Re: [Xen-users] Doubt on XEN memory management: please clarify

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Flavio wrote:

> Let's go with my XEN setup now.
> I have 8GB of RAM and I have seen that if I put dom0_mem=1024M 
> option, the dom0
> has 1GB of RAM for itself.
> What does this really mean?
The default is that when you boot, all the system RAM (less a bit 
used by Xen itself) will be 'available' to Dom0. As you start guests, 
then Dom0 will balloon down it's memory availability to free memory 
for the guest. This all works fine, and as long as you never force 
memory down too low then Dom0 won't complain - you can set a minimum 
below which Dom0 will not balloon and it would be a good idea to tune 
this to your requirements.
Now, for a couple of reasons this isn't ideal. The main reason is 
that certain buffers and structures used to keep track of "stuff" are 
sized at boot time based on the amount of memory available. So in 
your case, you start with 8G of RAM but potentially balloon down to 
as little as 1/4G (256M) - just 1/32 of the size. So you now have a 
system running in a small amount of RAM but with buffers and 
structures sized for a lot of RAM (about 32 times what it actually 
has available in this case).
Also, each time you start a guest (or alter a guest's memory 
allocation), then the host's memory configuration must be changed 
which takes a little time.
Lastly, these changes in memory allocation will (or may) affect 
performance in some areas - giving a variable system performance 
depending on how many guests are running and how much RAM they use.
So it's a good idea to specify how much memory to give to Dom0. It 
will then size things to suit (which in itself will reduce it's 
memory footprint). You can still balloon between high and low limits, 
or you can set a fixed allocation and have deterministic performance 
and minimum memory usage.
> This is confusing me, because I don't understand how
> to decide the amount of RAM to assign to the dom0. For example, what 
> if I assign
> 256M? What if 8192M?
8G isn't a good choise - see above about sizing of buffers and structures.

256M may or may not be enough - only you can only determine that for 
your setup, it depends on what is running on the system. If you run 
just the bare minimum of software/services on Dom0 then it may well 
be enough, but if you run other things (particularly memory hungry 
programs) it might not be enough. You need to pick a value (not too 
low, try starting at 512M and work down) and then look at how the 
system is running - look at how much memory is used for different 
things (cat /proc/memory) and particularly if it is swapping (very 
bad for performance).
Normally, you don't tend to do much in Dom0, so not having large 
caches isn't really an issue (it just delays things when stuff has to 
read from disk instead of cache when you issue a command) as long as 
all the stuff that's running "all the time" or "frequently" can stay 
in RAM.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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