bench: concurrent-read harness against h5py threads and processes

concurrent_read reads one shared File from 1-16 threads: every dataset
in full (distinct datasets per thread) and random hyperslabs of one
dataset, over a deflate and a contiguous file it generates (or reuses
while manifest.json matches). It reports decoded MB/s and scaling
efficiency, warm or --cold (posix_fadvise) page cache, sizes the decode
pool with --decode-threads, and writes JSON.

scripts/concurrent_read_h5py.py runs the same workload on the same files
with h5py threads or spawned processes (same splitmix64 data and slab
stream, checked at spot elements), and compare_concurrent_read.py prints
one table and refuses runs with different workloads. A smoke test runs
all three end to end on tiny files (h5py half honours CLAWHDF5_PYTHON /
CLAWHDF5_REQUIRE_INTEROP).

BENCHMARKS.md gets a "Concurrent reads" section with the commands, marked
not yet measured.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-bench/Cargo.toml

@@ -34,6 +34,10 @@ path = "src/bin/consolidation_efficiency.rs"
 name = "ephemeral_perf"
 path = "src/bin/ephemeral_perf.rs"
 
+[[bin]]
+name = "concurrent_read"
+path = "src/bin/concurrent_read.rs"
+
 [[bin]]
 name = "mpi_io_bench"
 path = "src/bin/mpi_io_bench.rs"
@@ -64,6 +68,10 @@ clawhdf5-io = { path = "../clawhdf5-io" }
 mpi = { version = "0.8", optional = true }
 serde = { workspace = true }
 serde_json = "1"
+# concurrent_read: size the decode pool (--decode-threads) and evict files
+# from the page cache (--cold, posix_fadvise). Both pure Rust / bindings only.
+rayon = "1"
+libc = "0.2"
 tempfile = { workspace = true }
 # Optional: libhdf5 C wrapper for side-by-side comparison (requires system libhdf5).
 # Enable with: cargo bench -p clawhdf5-bench --features libhdf5-compare