From: Eddy Caron Newsgroups: comp.parallel.mpi Subject: Mpich : Synchronous vs Asynchronous Date: Tue, 27 Apr 1999 00:28:24 +0200 Message-Id: <3724E888.58584A3E@neuronnexion.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Xref: ukc comp.parallel.mpi:4989 Hi, I perform a simple programs to evaluate overlap communication/computing. Proc 0 : | Proc 1 : | Send n vectors | Receive n Vector Receive 1 vector | Send 1 vector Compute | Compute And I perform this programs in two version on two kind of cluster. In each case the synchronous mode is better than asynchronous mode. Why there is no overlap communication/computing ? And why the first time to Isend for proc 0 is not egal to zero (All time are evaluate with MPI_Wtime) ? Thanks for all. *************** * Example 1 : * *************** Synchronous (PC cluster) ---------------------- Proc 0 : | Proc 1 : | Send n vectors 11,18s | Receive n Vector 11,18s Receive 1 vector 0,00s | Send 1 vector 0,00s Compute 19,91s | Compute 19,88s ------ ------ Total Time 31,09s 31,06s Asynchronous (PC cluster) ----------------------- Proc 0 : | Proc 1 : | ISend n vectors 31,54s | IReceive n Vector 0,35s IReceive 1 vector 0,00s | ISend 1 vector 0,00s Compute 19,88s | Compute 19,88s Waitall 0,01s | Waitall 11,31s ------ ------ Total Time 51,43s 31,54s *************** * Example 2 : * *************** Synchronous (Alpha Cluster) ------------------------- Proc 0 : | Proc 1 : | Send n vectors 3,08s | Receive n Vector 3,06s Receive 1 vector 0,00s | Send 1 vector 0,00s Compute 21,30s | Compute 21,30s ------ ------ Total Time 24,38s 24,36s Asynchronous (Alpha cluster) -------------------------- Proc 0 : | Proc 1 : | ISend n vectors 23,70s | IReceive n Vector 0,14s IReceive 1 vector 0,00s | ISend 1 vector 0,00s Compute 21,38s | Compute 21,39s Waitall 0,01s | Waitall 2,17s ------ ------ Total Time 45,09s 23,70s