[mpiwg-tools] QMPI API (After 2020-10-08 Discussion)

[Joachim Protze]

Hi Wesley,

most of the functions are as expected, just the last part makes me
wonder how this should work.

Am 08.10.20 um 22:54 schrieb Wesley Bland via mpiwg-tools:
> 
> int QMPI_Register_tool_name(char * tool_name, void (*
> init_function_ptr)(int tool_id));
> 
> tool_name - [IN] Character string - A unique string representing the
> name of the tool
> init_function_ptr - [IN] Function pointer - Pointer to a function that
> MPI will called before MPI is initialized
> 

Since you start operating with function pointers, I'm wondering whether
all the functions below should also be function pointers (with some
lookup mechanism) rather than public interface functions.

Also, is the tool always guaranteed to find the QMPI_Register_tool_name
function? I don't remember the name, so I didn't find documentation, but
I think that some linkers support encapsulated name spaces for dynamic
libraries.


> tool_storage - [IN] Address - A pointer to an address where the tool has
> data being stored for its particular instance
> 
> This function allows the tool to provide a storage object (tool_storage)
> which lets the tool store information specific to its instance of itself
> (in case there are multiple copies of a tool). The specific instance of
> the tool is identified with the tool_id value and should be matched with
> the one returned by the callback function that the tool registered with MPI.

From my perspective, this structure holds all state which a particular
instance of a tool has.


> *Interception*
> 
> When the tool's interception function is called, it can get any
> tool-specific information from the context object. The fields in the
> context object can be retrieved with these functions:
> 
> QMPI_Context_get_storage(QMPI_Context context, int tool_id, void **storage);
> 
> context - [IN] Opaque handle - A handle to a context object containing
> tool and function-specific information
> tool_id - [IN] Integer - A unique identifier for the tool
> storage - [OUT] Address - A pointer to an address where the tool has
> data being stored for its particular instance
> 
> Get the back storage object the tool provided
> to QMPI_Register_tool_storage that matches tool_id.

Where does this tool_id come from? Is it passed in as an argument to the
interception function? The tool does not know the id.

> 
> *Calling Additional QMPI Tools*
> 
> When the tool is done with its own interception, it should call this
> function to determine the next tool in the QMPI stack (which the tool
> itself is responsible for calling):
> 
> int QMPI_Get_function(int tool_id, enum QMPI_Functions_enum
> function_enum, void (** function_ptr)(void), QMPI_Context *
> next_tool_context);
> 
> tool_id - [IN] Integer - A unique identifier for the tool
> function_enum - [IN] Enum value - An enumerated value specifying which
> MPI function is being registered for interception.
> function_ptr - [OUT] Function pointer - A pointer to a function that
> should be called when an application calls the specified function.
> next_tool_context - [OUT] Opaque handle - A handle to a context object
> containing tool and function-specific information
> 
> This function also uses tool_id and function_enum to determine the
> current tool's ID and the enum value for the function being queried. It
> also provides a function pointer in a similar format to the registration
> function. Finally, the function returns the context object of the next
> tool in the call stack which should be used when calling the returned
> function.

Why does this function not need the current QMPI_Context context?
I would store a pointer to the function-table in this context and
implement QMPI_Get_function as a macro accessing this table.

I assume the storage for next_tool_context is stack memory from the
caller (the tool)?

I would assume, that his function also provides the next tool id, to
pass it with the function call.

Alternatively, the tool ID could be a field in next_tool_context. Then
we need a function to query the tool ID from the context. Also,
QMPI_Context_get_storage does not need the tool_id as input in this case.


Best
Joachim



-- 
Dipl.-Inf. Joachim Protze

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering
RWTH Aachen University
Seffenter Weg 23
D 52074  Aachen (Germany)
Tel: +49 241 80- 24765
Fax: +49 241 80-624765
protze at itc.rwth-aachen.de
www.itc.rwth-aachen.de

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