12/7/ Performance Optimizations for running NIM on GPUs Jacques Middlecoff NOAA/OAR/ESRL/GSD/AB Mark Govett, Tom Henderson Jim Rosinski

12/7/ Goal for NIM

12/7/ Optimizations to be discussed NIM: The halo to be communicated between processors is packed and unpacked on the GPU No copy of entire variable to and from the CPU About the same speed as the CPU Halo computation Overlapping communication with computation Mapped, pinned memory NVIDIA GPUDirect technology

12/7/ Halo Computation Redundant computation to avoid communication Calculate values in the halo instead of MPI send Trades computation time for communication time GPUs create more opportunity for halo comp NIM has halo comp for everything not requiring extra communication NIM next step is to look at halo comp’s that require new but less often communication

12/7/ Overlapping Communication with Computation Works best with a co-processor to handle comm Overlap communication with other calculations between when a variable is set and used. Not enough computation time on the GPU Calculate perimeter first then do communication while calculating the interior Loop level: Not enough computation on the GPU Subroutine level: Not enough computation time Entire dynamics: Not feasible for NIM

12/7/ Overlapping Communication with Computation: Entire Dynamics 14 exchanges per time step 3 iteration Runge Kutta loop Exchanges in the RK loop Results in a 7 deep halo Perimeter Interior Way too much communication More halo comp? Move exchanges out of RK loop? Considerable code restructuring required.

12/7/ Mapped, Pinned Memory: Theory Mapped, pinned memory is CPU memory Mapped so GPU can access it across PCIe bus Page-locked so the OS can’t swap it out Limited amount Integrated GPUs: Always a performance gain Discrete GPUs (what we have) Advantageous only in certain cases The data is not cached on the GPU Global loads and stores must be coalesced Zero-copy: Both GPU and CPU can access data

12/7/ Mapped, Pinned Memory: Practice Using mapped, pinned memory for fast copy SendBuf is mapped and pinned Regular GPU array (d_buff) is packed on GPU d_buff is copied to SendBuf Twice as fast as copying d_buff to a CPU array Pack the halo on GPU SendBuf = VAR Zero-copy 2.7X slower Why? Unpack halo on GPU VAR = RecvBuf Zero-copy unpack same speed but no copy

12/7/ Mapped, Pinned Memory: Results NIM horizontal, 96 vertical 10 processors Lowest value selected to avoid skew

12/7/ Mapped, Pinned Memory: Results

12/7/ NVIDIA GPUDirect Technology Eliminates the CPU in interprocessor communication Based on an interface between the GPU and InfiniBand Both devices share pinned memory buffers Data written by GPU can be sent immediately by InfiniBand Overlapping communication with computation? No longer a co-processor to do the comm? We have this technology but have yet to install it

12/7/ Questions?