[Xen-users] understanding dom0 usage memory

[Claudinei Matos <claudineimatos@xxxxxxxxxxxxxxxxx>]

Hi guys,

First of all, sorry for my poor english.

This is my first message at this list. I'm using Xen on several machines 
since last November.
Right now I'm having a specific problems with 2 servers: both are losing 
the network on dom0 but all guest systems are still running fine.
Looking at those dom0 logs I've discovered that sometimes linux kernel 
OOM-Killer tries to kill some important applications like sshd for e.g.
Using ps, top, pmap and free commands I can see that dom0 will try to 
eat as much memory as there is available, but I can't see all the 
process using this memory, even if I do sum the virtual memory size 
(VSZ) of each process, which is supposed to be as big as the total 
machine (dom0) memory AFAIK.
For example, I have 4 AMD64 systems running Gentoo Linux with Xen 3.1.1 
and linux sources 2.6.20-r6.
Two of those machines have 512MB for dom0 and the others have 430MB.
Those hypervisors are eating around 400MB to 500MB (from `free -m` 
'used' output) each one, and the only process I'm running on them are 
mdadm (for raid monitor) ntpd and sshd.
If I sum those machines' processes, I can see between 130 to 200 MB of 
VSZ, so where's the memory not being used? If I do sum RSS (resident 
memory) plus SHR (shared memory) for all process I'll get somethiing 
between 80MB to 120MB being used.
On a test machine with only 128MB available to dom0 running the same 
processes I can see around 117MB in use.
On other machines without Xen I can sum all process RSS plus SHR and 
I'll get almost the same value from `free -m` output.
My question is if the xen kernel part do try to allocate memory which I 
can't see on ps/top utils or if I maybe have a problem with my machines 
setup?
Another question is if it's OK to just allocate 128MB to dom0? As I said 
the only machine with 128MB is a test machine which is not used this much.
Thanks for any advice,

Claudinei Matos

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