From 75ceb98d187d63c3790f6b070d5aa54c6d3aabd9 Mon Sep 17 00:00:00 2001
From: ak2k <19240940+ak2k@users.noreply.github.com>
Date: Sat, 20 Jun 2026 12:08:20 -0400
Subject: [PATCH] perf(extract): compress shards with zstd instead of snappy
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Switch the Parquet writer from snappy to zstd (level 3). On a 184k-work
sample (all 19 relationship tables, 2026-03-29 partition) total shard size
drops 27% (61.6 MB -> 45.2 MB); at higher levels zstd:6 is -31% and zstd:9
-33%. Reads are no slower — pyarrow full-table read and a DuckDB group-by
over work_referenced_works are both within noise of snappy (smaller files
offset the marginally higher decode cost). Extraction is JSON-parse-bound,
so the added compression CPU is <5% of wall-clock at level 3.

The win is mostly storage and HuggingFace upload bandwidth at snapshot
scale. zstd is self-describing per file, so existing snappy shards stay
readable and only newly written shards use zstd — no re-extraction needed.

The worker memory model comment already assumed a zstd compressor; this
makes the code match it.
---
 sync/extract.py | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/sync/extract.py b/sync/extract.py
index 42e8ff7..9ae341f 100644
--- a/sync/extract.py
+++ b/sync/extract.py
@@ -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.
@@ -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
 
@@ -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: