Re: [Xen-users] Purpose of mem-max command

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Bear in mind that I'm not even an advanced user ...


On 11 Oct 2015, at 11:45, Marco Guazzone <marco.guazzone@xxxxxxxxx> wrote:

> What I don't understand is how to use it effectively.
> Could you provide a practical example of how to use it?

I don't know if it works on an already running VM - I've only used it in VM 
config files. For example, this is from one of my VM configs :
> memory  = '512'
> maxmem  = '2048'

This starts the VM with 512M of memory, but it can be increased to 2G using 
mem-set from the command line. Just starting it with 512M (without the maxmem 
line) means it's configured for 512M max and it can't be increased past that.

AIUI, when a machine (whether virtual or bare metal) starts up, certain memory 
structures (kernal tables) are setup based on the amount of memory installed. I 
assume that this constrains the machine from dynamically adding memory above 
what was present at startup - or in this context, configured by the maxmem 
argument to Xen. I know that in the past there has been some discussion around 
the configuration of Dom0 - and that there is a small memory saving from 
specifying it's max size at boot time as it then allocates these tables 
according to the smaller ram size configured instead of the full machine memory 
which it's never going to use.


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