[uClinux-dev] Minimum size of uCLinux and J9 compatibility

Jamie Lokier jamie at shareable.org
Tue Feb 3 06:32:48 EST 2009

Chris Gray wrote:
> On Monday 02 February 2009 22:01:12 Jamie Lokier wrote:
> > I didn't use Java because I thought it wouldn't fit, to be honest.
> >
> > There's about 10MB free on my 64MB device (32 allocated to video
> > coprocessors, away from Linux; the rest is used by Linux, utils etc.)
> > I found that's actually not enough when streaming from hard disk -
> > because Linux's page allocator can't handle it, playback struggles.
> >
> > So I thought adding Java would make it worse.
> It probably would have: you can certainly run Java applications in less than 
> 10 MB (or even 5 MB), but you need to keep them pretty simple (and when the 
> memory consumption starts to grow it can be hard to put your finger on just 
> why). OTOH I know people who are doing pretty complex stuff (including genetic 
> algorithms, would you believe) in something like 18 MB.

That's good to know.  My customer wants to use Java, I can use this to
say "no, bad idea!" :-)

Even if it fits in 5MB, the trouble is the system needs 10MB _free_ to
stream from hard disk reliably.  Linux 2.4 with the "page_alloc2"
fragmentation-avoidance allocator cannot release page-cache due to
streaming I/O smoothly, and with "page_alloc" (the normal Linux
allocator), it's smooth but quickly fragments so much that launching
new apps, such as shell scripts or telnet shells stops working.

> There are some aspects of Java which make it hard to keep the
> memory consumption real down low - the reflection data for example
> (all those strings!) and the frustrating lack of modularity in the
> class libraries. Yes you can compile stuff AOT and strip out unused
> methods and reflection data - but be careful because the Java
> libraries themselves use reflection quite a lot. The good news is
> that once you have a certain critical mass of commonly- used core
> classes from java.lang, java.io, java.net and java.util loaded the
> memory consumed by class libraries starts to stabilise a bit -
> provided of course that you don't start dragging in every cute
> open-source library you see on the web.

And of course, no MMU means no swapping in just those parts of the
libraries which are used, so you really need enough RAM.

-- Jamie

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