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

[Eike Waldt <waldt@xxxxxxxxxxxxx>]

On 2/14/19 11:31 PM, Peter wrote:
> This continues to be an issue on 4.11.  Trying to figure out how to
> calculate the VM size that will be supported on a host.
I am also aware of the issue... HVM domUs just "need more memory" from
the hypervisor.
There is some mempry overhead somewhere.

A rule of thumb I calculate in my environment is +3%.
That works out even for larger DomUs with e.g. 512 GB of RAM.


> 
> local dom0_free_mb=$(xl info | egrep "free_memory" | awk '{print $3}')
> 
> # start vm
> 
>   local dom0_free_after_start_mb="$(xl info | egrep "free_memory" | awk
> '{print $3}')"
>   local domU_mem_after_start_mb=$(sed  '/^ *memory *= *[0-9]*/!d;s/^
> *memory *= *//g' paramsfile)
>   mkdir -p /tmp
>   echo "dom0 mem pre change: ${dom0_free_mb:-0}MB; dom0 free mem post
> change: ${dom0_free_after_start_mb:-0}; domU memory:
> ${domU_mem_after_start_mb};  discrepancy: $((${dom0_free_mb:-0} -
> ${dom0_free_after_start_mb:-0} - ${domU_mem_after_start_mb:-0}))" | tee
> -a /tmp/memdiscrepancy.log
> 
> dom0 mem pre change: 125264MB; dom0 free mem post change: 283; domU
> memory: 124000;  discrepancy: 981
> 
> # xl info | grep xen_version
> xen_version            : 4.11.0
> 
> Regards, Peter
> 
> On 11/01/18 1:16 PM, Peter wrote:
>> 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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> 
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-- 
Eike Waldt
Linux Consultant & Trainer
Tel.: +49-175-7241189
Mail: waldt@xxxxxxxxxxxxx

B1 Systems GmbH
Osterfeldstraße 7 / 85088 Vohburg / http://www.b1-systems.de
GF: Ralph Dehner / Unternehmenssitz: Vohburg / AG: Ingolstadt,HRB 3537

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