Dear all, Firstly, apologies if this is the wrong place to post - I couldn't see a particularly relevant subgroup or forum for discussion, so if this is the wrong place, redirection is more than welcome.... I have a program. Actually my program is three programs coupled together. The entire application lives in MPI_COMM_WORLD, and each of the components operates mainly within its own private communicator. Another three communicators exist that handle the coupling between each pair of subcomponents. I run my program on a machine - it has some special features that allow me to alter how MPI behaves - I can take advantage of different protocols for point to point messaging, I can elect to use hardware acceleration for collectives, and various other things that radically change how MPI behaves and performs. I can switch these on and off, say, via the environment and via my MPI launcher program. For the purposes of tuning, I can isolate the three applications as well as the inter component communication patterns - I discover that the parametrisation needed for each to be optimal is rather different. Tuning for the whole ensemble leads to none of the individual parts performing optimally. After discussion with my MPI vendor, it rapidly became apparent that there was no way of articulating from outside the application that I wanted different (as yet uncreated) communicators on different task sets to behave differently in anything remotely approaching a sensible fashion - the communicators are completely opaque, and there is no way to reference them! Here's a simple example to illustrate < ---------- COMM_WORLD ---------------> < main app > � � � < post processing > --- long messages one way--> My machine has, say, two p2p messaging modes, low latency driven by the CPU, and high latency offloaded to supporting hardware. The main app is tightly synchronised and benefits from the low latency mode even though it consumes CPU cycles. Communications between the main app and the post processor is unidirectional and slow delivery does not hurt overall performance (but involving the CPU in dispatching them does). Optimal performance is acheived by setting comm_world to use the high latency offload mode, and comm_main_app to use the low latency CPU mode. An even simpler example would be a monolithic application, where optimal settings are not apparent until part way through execution - i.e. data dependent. As it stands there is no practical way of setting this up - even with the generous support of my vendor. This is in stark contrast to MPI-IO where I have data structures available to pass either vendor specific or generic hints allowing me to modify behaviour on a per-file basis, in application, at runtime. I really want to be able to perform a similar thing when I create communicators, such that when I perform, say, a comm_split, I can create unique characteristics for each of the new communicators. So I guess the first question is has any work been taking place in this area, and if not, how do I go about spurring the discussion? Best regards, Martyn