[Xen-users] Xen 4.10 pvhv2 xl info total memory - free memory not equal to sum of VM memory on host.

[Peter <xen@xxxxxxxxxxxxxxxxxx>]

Hi.

Just started using Xen 4.10 with PVH (was using 4.9 with PV).

I start up a Linux domU VM with a params file listing memory = $mb

Not using ballooning.

The amount of free memory reported by xl free after the change is less 
than the memory reported by xl free before the change minus the amount 
in the memory params setting.
i.e. each VM reduces xl info reported free memory by more than the 
amount listed in the params file.
Moreover the amount of extra memory is not constant per VM, but rather 
it varies.  And I am unclear what the variance is due to.
I would like to be able to calculate from the domU configs how much 
memory is going to be used on the dom0.  Else we can end up with some 
hosts trying to allocate more memory than they have.
Is this a bug?  Is there a way to calculate/estimate what that 
discrepancy should be?
Sample discrepancies:

dom0 mem pre domu start: 86597MB; dom0 free mem post domu start: 80926; 
domU memory: 5625;  discrepancy: 46
dom0 mem pre domu start: 80926MB; dom0 free mem post domu start: 74600; 
domU memory: 6272;  discrepancy: 54
dom0 mem pre domu start: 74600MB; dom0 free mem post domu start: 66533; 
domU memory: 8000;  discrepancy: 67
dom0 mem pre domu start: 66533MB; dom0 free mem post domu start: 58466; 
domU memory: 8000;  discrepancy: 67
dom0 mem pre domu start: 58466MB; dom0 free mem post domu start: 50370; 
domU memory: 8031;  discrepancy: 65
dom0 mem pre domu start: 50370MB; dom0 free mem post domu start: 36081; 
domU memory: 14176;  discrepancy: 113
dom0 mem pre domu start: 36081MB; dom0 free mem post domu start: 20249; 
domU memory: 15706;  discrepancy: 126
Different host showing total differences:

# xl info | egrep 'total_memory|free_memory'a
total_memory           : 49143
free_memory            : 28835

Difference is 20308

# xl list | awk '{print  $2 " " $3}'
ID Mem
0 2048
1 1899
2 3464
3 4095
4 8115

Total is 19621

# xl list --long | grep shadow
                "shadow_memkb": 15200,
                "shadow_memkb": 29768,
                "shadow_memkb": 36864,
                "shadow_memkb": 66976,
Total is 145MB



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