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Batch-allocate per-record objects; reuse CSV writer field buffer

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After batch-arena field allocation, profiling cat over 1M-record CSV showed
the remaining ~5M allocations were almost entirely per-record (one each):
the Mlrmap struct, the RecordAndContext wrapper, the CSV writer's []string,
and the go-csv parser's own buffers.

Address the first three:

- mlrval.RecordArena gains NewRecord(), vending the Mlrmap struct itself from
  a per-batch slab (respecting --no-hash-records). Rolled out to every
  line-based reader (CSV, CSV-lite, TSV, DKVP, NIDX, PPRINT, XTAB, DKVPX) in
  place of NewMlrmapAsRecord.

- The CSV reader batch-allocates RecordAndContext wrappers from a per-batch
  slab instead of one heap object per record (comment/output-string entries
  still allocate individually, but they are rare).

- RecordWriterCSV reuses a single fieldsBuffer []string across records instead
  of allocating one per Write; WriteCSVRecordMaybeColorized consumes it
  synchronously and the writer is single-goroutine, so this is safe.

Effect (big.*, 1M records, cat, best of 5):
  csv   0.26 -> 0.22
  dkvp  0.51 -> 0.45  (Mlrmap slab)

For CSV, cat's allocation-object count drops ~5.0M -> ~2.1M. The remaining
~2M are the go-csv parser's per-record backing string and field slice, which
are intrinsic to parsing and would require a zero-copy/batch-slab parser
rework. A CPU profile of cat now shows it is I/O-bound (syscall ~56%, bufio
read+flush), with allocation/GC down to ~10% -- i.e. further allocation
trimming no longer moves cat's wall-clock. GOGC=off confirms (no change).

Verified: go test ./pkg/... and full regression suite pass; output is
byte-identical across all formats including record-retaining verbs (tac),
hashed and --no-hash-records.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
John Kerl 2026-06-19 12:27:22 -04:00
parent 55287b0b5c
commit d2acbfeea3
9 changed files with 60 additions and 14 deletions
Download patch file Download diff file Expand all files Collapse all files

6
pkg/input/record_reader_pprint.go
View file

@ -239,7 +239,7 @@ func (reader *RecordReaderPprintFixedSplit) getRecords(
}
}
record := mlrval.NewMlrmapAsRecord()
record := arena.NewRecord()
if !reader.readerOptions.AllowRaggedCSVInput {
for i, field := range fields {
arena.PutDeferred(record, reader.headerStrings[i], field, dedupeFieldNames)
@ -472,7 +472,7 @@ func getRecordBatchExplicitPprintHeader(
return
}
record := mlrval.NewMlrmapAsRecord()
record := arena.NewRecord()
if !reader.readerOptions.AllowRaggedCSVInput {
for i, field := range fields {
arena.PutDeferred(record, reader.headerStrings[i], field, dedupeFieldNames)
@ -596,7 +596,7 @@ func getRecordBatchImplicitPprintHeader(
}
}
record := mlrval.NewMlrmapAsRecord()
record := arena.NewRecord()
if !reader.readerOptions.AllowRaggedCSVInput {
for i, field := range fields {
arena.PutDeferred(record, reader.headerStrings[i], field, dedupeFieldNames)