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


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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