to keep it compatible with existing behaviour,
    e.g. by enabling that free-after-reading mode only if the kernel
    is actually reading from initrd. As a side-effect, such a change
    also improves memory usage of initrd in all other cases.

Assuming you've added that free-after-reading extension, let's have a
quick look at the memory usage:

before boot (on disk/flash) kernel.gz+initrd.gz+ramfs.tar.gz
kernel uncompresses itself kernel+kernel.gz+initrd.gz+ramfs.tar.gz
kernel starts kernel+data+initrd.gz+ramfs.tar.gz
kernel builds initrd, approx. kernel+data+initrd*X+initrd.gz*(1-X)+
initrd runs kernel+data+initrd+pgm+ramfs.tar.gz
populating ramfs, approx. kernel+data+initrd+pgm+ramfs*X+
pivot_root to ramfs kernel+data+initrd+pgm+ramfs
if we clean up now kernel+data+pgm+ramfs
exec init kernel+data+init...+ramfs

kernel(.gz) = regular kernel, without initrd-arch extensions
initrd(.gz) = the tiny environment that populates ramfs
ramfs(.tar.gz) = the actual ramfs content. ramfs.tar.gz is appended to
pgm = program that populates ramfs
X = fraction of work done, 0 <= X <= 1
init... = init and everything it launches

- Werner

 / Werner Almesberger, ICA, EPFL, CH /

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