[Pvfs2-developers] fix BMI multiplexing of multiple methods

[walt]

I'm not clear what level you propose this at - the BMI interface, or the 
interface to the BMI method (internal to BMI).  I'm not wild about 
changing the BMI interface, particularly to use callbacks.  While these 
may seem superior to some of us, they are more complex and it really 
starts to mess with the extensibility.  Changes within BMI aren't a 
concern.  By the same light, the CONTEXT method seems a bit much.

Walt

Sam Lang wrote:
> 
> Hi All,
> 
> Right now if multiple methods are enabled in BMI, we tend to get poor 
> performance from the "fast" network, because BMI_testcontext iterates 
> through all the active methods calling testcontext for each one.  It 
> tries to be smart about which methods get scheduled ;-) to prevent 
> starvation, but it treats all the methods fairly, which tends to make 
> tcp (the slow one) hog the time spent in testcontext.  I have a few 
> ideas for this, so I'll go ahead and propose them and let you all shoot 
> them down or propose others.
> 
> Option CALLBACK:  Instead of returning completion as a list in 
> testcontext, we allow a BMI context to be constructed with a callback, 
> and on completion of operations, the callback is called.  This allows 
> each method to drive its own operations, and notify the consumer of 
> completion immediately.  There would still need to be a testcontext call 
> for methods that only service operations during that call.  The changes 
> might not be that significant, the BMI_open_context call could just take 
> an extra parameter that was the callback function.  If the parameter is 
> null, we just use the completion list as before.
> 
> Option CONTEXT:  Require separate contexts for separate methods.  This 
> pushes the problem up to the application, probably not where it belongs, 
> since active methods are opaque from the BMI api.
> 
> Option POLL_PLAN:  Modify the construct_poll_plan function in bmi that 
> already tries to be fair, so that its aware of the performance 
> discrepancy between methods.  Maybe it can just skip tcp every other 
> time for example.  This is probably the easiest, since it doesn't 
> require API changes and the like.
> 
> -sam
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