Re: [uml-devel] memory

[Jeff Dike <jdike@addtoit.com>]

Vaibhavs@noida.hcltech.com said:
> I have to build a simulator, to check the scalability limits in memory
> management of linux. For this, the simulator  will provide a large
> amount of RAM (in GBs), even if the amout of physical memory is only
> 512MB. 
>
> Can UML be used for this purpose? I had seen the "mem" option in the
> kernel switches option of UML. Can UML simulate such a case? 

Yes, it should be fine for what you want.

> If it
> does, how UML handles this? Specifically, can the UML kernel allocate
> all this amount of memory

It'll allocate a temp file of the appropriate size.  If it's larger than what
it can use for physical memory, the rest will be allocated as highmem.  If
you leave 2-level pagetables on, then you're limited to 4G. If you turn on
3-level pagetables, then you get up to 128G.  

doug@easyco.com said:
> UML runs the client using a single user mode memory block as  the
> entire client's core.  Thus the clients core size is limited to what
> a single task can allocate as plain memory.

No, it only maps the physical memory as a block.  The highmem part is mapped
on demand, as with i386.

rob@landley.net said:
> You could run out of virtual address space, though.  Trying to map
> more than a  few gigabytes on a 32 bit machine is unlikely to work. 

With highmem, it's not all mapped at once.

rob@landley.net said:
> Not  unless it's unmapping and remapping multiple mmaps.  

It is.

> (There's
> large file  support, but trying to mmap a 5 gig chunk out of a large
> file can't work:  

It does it a page at a time.

> what would that mean?  How could you generate an
> offset into the last meg?   

The LFS interfaces take 64-bit offsets.

> What would the pointer _be_?)

What pointer?

> The parent kernel's highmem support still doesn't provide more than 4
> gigabytes per application, and the UML kernel is one application.

The host kernel's highmem support is totally irrelevant.  There doesn't even
need to be highmem support on the host.

				Jeff



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