Re: [Libmesh-users] Question about unique_ids

[David K. <dav...@ak...>]

On Wed, Jan 10, 2018 at 3:41 PM, Roy Stogner <roy...@ic...>
wrote:

>
> On Wed, 10 Jan 2018, David Knezevic wrote:
>
> I'd like to use unique_ids to identify elements and nodes without worrying
>> about renumbering, but before I do that I'd like to understand a bit more
>> about how unique_ids are defined.
>>
>> In particular, what is the idea behind how we generate the unique_ids?
>>
>
> Basically we have two constraints:
>
> 1. We want to never ever reuse a unique_id for two different
> DofObjects of the same category (Nodes, Elems).
> 2. We want to assign a unique_id to each DofObject as soon as possible.
>
> And that leads us to a few complications:
>
> 1. Our unique_id values will often be sparse:  When adaptively
> coarsening we delete objects, and we *don't* give their replacements
> the same unique_id if/when we adaptively refine the same area again.
> When building distributed meshes, we add objects on one processor
> without communicating with other processors, so each processor "owns"
> an interleaved subset of unique_id space, and may leave some of that
> space blank.
> 2. Our unique_id values are really only well-defined *when* they're
> created, and that creation may depend on mesh type and partitioning.
> So:
>
> One concrete question I have is if I create two Mesh objects by reading in
>> the same mesh exodus file twice, will those two meshes have matching
>> unique_ids? I guess they will simply because "add_node" and "add_elem"
>> will
>> be called in the same order during those two read operations.
>>
>
> If both reads are using the same libMesh version, and either both are
> to a ReplicatedMesh or both are to a DistributedMesh, then currently
> the answer is yes.  I don't expect that to change, though I haven't
> really thought about it before.
>
> Please don't count on unique_id numbering remaining the same from
> version to version, though!  If you need to open a mesh next year with
> the same unique_id numbering it has this year, save that mesh in XDR
> or XDA (where we write out unique_id), not Exodus.



Thanks for the explanations, very helpful!

David