[mpiwg-tools] Open MPI pvar scheme

[Jeff Squyres (jsquyres)]

On Nov 13, 2013, at 6:13 PM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:

> Let me comment on the problem in your slides of last teleconf:
> Slide 2: "Problem: multiple network devices on a server – want to return the number of sends on each device but don’t want to create a new variable for each of them.
>   Solution: have each performance variable return an array of values; one for each underlying network device"
> 
> That basically says " (an MPI implementor) creates a multi-element pvar (i.e., pvar with count > 1)"
> 
> Slide 3: "Problem: But how do you know which array slot maps to which underlying Linux device?
>   Solution: the btl_usnic_devices state performance variable."
> 
> I suppose "YOU"  in the problem is an MPI_T user, for example, a tool developer. The question is distilled into "the user wants to know names of elements in a multi-element pvar". 

We didn't talk about this on the call today.

I don't think I agree with your above statement.  It's not the *name* of the pvar that is the issue -- a pvar already has a well-defined name.  It's what each element of a pvar value *corresponds to*.  In this case, we have a pvar with N elements in its value, and the implementation is corresponding each element to a Linux device (and it is desirable for the MPI_T user to know what that correspondence is).  

We therefore want a way to express that relationship: that each element has a string corresponding to it (that is not necessarily a "name" or "subname").  However, not all pvars have this kind of relationship.

-- 
Jeff Squyres
jsquyres at cisco.com
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