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Hi,

In general, you can see 'reserved_host_memory_mb' as your estimation of 
how much memory your hypervisor needs. From the point of view of Nova, 
your system is consuming the amount you set in 'reserved_host_memory_mb' 
[0].

Then, the field 'free_ram_mb' (the one that is checked when VMs are 
being scheduled) is calculated using the memory consumed by the 
instances and the value of reserved memory [1].

Said that, the scheduler won't see the real free mem (700Mb in your 
case), it will see the calculated value. I would suggest you take a look 
to the value of "free_ram_mb" on your hypervisor (the value in the 
database: like "nova hypervisor-show", not the one in the compute logs), 
and then... you compare. If you're expection 700Mb and you see 900Mb, 
you should tweak a bit more your 1024Mb or reserved memory and increase it.

It's a bit difficult to follow the flow in the code, but these links are 
quite useful to understand how it works.

[0] 
https://github.com/openstack/nova/blob/master/nova/compute/resource_tracker.py#L695 

https://github.com/openstack/nova/blob/master/nova/compute/resource_tracker.py#L708
[1] 
https://github.com/openstack/nova/blob/master/nova/compute/resource_tracker.py#L532


On 11/27/2014 01:55 PM, mad Engineer wrote:
> it reports "Free ram (MB): 425"
> but free -m has different result.
>
> Is it working for you.Are you using icehouse
>
> On Thu, Nov 27, 2014 at 6:16 PM, George Mihaiescu <lmihaiescu at gmail.com> wrote:
>> Then I would enable debug and verbose in nova.conf and restart the scheduler
>> service. In this way you should see in the logs the entire scheduler logic
>> and what resources it thinks your host has.
>>
>> On 27 Nov 2014 06:20, "mad Engineer" <themadengin33r at gmail.com> wrote:
>>> George,
>>>                overcommit of RAM is 1  and that is working.However
>>> instances are still getting created with available free memory <
>>> reserved_host_memory_mb
>>>
>>> On Thu, Nov 27, 2014 at 4:33 PM, George Mihaiescu <lmihaiescu at gmail.com>
>>> wrote:
>>>> Depending on your overcommit ratio, the scheduler can schedule instances
>>>> using more virtual memory than the available physical memory on the host,
>>>> 700 MB in your case.
>>>>
>>>> On 27 Nov 2014 05:36, "mad Engineer" <themadengin33r at gmail.com> wrote:
>>>>> hi all i have set
>>>>> reserved_host_memory_mb in nova.conf of controller and compute and
>>>>> restarted necessary services.
>>>>> i am expecting scheduler to not pickup host that has less than what is
>>>>> reserved_host_memory_mb
>>>>> in my example i put reserved_host_memory_mb = 1024
>>>>> and free RAM in compute node is 700 Mb.
>>>>> But still scheduler chooses this host and created a new instance on it.I
>>>>> am expecting scheduler to show "No valid Hosts found" as it has less ram
>>>>> than reserved_host_memory_mb = 1024
>>>>>
>>>>> Can some one help me understand this value
>>>>>
>>>>> Thanks
>>>>>
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