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On 12/21/2018 03:45 AM, Rui Zang wrote:
> It was advised in today's nova team meeting to bring this up by email.
> 
> There has been some discussion on the how to track persistent memory 
> resource in placement on the spec review [1].
> 
> Background: persistent memory (PMEM) needs to be partitioned to 
> namespaces to be consumed by VMs. Due to fragmentation issues, the spec 
> proposed to use fixed sized PMEM namespaces.

The spec proposed to use fixed sized namespaces that is controllable by 
the deployer, not fixed-size-for-everyone :) Just want to make sure 
we're being clear here.

> The spec proposed way to represent PMEM namespaces is to use one 
> Resource Provider (RP) for one PMEM namespace. An new standard Resource 
> Class (RC) -- 'VPMEM_GB` is introduced to classify PMEM namspace RPs. 
> For each PMEM namespace RP, the values for 'max_unit', 'min_unit', 
> 'total' and 'step_size` are all set to the size of the PMEM namespace. 
> In this way, it is guaranteed each RP will be consumed as a whole at one 
> time.
 >
> An alternative was brought out in the review. Different Custom Resource 
> Classes ( CUSTOM_PMEM_XXXGB) can be used to designate PMEM namespaces of 
> different sizes. The size of the PMEM namespace is encoded in the name 
> of the custom Resource Class. And multiple PMEM namespaces of the same 
> size  (say 128G) can be represented by one RP of the same 

Not represented by "one RP of the same CUSTOM_PMEM_128G". There would be 
only one resource provider: the compute node itself. It would have an 
inventory of, say, 8 CUSTOM_PMEM_128G resources.

> CUSTOM_PMEM_128G. In this way, the RP could have 'max_unit'  and 'total' 
> as the total number of the PMEM namespaces of the certain size. And the 
> values of 'min_unit' and 'step_size' could set to 1.

No, the max_unit, min_unit, step_size and total would refer to the 
number of *PMEM namespaces*, not the amount of GB of memory represented 
by those namespaces.

Therefore, min_unit and step_size would be 1, max_unit would be the 
total number of *namespaces* that could simultaneously be attached to a 
single consumer (VM), and total would be 8 in our example where the 
compute node had 8 of these pre-defined 128G PMEM namespaces.

> We believe both way could work. We would like to have a community 
> consensus on which way to use.
> Email replies and review comments to the spec [1] are both welcomed.

Custom resource classes were invented for precisely this kind of use 
case. The resource being represented is a namespace. The resource is not 
"a Gibibyte of persistent memory".

Best,
-jay

> Regards,
> Zang, Rui
> 
> 
> [1] https://review.openstack.org/#/c/601596/
>