Progress Toward Accelerating CAM-SE. Jeff Larkin Along with: Rick Archibald, Ilene Carpenter, Kate Evans, Paulius Micikevicius, Jim Rosinski, Jim Schwarzmeier, Mark Taylor

Background In 2009 ORNL asked many of their top users: What sort of science would you do on a 20 Petaflops machine in 2012? – Answer to come on next slide Center for Accelerated Application Research (CAAR) established to determine: – Can a set of codes from various disciplines be made to effectively use GPU accelerators with the combined efforts of domain scientists and vendors – Each team has a science lead, code lead, members from ORNL, Cray, Nvidia, and elsewhere

CAM-SE Target Problem 1/8 degree CAM, using CAM-SE dynamical core and Mozart tropospheric chemistry. Why is acceleration needed to “do” the problem? – When including all the tracers associated with Mozart atmospheric chemistry, the simulation is too expensive to run at high resolution on today’s systems. What unrealized parallelism needs to be exposed? – In many parts of the dynamics, parallelism needs to include levels (k) and chemical constituents (q).

Profile of Runtime % of Runtime

Next Steps Once the dominant routines were identified, standalone kernels were created for each. Early efforts tested PGI & HMPP directive, plus CUDA C, CUDA Fortran, and OpenCL Directives-based compiler were too immature at the time – Poor support for Fortran modules and derived types – Did not allow implementation at a high enough level CUDA Fortran provided good performance while allowing us to remain in Fortran

Identifying Parallelism HOMME parallelizes both MPI and OpenMP over elements Most of the tracer advection can also parallelize over tracers (q) and levels (k) – Vertical remap is the exception, due to vertical dependence in levels. Parallelizing over tracers and sometimes levels while threading over quadrature points (nv) provides ample parallelism within each element to utilize GPU effectively.

Status Euler_step & laplace_sphere_wk were straightforward to rewrite in CUDA Fortran Vertical Remap was rewritten to be more amenable to GPU (made it vectorize) – Resulting code is 2X faster on CPU than original code and has been given back to the community Edge Packing/Unpacking for boundary exchange needs to be rewritten (Ilene talked about this already) – Designed for 1 element per MPI rank, but we plan to run with more – Once this is node-aware, it can also be device-aware and greatly reduce PCIe transfers Someone said yesterday: “As with many kernels, the ratio of FLOPS per by transfer determines successful acceleration.”

Status (cont.) Kernels were put back into HOMME and validation tests were run and passed – This version did nothing to reduce data movement, only tested kernel accuracy – In process of porting forward to current trunk and do more intelligent data movement Currently reevaluating directives now that compilers have matured – Directives-based vertical remap now slightly outperforms hand-tuned CUDA – Still working around derived_type issues

Challenges Data Structures (Object-Oriented Fortran) – Every node has an array of element derived types, which contains more arrays – We only care about some of these arrays, so data movement isn’t very natural – We must essentially change many non-contiguous CPU arrays into a contiguous GPU array Parallelism occurs at various levels of the calltree, not just leaf routines, so compiler must be able to inline leaves in order to use directives – Cray compiler handles this via whole program analysis, PGI compiler may support this via inline library

Challenges (cont.) CUDA Fortran requires everything live in the same module – Must duplicate some routines and data structures from several module in our “cuda_mod” – Insert ifdefs that hijack CPU routine calls and forward the request to matching cuda_mod routines – Simple for user, but developer must maintain duplicate routines – Hey Dave, when will this get changed? ;)

Until the Boundary Exchange is rewritten, euler_step performance is hampered by data movement. Streaming over elements helps, but may not be realistic for the full code.

With data transfer, laplace_sphere_wk is a wash, but since all necessary data is already resident from euler_step, kernel only time is realistic.

Vertical remap rewrite is 2X faster on the CPU and still faster on GPU. All data already resident on device from euler_step, so kernel-only time is realistic.

Future Work Use CUDA 4.0 dynamic pinning of memory to allow overlapping & better PCIe performance Move forward to CAM5/CESM1 – No chance of our work being used otherwise Some additional, small kernels are needed to allow data to remain resident – Cheaper to run these on the GPU than to copy the data Reprofile with accelerated application to identify next most important routines – Chemisty implicit solver is expected to be next – Physics is expected to require mature, directives-based compiler – Rinse, repeat

Conclusions Much has been done, much remains For a fairly new, cleanly written code, CUDA Fortran was tractable. – HOMME has very similar loop nests throughout, that was key to making this possible – Still results in multiple code paths to maintain, so we’d prefer to move to directives for the long-run We believe GPU accelerators will be beneficial for the selected problem – We hope that it will also benefit a wider audience (CAM5 should help this)