Fixed a output seismograph file bug in GPU mode

[MengYuanzhuo]

• Fixed a bug in GPU mode where, when NTSTEP_BETWEEN_OUTPUT_SAMPLE was not 1, the output seismograph file appended the previous data at the end, thereby overwriting the original data.

• Before repair, single-channel data when NTSTEP_BETWEEN_OUTPUT_SAMPLE = 1.

• Before repair, single-channel data when NTSTEP_BETWEEN_OUTPUT_SAMPLE = 10.

• (There is some repetition of the data above at the end of the data, which overwrites the original data.)

• After repair, single-channel data when NTSTEP_BETWEEN_OUTPUT_SAMPLE = 10.

[Copilot]

Pull request overview

Fixes incorrect seismogram output in GPU mode when seismograms are written in chunks (e.g., when NTSTEP_BETWEEN_OUTPUT_SAMPLE is not 1) by ensuring device-side seismogram buffers are flushed to host before file output.

Changes:

• Flush GPU seismogram buffers to host immediately before write_seismograms_to_file() runs.
• Adjust CUDA seismogram transfer logic to copy only the “valid” portion of the buffer.
• Add a CPU-only stub for the new CUDA flush entry point for non-CUDA builds.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/specfem2D/write_seismograms.F90	Calls a new GPU flush routine before writing seismograms to disk.
src/gpu/write_seismograms_cuda.cu	Implements flush_seismograms_cuda() and changes device→host transfer strategy.
src/gpu/specfem2D_gpu_cuda_method_stubs.c	Adds a no-op stub for flush_seismograms_cuda() when CUDA is not enabled.

Comments suppressed due to low confidence (1)

src/gpu/write_seismograms_cuda.cu:276

• The subsequent host->Fortran copy loops over j=0..nlength_seismogram-1 for every receiver even when only valid_length samples were copied from the device. This does extra work and also makes the earlier host-buffer zeroing necessary. Consider copying/converting only the first valid_length samples (or seismo_current samples in flush), leaving the unused tail untouched.

    // copies values into host array
    for (int irec=0; irec < mp->nrec_local; irec++){
      for (int j=0; j < nlength_seismogram; j++){
        sisux[j + nlength_seismogram * irec] = (double) h_seismo[j + nlength_seismogram * irec];
        sisuz[j + nlength_seismogram * irec] = (double) h_seismo[j + nlength_seismogram * irec + size];
      }

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[danielpeter]

thanks for the contribution! 🙏