# Profiling Fortran code scope-profiler ships a Fortran module with the same region model as the Python API. A Fortran program marks regions, writes a small trace file per rank, and `scope-profiler import-native` turns those traces into the usual HDF5 output — so a Fortran run gets the same summaries, charts, exporters and `plot` workflow as a Python one. The module is deliberately undemanding: plain Fortran 2008 with `iso_c_binding`, one file, no preprocessor flags, no HDF5, no MPI, nothing to link beyond libc. The trace format is shared with {doc}`the C API `, so a program built from both lands in a single profile. ## Getting the source It ships inside the installed package: ```bash gfortran -c $(python -c "import scope_profiler.native_trace as t; print(t.fortran_source_path())") ``` or from a checkout, `src/scope_profiler/fortran/scope_profiler.f90`. A `Makefile` and a runnable `example.f90` sit next to it. ## Marking regions ```fortran program simulation use scope_profiler implicit none integer :: step, solve, assemble call sp_init("profile") ! output prefix solve = sp_region("solve") ! resolve names once... assemble = sp_region("assemble") do step = 1, 1000 call sp_begin(assemble) ! ...then use the handles call assemble_matrix() call sp_end(assemble) call sp_begin(solve) call solve_system() call sp_end(solve) end do call sp_finalize() ! writes profile_rank00000.spt end program simulation ``` `sp_begin_name("solve")` / `sp_end_name("solve")` take the name directly, which reads better in cold code; they look the name up on every call, so prefer the handle form in hot loops. Regions may nest, and a region may re-enter itself recursively — each entry reserves its own slot, exactly as in the Python API. Nesting must be *proper*: a region entered inside another has to be left before it. Post-processing reconstructs the call graph from the recorded intervals, so a `sp_begin`/`sp_end` pair that crosses another region's boundary — rather than sitting inside or outside it — has no call stack to recover, and reading the trace back raises `NestingError`. The timings themselves are still recorded and readable; it is the nesting, the flame chart and the exclusive times that cannot be derived. ## The API | Call | Purpose | | --- | --- | | `sp_init(prefix [, rank])` | Start profiling. `rank` (default 0) gives each MPI rank its own trace file. | | `sp_region(name)` | Handle for a region name, created on first use. | | `sp_begin(id)` / `sp_end(id)` | Enter and leave a region. | | `sp_begin_name(name)` / `sp_end_name(name)` | The same, looking the name up each time. | | `sp_num_calls(id)` | How often a region was entered; readable after `sp_finalize()`. | | `sp_now_ns()` | The raw clock, in nanoseconds. | | `sp_is_active()` | Whether profiling is running. | | `sp_finalize()` | Write `_rank.spt` and stop. | ## Under MPI Pass each rank's id so the traces do not collide: ```fortran call MPI_Comm_rank(MPI_COMM_WORLD, my_rank, ierr) call sp_init("profile", rank=my_rank) ``` Every rank writes its own file; nothing is communicated, so `sp_finalize()` is not collective and a rank that dies takes only its own trace with it. The importer merges whatever it finds: ```bash mpirun -n 128 ./simulation scope-profiler import-native . -o profiling_data.h5 scope-profiler plot default profiling_data.h5 -o figures ``` ## From Python ```python from scope_profiler.native_trace import load_traces, convert_traces results = load_traces("run_dir") # -> ProfilingResults, as usual results.print_summary() results.to_dataframe() convert_traces("run_dir", "profiling_data.h5", label="128 ranks") ``` `load_traces` returns the very same `ProfilingResults` a Python run produces, so every method, `plot_*` function and exporter works on it unchanged. ## One timeline with Python The module reads the same OS clock CPython's `time.perf_counter_ns()` uses — `CLOCK_MONOTONIC` on Linux, `CLOCK_UPTIME_RAW` on macOS — and picks it by probing at run time rather than by compile-time platform macros, which gfortran does not reliably define. That means Fortran and Python regions recorded in the same process tree share an epoch: a Python driver's timestamps and its Fortran kernels' timestamps are directly comparable, and can be read on one timeline. ## Python calling Fortran The case this is really for: a Python driver over Fortran kernels, where both sides mark regions and you want **one** profile. Because the two APIs read the same clock, their timestamps are directly comparable — the Fortran regions nest inside the Python region that called them, and `call_stack()` sees a single tree spanning both languages. Build the kernels however you already do (f2py, ctypes, Cython, a hand-written extension), compiling `scope_profiler.f90` in alongside them: ```bash python -m numpy.f2py -c scope_profiler.f90 kernels.f90 -m kernels --backend meson ``` Then let `finalize()` fold the Fortran trace in: ```python import kernels from scope_profiler import ProfileManager ProfileManager.setup(file_path="profiling_data.h5") kernels.kernels.start_profiling("trace", rank) # the Fortran sp_init for step in range(nsteps): with ProfileManager.profile_region("python:step"): kernels.kernels.solve(n) # records its own regions kernels.kernels.stop_profiling() # sp_finalize writes the trace ProfileManager.finalize(native_traces=".") # ...and it lands in the h5 ``` ```text region ranks calls total [s] avg [s] ------------------------------------------------------------ python:step 1 3 0.00148475 0.000494917 python:call_fortran 1 3 0.00147529 0.000491764 fortran:factorize 1 3 0.000860209 0.000286736 fortran:assemble 1 3 0.000593416 0.000197805 ``` Two rules: - **Call the Fortran `sp_finalize()` first.** Its trace has to exist by the time `finalize()` reads it. - **Give the two sides distinct region names.** A name recorded by both raises, rather than silently double-counting a wrapper and the region inside it. A `python:` / `fortran:` prefix is the simplest convention. Under MPI each rank folds in the trace matching its own rank, so pass the rank to `sp_init` and nothing else changes — the usual single output file comes out the other end. ### Combining afterwards If the two halves were profiled separately, merge them after the fact: ```bash scope-profiler import-native traces/ --merge python_only.h5 -o combined.h5 ``` or in Python: ```python from scope_profiler import merge_results, read_h5 from scope_profiler.native_trace import load_traces combined = merge_results(read_h5("python_only.h5"), load_traces("traces/")) ``` ## The trace format `sp_finalize()` writes `_rank.spt`, a small binary file: ``` char[8] "SCOPEPRF" int32 format version (1) int32 rank int64 number of regions per region: int32 length of the name in bytes char[] name int64 number of calls int64[] start timestamps, nanoseconds int64[] end timestamps, nanoseconds ``` Native endianness; the reader detects and handles both. 16 bytes per recorded call, the same as the Python side. ## Limitations - **Not thread safe.** A region must be entered and left by the same thread. OpenMP threads inside a region are fine — wrap the whole parallel construct in one `sp_begin`/`sp_end` from the master thread. - **No LIKWID counters.** Hardware counters are collected only by the Python API; a Fortran trace carries timings alone. - **Names are truncated** at `SP_MAX_NAME` (128) characters. - **Recursion depth** per region is capped at `SP_MAX_DEPTH` (64); deeper nesting is reported on stderr and left untimed rather than corrupting the buffer. - **A region still open at `sp_finalize()`** is reported on stderr and dropped, rather than written with a missing end time.