On May 11, 2005, at 6:52 PM, Angela Kahealani wrote:
On Wed, 2005-05-11 14:37, Charles Lockhart wrote:
Sorry, I meant to say with no swap space at all, not just without a
swap partition.
Many embeded Linux distros use only real RAM, not virtual RAM,
you are perhaps referring to 'uClinux' .vs linux. The former being
'linux for systems
without MMUs', though even thats not true any more, given that most of
'uClinux' is now
part of the 'normal' kernel tree.
(Nearly) the only reason to turn off support for the MMU is that you
don't have one. Most modern
SoCs implement a MMU.
Also, note that swapping != virtual memory. Swaping was part of the
original Unix design, long before
the requisite hardware support showed up to enable 'virtual memory'.
Back then, if you were running
out of memory, the kernel would swap out entire processes to the swap
area. These days the kernel
will first try to put cold dirty pages into the swap area. See
previous message.
and have no swap... you may have to recompile or even recode some
applications to not expect virtual memory.
Most applications can't tell the difference.
http://www.uclinux.org/pub/uClinux/FAQ.shtml
Q. Does uClinux support multitasking? What limitations are imposed
by not
having a MMU?
A. uClinux absolutely DOES support multi-tasking, although there
are a
few things that you must keep in mind when designing programs...
1. uClinux does not implement fork(); instead it implements
vfork().
This does not mean no multitasking, it simply means that the
parent blocks until the child does exec() or exit(). You
can still
get full multitasking.
note that most uClinux systems 'emulate' fork() as vfork().
2. uClinux does not have autogrow stack and no brk(). You need
to use
mmap() to allocate memory (which most modern code already
does).
There is a compile time option to set the stack size of a
program.
note here that there is a trivial malloc() library that uses mmap().
3. There is no memory protection. Any program can crash another
program or the kernel. This is not a problem as long as you
are
aware of it, and design your code carefully.
4. Some architectures have various code size models, depending
on how
position independance is achieved.
See also:
http://www.cyberguard.info/snapgear/tb20020530.html
