gather_storage merged only runs that touch, so a strided selection of a
contiguous dataset over a Storage became one range (and one owned Vec) per
element: a stride-2 read of 32M f32 through File::open_storage made
16,777,232 read_at calls, took 2.0 s and peaked at 2.09 GB.
The selection is now walked twice. The first walk checks the runs and plans
spans: runs in increasing order at most 4 KiB apart (GATHER_GAP_BYTES) are
read as one span up to 8 MiB (GATHER_SPAN_BYTES; a longer run is split), so
nothing is stored per run. The spans are fetched in RAW_BATCH_BYTES batches
while the second walk copies each run out of its span. Same checks and
errors as before.
The same read is now 32 reads and 0.31 s (File::open: 0.08 s).
contiguous_read_interop: every h5py-checked selection is also read through
File::open_storage and must give libhdf5's bytes; a new test bounds the
range reads of strided, blocked, column and point selections (stride 2: at
most 1 data read; 563,200 before).
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
