[Xen-devel] Free memory reported but not used in new domain creation?

["G.R." <firemeteor@xxxxxxxxxxxxxxxxxxxxx>]

Hi guys,
I need your help understanding this behavior:

My host has 8GB RAM installed. Xen reports about 1.8G free memory with
three domains running.

xl info|grep memory
total_memory           : 7887
free_memory            : 1833

xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   768     8     r-----     650.2
nas                                          1  3072     2     -b----     704.0
windom                                       3  2047     4     r-----     520.7

Now, when I tried to launch a forth domain with 1G memory assigned,
here is the result:

xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   380     8     r-----     654.1
nas                                          1  3072     2     -b----     706.7
windom                                       3  2047     4     r-----     529.8
freebsd                                      5  1024     1     -b----       0.7

xl info|grep memory
total_memory           : 7887
free_memory            : 1184

As you can see, hypervisor is ballooning memory from dom0 instead of
allocating from free_memory. If I increase the guest memory of the new
domain to 1.5G or 2G. It simply fails to launch, saying something like
dom0 memory is below minimal threshold.

This gives me the impression that the free_memory is fake. Is it an
expected behavior?
What are the remaining 1GB memory for?

PS: I'm on the xen 4.3.0 release.

Thanks,
Timothy

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