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perf(extract): compress shards with zstd instead of snappy #1

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perf(extract): compress shards with zstd instead of snappy #1

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32 changes: 29 additions & 3 deletions sync/extract.py
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,17 @@
_PROGRESS_INTERVAL = 1_000_000
_DEFAULT_WORKERS = 6

# Parquet compression for extracted shards. zstd at a low level is ~30% smaller
# than snappy on this data (mostly integer ID columns that Parquet already
# dictionary/RLE-encodes, plus the text-heavy abstracts table) while reading at
# least as fast — the smaller files mean less I/O on every downstream scan and,
# at snapshot scale, materially less storage and HuggingFace upload bandwidth.
# Level 3 is the balance point: levels above it buy only a few more percent for
# a growing CPU cost, and extraction is JSON-parse-bound so the codec barely
# moves wall-clock either way.
_PARQUET_COMPRESSION = "zstd"
_PARQUET_COMPRESSION_LEVEL = 3

# Memory model: reserve ~2 GB for the OS and assume each worker peaks
# around 6 GB resident (zstd compressor + PyArrow batch + Python heap).
# Empirically, 8 workers on 16 GB causes swap thrashing; 8 on 64 GB is fine.
Expand Down Expand Up @@ -899,7 +910,12 @@ def _ensure_writer(self) -> pq.ParquetWriter:
write_dir = self.staging_dir if self.staging_dir else self.output_dir
write_dir.mkdir(parents=True, exist_ok=True)
out_path = write_dir / self._shard_name
self._writer = pq.ParquetWriter(out_path, self.schema, compression="snappy")
self._writer = pq.ParquetWriter(
out_path,
self.schema,
compression=_PARQUET_COMPRESSION,
compression_level=_PARQUET_COMPRESSION_LEVEL,
)
self._current_path = out_path
return self._writer

Expand All @@ -925,13 +941,23 @@ def close(self) -> None:
out_path = write_dir / self._shard_name
# Write empty parquet as completion marker
if self.schema is not None:
with pq.ParquetWriter(out_path, self.schema, compression="snappy") as w:
with pq.ParquetWriter(
out_path,
self.schema,
compression=_PARQUET_COMPRESSION,
compression_level=_PARQUET_COMPRESSION_LEVEL,
) as w:
pass # empty file with schema
else:
# No data was ever written — create a minimal empty parquet
empty_schema = pa.schema([pa.field("_placeholder", pa.int64())])
empty_table = pa.table({"_placeholder": pa.array([], type=pa.int64())})
pq.write_table(empty_table, out_path, compression="snappy")
pq.write_table(
empty_table,
out_path,
compression=_PARQUET_COMPRESSION,
compression_level=_PARQUET_COMPRESSION_LEVEL,
)
self._current_path = out_path

if self._writer is not None:
Expand Down