[mpiwg-tools] MPI_T question about cvar value guarantees

[Jeff Squyres (jsquyres)]

We discussed this in the Chicago Feb 2016 MPI Forum meeting, during the tools WG.

The consensus was that we don't need to add any text to address this.  The value of a variable can change at any time -- even 2 consecutive calls to cvar_read could return different values if someone else wrote a new value.

So: no guarantees are given about the read values.  No text to add here.


> On Feb 4, 2016, at 12:27 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com> wrote:
> 
> Nathan Hjelm and I stumbled across a question regarding MPI_T today: what guarantees does MPI_T provide about control variable values when MPI_T is finalized and MPI is either not initialized or is finalized?
> 
> For example:
> 
> ```C
> // Block A
> MPI_T_Init(...);
> MPI_T_Cvar_read(..., &value_pre);
> MPI_T_Cvar_write(...);
> MPI_T_Finalize(...);
> 
> // ...dead zone where neither MPI_T nor MPI are initialized...
> 
> // Block B
> MPI_Init(...)
> MPI_T_Init(...);
> MPI_T_Cvar_read(..., &value_post);
> assert(value_pre == value_post);
> ```
> 
> Is it guaranteed that the assert() in block B must be true?
> 
> Here's why I ask -- consider the following two scenarios:
> 
> SCENARIO 1:
> ===========
> 
> - Block A causes a dynamically-loaded component to be loaded and initialized
> - The cvar from that dynamically-loaded component is set to a default value
> - The cvar is then read into value_pre
> - The cvar is then assigned a non-default value
> - MPI_T is then finalized
> - Since MPI is not initialized, the dynamically-loaded component is unloaded (so that the process can be valgrind clean if it exits)
> 
> - Block B causes the same dynamically-loaded component to be loaded and initialized
> - The cvar from that dynamically-loaded component is set to a default value
> - The cvar is then read into value_post
> - value_pre and value_post are both the same (default) value -- the assert passes
> 
> SCENARIO 2:
> ===========
> - Block A causes a statically-loaded component to be initialized
> - The cvar from that statically-loaded component is set to a default value
> - The cvar is then read into value_pre
> - The cvar is then assigned a non-default value
> - MPI_T is then finalized
> - Since the component is static, it is *not* unloaded
> 
> - Block B causes nothing to happen to the component (because it's statically loaded)
> - The cvar from that statically-loaded component ***is still set to the non-default value***
> - The cvar is then read into value_post
> - value_pre and value_post are different values -- the assert fails
> 
> -- 
> Jeff Squyres
> jsquyres at cisco.com
> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
> 


-- 
Jeff Squyres
jsquyres at cisco.com
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/