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Proposal (More) Flexible RMA Synchronization for MPI-3 Hubert Ritzdorf NEC–IT Research Division
© NEC Europe Ltd 2008 The Problem Process (I)Process J MPI_Puts MPI_Gets How can Process J decide that Process (I) has finished RMA transfer so that Process J can access/change the data ? and others Shared Memory: Atomic counters, ready flags, semaphores, … Its not necessarily defined, which process provides the data, decrements the counter, sets the flag, … MPI-2: Collective approach (Fence, Complete/Wait) on Windows The group has to be clear in advance; not very flexible New Proposal
© NEC Europe Ltd 2008 Idea Window: Large amount of (shared) memory Atomic counter (synchronization counters): To independently handle parts of the (shared) memory New MPI Type MPI_Sync: MPI Synchronization counter Special Allocate: MPI_Win_Sync_alloc_objects (n_sync, sync_counters, win, info) MPI_Win_Sync_free_objects (n_sync, sync_counters, win) to be performed by the owner of the window win
© NEC Europe Ltd 2008 Problem How to transfer the info on MPI_Sync counters Every process (in win comm) should be able to get access Access to counter may change in run-time Processes must have required info to ``attach to the counter Idea: New MPI Datatype MPI_Handle_sync Transfer data via MPI pt2pt and collectives Possible alternative : Create n_sync counters in MPI_Win_create Pro: User doesnt see them; Contra: libraries, less flexibility, scalability
© NEC Europe Ltd 2008 Usage MPI_Win_sync_ops_init (target_rank, sync_mode, sync_counter, win, info, req) Init wait for completion of RMA operations corresponding to sync_mode : Bit vector or of MPI_MODE_WIN_PUT, MPI_MODE_WIN_GET, MPI_MODE_WIN_ACCUMULATE increment/decrement counter sync_counter on process target_rank On processes which perform RMA operations
© NEC Europe Ltd 2008 Usage MPI_Win_sync_object_init (sync_counter, count, win, info, req) On process where counter is located Init wait for count completions (atomic increments) of requests created by MPI_Win_sync_ops_init Designed as persistent request for frequent re-use All possibilities of MPI Test/Wait functions Works on cache coherent and non cache coherent systems Can be implemented by pt2pt functions
© NEC Europe Ltd 2008 Example MPI_Win_sync_object_init (sync_counter, 8, win, info, req); MPI_Start (req) MPI_Win_sync_ops_init (target_rank MPI_MODE_PUT, sync_counter, win, info, req); MPI_Start (req) 8 neighboursTarget MPI_Put (target, …) MPI_Wait (req, …)
© NEC Europe Ltd 2008 Wait for completion of which RMA ops ? With current MPI-2 Interface: Wait for completion of all RMA requests since last synchronization Additional functions for MPI-3 interface (useful for threads and several synchronization counters) Add synchronization counter request to RMA functions for example: MPI_Iput (…, win, req), MPI_Iget (…, win, req), … Not really save programming style. Difficult for threads or several counters
© NEC Europe Ltd 2008 Window as Shared Memory More dynamic usage of window required (for example for dynamic process spawning) MPI_Win_change_comm (new_comm, peer_rank, win) Allow processes to join or leave an already existing window.
© NEC Europe Ltd 2008 Possible Extensions Synchronization counters == atomic counters Wait/Test that counter completed on remote node Change final counter value (increment, decrement) Automatic restart of request if final counter value is reached and test/wait function was executed
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