Limit dom0 memory to 4GB

[marmarek]

Apparently Linux kernel reserves some internal memory structures based on maximum memory, not the actual one. This is waste of resources, especially on big systems, because normally most of memory is assigned to AppVMs.
This is also along the lines of Xen best practices.

Related to #1136

[qubesuser]

Sounds good, although in addition to 4GB, 8GB and 16GB also seem reasonable candidates for the value to use.

Longer term, it would be nice to get memory hotplugging working for both dom0 and VMs in a way that results in not wasting memory for page structures for unassigned RAM and ballooning instantly, and then maybe remove the dom0 limit again, but I'm not sure if that's feasible.

[qubesuser]

Sounds good, although in addition to 4GB, 8GB and 16GB also seem reasonable candidates for the value to use.

Longer term, it would be nice to get memory hotplugging working for both dom0 and VMs in a way that results in not wasting memory for page structures for unassigned RAM and ballooning instantly, and then maybe remove the dom0 limit again, but I'm not sure if that's feasible.

[marmarek]

On Sat, Oct 10, 2015 at 02:30:08PM -0700, qubesuser wrote:

Sounds good, although in addition to 4GB, 8GB and 16GB also seem reasonable candidates for the value to use.

The point is you don't need too much memory in dom0 - all the user
applications are running in VMs. Dom0 is only for running X server,
window manager and some management scripts. In the future we'll split
even that (X server) to a separate GUI domain, so dom0 will be even
smaller.

Longer term, it would be nice to get memory hotplugging working for both dom0 and VMs in a way that results in not wasting memory for page structures for unassigned RAM and ballooning instantly, and then maybe remove the dom0 limit again, but I'm not sure if that's feasible.

I think this would be mostly useful for VMs. BTW currently because of
this overhead maxmem cannot be bigger than initial mem (aka "memory")
times 10:
QubesOS/qubes-core-admin@bf21730

So if we get the memory hot plugging working, we probably could get rid
of that limit.

Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

[marmarek]

On Sat, Oct 10, 2015 at 02:30:08PM -0700, qubesuser wrote:

Sounds good, although in addition to 4GB, 8GB and 16GB also seem reasonable candidates for the value to use.

The point is you don't need too much memory in dom0 - all the user
applications are running in VMs. Dom0 is only for running X server,
window manager and some management scripts. In the future we'll split
even that (X server) to a separate GUI domain, so dom0 will be even
smaller.

Longer term, it would be nice to get memory hotplugging working for both dom0 and VMs in a way that results in not wasting memory for page structures for unassigned RAM and ballooning instantly, and then maybe remove the dom0 limit again, but I'm not sure if that's feasible.

I think this would be mostly useful for VMs. BTW currently because of
this overhead maxmem cannot be bigger than initial mem (aka "memory")
times 10:
QubesOS/qubes-core-admin@bf21730

So if we get the memory hot plugging working, we probably could get rid
of that limit.

Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?