Re: Fwd: [Xen-users] Why limit dom0's memory?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > So, it appears that the dom0 basically just consumes all memory not
> > used by domUs, and will release that memory when it is needed by a new
> > guest.  If that is true, then I don't see the advantage of forcing the
> > dom0 to a hard memory limit, but there must be one, since the option
> > is there and people are using it (or so google tells me.)
> >
> > Do I have this wrong, or am I not understanding something?
>
> AFAIK what some people is doing is setting a level of memory for Dom0
> for it to never go down that level (giving memory to the guests). That
> is, if you set the memory to 1 GB, Dom0 will allocate all your memory
> except for that 1 GB. It's a way of ensuring that Dom0 will never run
> out of memory.

More information / clarification:

The dom0_mem parameter on the Xen command line is used to set how much memory 
dom0 is allocated when the system starts.  This is instead of it being given 
all free memory in the system.

By default, dom0 shrinks itself as required to free up space for domUs that 
the administrator starts.  Setting dom0_mem does not stop dom0 from shrinking 
itself below the size in dom0_mem later on in order to accommodate guests, 
for instance if you start more than anticipated.  All it means is that dom0 
may not have to immediately shrink to make space for guests, since its not 
using all the memory to start with.

The parameter dom0-min-mem in /etc/xen/xend-config.sxp is used to set the 
smallest size dom0 will shrink itself to in order to accommodate domUs.  
Setting it to 0 disables auto-ballooning so that dom0 won't shrink itself 
automatically at all.

A number of people have had problems with the autoballooning behaviour, since 
in some cases in the past ballooning dom0 down significantly has caused 
instability.  Starting dom0 up at the right size to begin with is, for 
various reasons, *not* equivalent to starting it up with all memory and 
resizing it later.  Heavy resizing confused Linux sometimes and made things 
break.  Setting dom0_mem and disabling autoballooning eliminated instability 
for some in these situations.

I'm not clear whether such instabilities are so common now but I still 
recommend that if you know what size you want dom0 to be you may as well set 
dom0_mem leave it there!

Cheers,
Mark


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