On May 23, 2008, at 9:56 AM, Josh Hursey wrote:
Unfortunately, as Jeff pointed out, the behavior of frameworks and components in determining component selection is not consistent in the codebase. The mca_base_select() commit made things much better, but there are still frameworks that do not (or cannot) use it, and there are some behaviors that are just not well defined. Consistency issues lead to user (and developer) confusion and degrade the image of the project. For exactly those reasons I want to talk about a number of such issues in one of our technical meeting this summer (this issue is currently scheduled for the July meeting). The goal is to come out of that meeting with a coding standard behavior for components during open/selection/close. Frameworks and components can diverge from this base standard, but then it is the responsibility of the component writer to make sure this is clearly communicated to users about expectations.
This is a pretty strong statement and some examples are welcomed. Anyway, we already have a coding standard for the component manipulation, and apparently there are cases when we need a hand crafted selection logic (such as collectives as Josh pointed it out). The ^component is managed at the bottom layer, where we create the list of components to be opened, so this is consistent across the board.
To answer your question though, an individual component can determine
what to return for the {priority,module} pair based on anything it
wishes. For instance the SLURM PLM component will return NULL if it
does not see the correct environment variables, and a working module
if it does. Collectives are a special type of framework so the
selection logic there is specialized, meaning it does not use the
mca_base_select function, but uses a more custom version of select.
If you supply "^component" then the component is never opened and thus
never queried during selection. If you specify 0 for the priority of
the hierarch component the the component is opened, and will just
return NULL during selection. If you specify > 0 for the priority then
the hierarch component will return a module to the selection code.
This module will be used if the hierarch component has the 'best'
priority, otherwise the hierarch component should be closed
[hierarch_component_close] at the end of the selection code.
Determining the 'best' priority and whether or not the components are
closed at the end of selection is determined by the coll/base select
function.
I think I may have just made things seem more complex than they
probably are.
I don't think so. For me the process is straightforward. Here are the possible scenarios: 1) ^component behave as if the corresponding file (i.e. shared library) is not available. 2) init returning a NULL module, means that this component do not desire to be selected. There is no need to clarify the reason why, the outcome is that the component selected to be ignored. 3) returning a non NULL module and a priority allow the selection logic to include the specified module in the selection process. Of course the selection process is different for some framework, but this is to be expected. Keep in mind that while there are one-to-one framework (such as the IO subsystem and the PML) and many-to-one framework (such as the BTLs and the collectives) the priority always allow the selector to order the modules based on the decreasing priority. Then, based on the type of the framework (one-to-one or many- to-one), the selector pick the first or all modules from the list and close the others. As I said ... straightforward :)
george.
-- Josh On May 23, 2008, at 8:28 AM, Jeff Squyres wrote:I think that technically, the component can do whatever it wants (e.g., look at its priority, see 0, and decide to return NULL).However, to be consistent, we should decide on a specific behavior andmake it uniform to all components. I'd opt for the ^foo notation to disable a component. On May 23, 2008, at 8:14 AM, Rolf Vandevaart wrote:This mostly makes sense. But let me probe a little more. Can acomponent return NULL if it looks at its priority and the priority isless than or equal to 0? For example, currently the hierarch component returns NULL when its priority is equal or less than 0. This means that as a user when I set the priority to 0 I am indicating that I do not want the hierarch component selected at all. Or, is the priority only used to specify relative behavior. So, it is not to be used to completely deselect a component. To deselect, you would need to use the ^component format. That is where I am confused. Rolf Josh Hursey wrote:Yeah (Sorry I didn't reply earlier). Each component is asked for at least two items of information: priority (int), and module (struct *). The priority can range from [INT_MIN | INT_MAX] with the highest priority selected, even if that priority is negative. If the component does not want to be selected then it should return NULL for the module value. This indicates to the selection logic that no matter what the priority is set to the component should not be a candidate for selection. So a component is selectable if it returns a non-NULL value for the module struct, and is not selectable if it returns NULL. The priority only indicates relative rank between all available components. Does that make sense? I should probably add this comment to the mca_base_select function to preserve it. I'll make a bug for it so it doesn't get lost. -- Josh On May 23, 2008, at 7:14 AM, Jeff Squyres wrote:We may not have this uniform throughout the code base -- this is one of the things we wanted to talk about in the Bay area meeting. Ibelieve that the allowable range for priorities should be [0, 100],and that if you don't want to be selected, you should return NULL (or use some other mechanism to indicate that you didn't want to be selected). That was the original intent of the MCA selection mechanisms, at least. Josh -- is this consistent with what you found when you consolidated a lot of this stuff? On May 22, 2008, at 11:30 AM, Rolf vandeVaart wrote:I know there was some recent discussion about priority of components, but I wanted to double check. I am trying to understand what priority = 0 means. My assumption is the following: priority >= 0 means the component is selectable priority < 0 means the component is not selectable I ask this because in some of the collective code it looks like a priority = 0 means not selectable. Not a big deal, but I am trying to fix a memory leak and I need to get this piece right. And I assume thatpriority < 0 will give one the same behavior as ^component but thecode paths within Open MPI would be different. Rolf _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel-- Jeff Squyres Cisco Systems _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel_______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel-- ========================= rolf.vandeva...@sun.com 781-442-3043 ========================= _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel-- Jeff Squyres Cisco Systems _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel_______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel
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