Cartesian Biweekly 2021 04 13

2021/04/13 GT4Py bi-weekly

To Dos

Review To Dos from previous sync

[x] Will ensure CSCS can compile microphysics code — working on it

Agenda

Progress on horizontal regions [@Johann D]

Detected via loop in MergeBlocksPass. See https://github.com/jdahm/gt4py/blob/horizontal-regions-gtstencil/src/gt4py/analysis/passes.py#L1028

Example:

def example(out: Field[int], out2: Field[int]):  
    with computation(FORWARD), interval(...):  
        tmp = 1  
        tmp2 = 2  
        out = tmp2[1, 0, 0]  
        with horizontal(region[I[0], :]):  
            out = tmp[1, 0, 0]  
        out2 = out + tmp2  

allocated_fields = {out, out2, tmp}

def example(out: Field[int], out2: Field[int]):  
    with computation(FORWARD), interval(...):  
        tmp = 1  
        tmp2 = 2  
# --- sync ---  
        out = tmp2[1, 0, 0]  
# --- sync ---  
        with horizontal(region[I[0], :]):  
            out = tmp[1, 0, 0]  
        out2 = out + tmp2  

allocated_fields = {out, out2, tmp, tmp2}

Re-run algorithm → same as last allocated_fields, so loop exits.

Understanding of corner and edge problem [Felix]

• Present solution in new parallel model. Does it fix all problems or was the simplified testcase too simple?
• Present plain single-stencil-GT4Py implementation

Status update: FV3core [@Eddie D]

• Show performance plot (link)

Profiling GT4Py [@Oli F]

Had to use add_profile_info backend option, nsight systems, and latest CUDA version (11).
Would like share some insights next meeting.

Small PRs / performance related

• gtscript.external_assert function instead of assert keyword
• StencilObject changes
• Make AccessIntent an IntFlag and add READ, WRITE
• Add device_sync backend option

Lagrangian Remapping

Algorithm in initial FV3 mapz GridTools port used a while loop and a read with a variable vertical offset. See: https://github.com/VulcanClimateModeling/GT_FV3/blob/master/fv_mapz/fv3_map_stencils.hpp#L136

Our new plan is to try this route instead of writing to an absolute index.

Simplified frontend example:

def stencil(k_offset: Field[int, IJ], pe1: Field[float], pe2: Field[float], ...):  
  with computation(FORWARD), interval(...):  
    l = k_offset + 1  
    qsum = pe1[0, 0, l+1] - pe2 + ...  
    while pe1[0, 0, l+1] < pe2[0, 0, 1]:  
      qsum += dp1[0, 0, l] * q[0, 0, l]  
      l = l + 1  

Changes to Ref nodes:

    Ref         = VarRef(name: str, [index: int])  
                | FieldRef(name: str, offset: Dict[str, int | Expr])  
                # Horizontal indices must be ints