Re: [Qemu-devel] Virtual memory question

[Stefan Hajnoczi]

On Mon, Aug 9, 2010 at 7:49 PM, Dennis <address@hidden> wrote:
> I have an application that takes up over 2 GB of memory but is
> otherwise demanding virtually no other resources at all.
> Running multiple instances of the application using physical memory
> isn't an option. Would it be hard to change Qemu's
> physical memory mapping to a file on disk ? (Not Qemu-KVM, just stock Qemu).

It's not 100% clear what you are trying to do, but are you aware that
guest "physical" memory can be swapped on the host?  If you put two
VMs on one host which use more memory than available host RAM, some of
their pages will get pushed out to swap, like any normal userspace
process.  Two things to consider in a memory overcommit scenario are
reduced performance due to swapping and allocating enough swap so the
out-of-memory killer does not terminate the QEMU process.

Also, have you looked at Kernel Samepage Merging (KSM)?  KSM detects
guest physical pages that contain the same contents and merge them
into a single page, saving memory.  This is appropriate if you run
multiple VMs based off the same disk image and expect they will be
executing the same code.  For more info, see
http://kernelnewbies.org/Linux_2_6_32#head-d3f32e41df508090810388a57efce73f52660ccb.

Stefan