Re: Page-aligned executables (Was re: [9fans] Non-stack-based calling conventions)

["ron minnich" <rminnich@gmail.com>]

On Feb 18, 2008 8:53 AM, Joel C. Salomon <joelcsalomon@gmail.com> wrote:

> Ron mentioned that tidbit in his IWP9 talk about booting Plan 9 under
> lguest.  Lguest (or was it another virtualizer?) uses mmap so it can
> load any arbitrary number of client kernels; 9l-produced ELF files
> break that model.

And with the execute in place support (XIP) it gets more interesting,
and worth understanding, for reasons I'll mention below.

IF you have an ext2-based RAM file system, and XIP is compiled in,
then you can do the following: instead of mmap'ing the file, ext2
essentially says: "here's a pointer to the data you wanted, it's in
memory, just use it, don't page it in because that would be a copy
from memory to memory".

Why's it matter? First, boot time speed. A talk I saw on a Linux-based
car computer stated that their boot time goal was one second; Linux
was only allowed a very small fraction of this time to be ready (200
milliseconds IIRC); XIP made a difference between meeting the time
goals and not meeting them. Second, memory: on a 32 MB node, you
really don't want two copies of every binary.

So it's interesting to think about whether Plan 9 could boot a node in
200 ms on a StrongARM (500 mhz? I think so) or not. But I think XIP on
a small RAM disk would not be impossible.
ron