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Subject: [OMPI users] Infiniband performance Problem and stalling
From: Randolph Pullen (randolph_pullen_at_[hidden])
Date: 2012-08-31 03:53:02

(reposted with consolidated information) I have a test rig comprising 2 i7 systems 8GB RAM with Melanox III HCA 10G cards running Centos 5.7 Kernel 2.6.18-274 Open MPI 1.4.3 MLNX_OFED_LINUX-1.5.3- (OFED-1.5.3- On a Cisco 24 pt switch   Normal performance is: $ mpirun --mca btl openib,self -n 2 -hostfile mpi.hosts  PingPong results in:  Max rate = 958.388867 MB/sec  Min latency = 4.529953 usec and: $ mpirun --mca btl tcp,self -n 2 -hostfile mpi.hosts  PingPong Max rate = 653.547293 MB/sec  Min latency = 19.550323 usec   NetPipeMPI  results show a max of 7.4 Gb/s at 8388605 bytes which seems fine. log_num_mtt =20 and log_mtts_per_seg params =2   My application exchanges about a gig of data between the processes with 2 sender and 2 consumer processes on each node with 1 additional controller process on the starting node. The program splits the data into 64K blocks and uses non blocking sends and receives with busy/sleep loops to monitor progress until completion. Each process owns a single buffer for these 64K blocks.     My problem is I see better performance under IPoIB then I do on native IB (RDMA_CM). My understanding is that IPoIB is limited to about 1G/s so I am at a loss to know why it is faster.   These 2 configurations are equivelant (about 8-10 seconds per cycle) mpirun --mca btl_openib_flags 2 --mca mpi_leave_pinned 1 --mca btl tcp,self -H vh2,vh1 -np 9 --bycore prog mpirun --mca btl_openib_flags 3 --mca mpi_leave_pinned 1 --mca btl tcp,self -H vh2,vh1 -np 9 --bycore prog   And this one produces similar run times but seems to degrade with repeated cycles: mpirun --mca btl_openib_eager_limit 64 --mca mpi_leave_pinned 1 --mca btl openib,self -H vh2,vh1 -np 9 --bycore  prog   Other  btl_openib_flags settings result in much lower performance.  Changing the first of the above configs to use openIB results in a 21 second run time at best.  Sometimes it takes up to 5 minutes. In all cases, OpenIB runs in twice the time it takes TCP,except if I push the small message max to 64K and force short messages.  Then the openib times are the same as TCP and no faster.   With openib: - Repeated cycles during a single run seem to slow down with each cycle (usually by about 10 seconds). - On occasions it seems to stall indefinitely, waiting on a single receive.    I'm  still at a loss as to why.  I can’t find any errors logged during the runs. Any ideas appreciated.   Thanks in advance, Randolph