Re: [uml-devel] memory

[Rob Landley <rob@landley.net>]

On Monday 24 January 2005 03:13 am, Vaibhav  Sharma, Noida wrote:
> According to the design of UML, it has its own VM system. If I have to
> check the scalability limits of the "parent kernel" for memory management,
> will UML be useful..?

Jeff Dike already answered this.  (He's the UML maintainer.)  UML _does_ remap 
a different chunk out of the physical memory backing file when it schedules a 
different process.  UML uses mmap to get its memory, so as long as the parent 
kernel has Large File Support (which lets a 32 bit machine use 64 bit file 
offsets, this is pretty widely deployed because people need a single file to 
be larger than INT_MAX, which is 2 gigabytes since file offsets are signed 
for some reason), and the UML instance is configured to use highmem (which is 
what unmaps and remaps the memory on a schedule) and at least 3 level page 
tables (so it doesn't run out of PTEs), UML shouldn't be limited to 32 bits 
of (simulated) physical memory.  Not that I've tried this. :)

An individual process running under a 32-bit UML would still only be able to 
access 4 gigabytes or less.  (It has a 32 bit virtual address space, after 
all...)  And since I'm unaware of any version of UML using the 4 gig/4 gig 
patch, I suspect UML might fill up its kernel memory area with page table 
entries eventually...  (Was Dave Jones's shared page table patch ever 
integrated?  Are linux page tables swappable?)

> When I debug the UML kernel when a process requests a large amount of
> memory, I will only get the flow used by UML for its VM. Is the code/design
> of UML very different from the parent kernel..?

UML uses mmap on a file to simulate an MMU.  It shouldn't care what kind of 
physical memory the parent kernel has.  But Jeff's the expert here.

However running 'find arch/um -name "*.c" | xargs grep mmap' in your linux 
source directory should get you started.

> (I may be totally wrong in this mail, please do correct me...)

I'm not the world's greatest source of info here either, I've only been 
playing with UML for a little while...

Rob


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