Re: [Xen-devel] [RFC] new totalmem= boot parameter

[Sarina Canelake <sarina.canelake@xxxxxxxxxx>]

On Mon, Jul 19, 2010 at 07:11:19PM +0100, Keir Fraser wrote:
> On 19/07/2010 18:56, "Sarina Canelake" <sarina.canelake@xxxxxxxxxx> wrote:
> 
> > We have a need for ensuring the total RAM available to [Xen / the kernel] at
> > boot is X MB because there are situations in which you wish to limit the
> > amount of RAM available to a box. The existing mem= option doesn't work
> > because it limits the maximum physical address, NOT the amount of available
> > RAM. Many, if not all, systems contain a substantial memory hole below 4 Gb,
> > typically a 0.5 or 1 Gb hole from 3-4 Gb. Thus, on a system with 6 Gb of 
> > RAM,
> > requesting mem=4096M will yield a box with maximum physical address in the 4
> > Gb neighborhood but perhaps only 3 or 3.5 actual gigs of RAM available.
> 
> It doesn't sound *very* useful. But then neither is mem= really. We can add
> something like this if you really need it. So what's the motivation?
> 

I found it useful while I was testing various core dumping capabilities.
Using a boot-time argument to limit memory eliminates the need for pulling 
out DIMMs (which I couldn't do anyways, as the machines I was working
on are remote). However mem= didn't suffice for this purpose
beyond 3 Gb since, as I mentioned, it limits the physical address 
rather than the amount of RAM, which is what I thought it was 
supposed to do. Hence the implementation of totalmem=, which made my 
16Gb+ boxes capable of imitating various, specific smaller configurations.

Alternatively, if mem= isn't used very frequently, perhaps it wouldn't 
be a bad idea to simply update the functionality of mem= to limit the 
total memory rather than the physical address.

Sarina

> 
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