Re: [Xen-API] VMs are using more memory than reported and expected

[Brian Menges <bmenges@xxxxxxxxxx>]

Depending on the VM booted, there is memory overhead:

memory-overhead ( RO): 4194304

Here's how I found my memory overhead for the Debian 6.0 (Squeeze) 64-bit image:

[root@xs-01 ~]# xe template-list |grep -i 'squeeze' -B1
uuid ( RO)                : b0f02f5d-d581-0e0a-8f85-40c3a65d660e
          name-label ( RW): Debian Squeeze 6.0 (32-bit)
    name-description ( RW): Template that allows VM installation from Xen-aware 
Debian-based distros. To use this template from the CLI, install your VM using 
vm-install, then set other-config-install-repository to the path to your 
network repository, e.g. http://<server>/<path>
In order to install Debian Squeeze from CD/DVD the multi-arch ISO image is 
required.
--
uuid ( RO)                : 2122992d-d430-e380-625e-d6049cfae018
          name-label ( RW): Debian Squeeze 6.0 (64-bit)
    name-description ( RW): Template that allows VM installation from Xen-aware 
Debian-based distros. To use this template from the CLI, install your VM using 
vm-install, then set other-config-install-repository to the path to your 
network repository, e.g. http://<server>/<path>
In order to install Debian Squeeze from CD/DVD the multi-arch ISO image is 
required.
[root@xs-01 ~]# xe template-param-get uuid=2122992d-d430-e380-625e-d6049cfae018 
param-name=memory-overhead
4194304

As you can see, there's a few megs of overhead for just this VM, then there's 
the ballooning range for dom0 that you have to consider, which I'm not 
positive, but I think it may be 2G and you seem to have taken that into 
account. The overhead range appears to be that last 1gb, and I'm sure other 
things get crammed in there too like your ramdisk perhaps, xen netback buffers, 
video card buffers, etc.

- Brian Menges
Principal Engineer, DevOps
GoGrid | ServePath | ColoServe | UpStream Networks


-----Original Message-----
From: xen-api-bounces@xxxxxxxxxxxxx [mailto:xen-api-bounces@xxxxxxxxxxxxx] On 
Behalf Of Alexandre Kouznetsov
Sent: Wednesday, May 08, 2013 16:11
To: xen-api@xxxxxxxxxxxxx
Subject: [Xen-API] VMs are using more memory than reported and expected

Hello.

I have a host machine that has 32GB RAM and runs XCP 1.6. I expected to be able 
to allocate up to 5 VM's using 6GB of RAM each, but XCP complains saying it 
can't start the last VM because of RAM shortage. It looks like somebody is 
eating more RAM that it should.

My goal is to run 5 equal VM's on this host. As last resource, I may calculate 
the appropriate RAM size to make them all fit (something around 5.8G), but this 
is issue is really unexpected. I hoped to give 30GB to VMs and have up to 2GB 
left for Dom0 and the roundings.


I have inspected the RAM usage with "xl list" and "xentop" commands. As you can 
see, I had to set one VM to use 5GB instead of 6GB of RAM.
(hope the mail does not breaks the formatting too much)

"xl list" output:
Name                      ID   Mem VCPUs     State    Time(s)
Domain-0                   0   741     4     r-----  1394433.5
VM1                        1  6144     4     r-----  592272.6
VM2                        2  6144     1     -b----   43681.0
VM3                        3  6144     1     -b----    6635.9
VM4                        4  6144     4     r-----   46929.5
VM5                        5  5120     4     -b----   29723.3

Xentop's output is rather large, but this are the numbers:
Mem: 33507896k total, 32194356k used, 1313540k free
NAME      MEM(k)
Domain-0  759040
VM1      6291456
VM2      6291456
VM3      6291456
VM4      6291456
VM5      5242880

So, If I sum VM's memory consumption one by one (6 machines including Dom0), it 
gives me 31167744, while xentop reports "32194356k used", almost 1GB is missing.
xentop says I have "1313540k free", but if I set all 5 VMs to 6GB, the last one 
can't start.

Please advice, what am I missing?
How came that the running domains have all less than 30GB assigned, but I'm 
unable to assign the other 2GB?
Why can't I at least allocate the "free" gigabyte that xentop reports?

Thank you.

--
Alexandre Kouznetsov


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