LineReader: Reuse temporary buffer to reduce per-line allocation (#27782

)

## What does this PR do?

Previously, the `LineReader` would allocate a []byte of size `config.BufferSize` before decoding each line. The underlying array's size allocation is fixed, so `outBuffer.Append` retains all of it even when the appended bytes are much shorter.

With this change, we store a single `tempBuffer []byte` which is reused across lines anywhere we need temporary storage. Converting to `outBuffer.Write` forces the buffer to copy data out of tempBuffer, but is able to only allocate space for the written bytes.

## Why is it important?

In our production environment, we run beats with k8s-enforced memory limits and are trying to resolve OOMs. The LineReader code path contributes a significant amount of memory allocation. The benchmarks added in bench_test.go show this reduces the memory profile with various line lengths:

```
goos: darwin
goarch: amd64
pkg: github.com/elastic/beats/v7/libbeat/reader/readfile
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz

name                                 old time/op          new time/op          delta
EncoderReader/buffer-sized_lines-16           125µs ± 3%            94µs ± 9%  -24.55%  (p=0.008 n=5+5)
EncoderReader/short_lines-16                 52.6µs ± 4%          36.3µs ±10%  -30.88%  (p=0.008 n=5+5)
EncoderReader/long_lines-16                  1.82ms ± 2%          1.70ms ±10%     ~     (p=0.151 n=5+5)
EncoderReader/skip_lines-16                   133µs ± 3%           140µs ± 8%     ~     (p=0.151 n=5+5)

name                                 old alloc/op         new alloc/op         delta
EncoderReader/buffer-sized_lines-16           442kB ± 0%           239kB ± 0%  -46.07%  (p=0.000 n=4+5)
EncoderReader/short_lines-16                  118kB ± 0%            15kB ± 0%  -87.27%  (p=0.008 n=5+5)
EncoderReader/long_lines-16                  8.73MB ± 0%          7.63MB ± 0%  -12.62%  (p=0.000 n=4+5)
EncoderReader/skip_lines-16                   270kB ± 0%           220kB ± 0%  -18.58%  (p=0.008 n=5+5)

name                                 old allocs/op        new allocs/op        delta
EncoderReader/buffer-sized_lines-16             718 ± 0%             519 ± 0%  -27.72%  (p=0.008 n=5+5)
EncoderReader/short_lines-16                    522 ± 0%             421 ± 0%  -19.35%  (p=0.008 n=5+5)
EncoderReader/long_lines-16                   2.65k ± 0%           1.58k ± 0%  -40.54%  (p=0.008 n=5+5)
EncoderReader/skip_lines-16                     420 ± 0%             419 ± 0%   -0.24%  (p=0.008 n=5+5)
```

CHANGELOG.next.asciidoc

@@ -747,6 +747,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Added support for parsing syslog dates containing a leading 0 (e.g. `Sep 01`) rather than a space. {pull}27775[27775]
 - Add base64 Encode functionality to httpjson input. {pull}27681[27681]
 - Add `join` and `sprintf` functions to `httpjson` input. {pull}27735[27735]
+- Improve memory usage of line reader of `log` and `filestream` input. {pull}27782[27782]
 
 
 *Heartbeat*

libbeat/reader/readfile/bench_test.go

@@ -0,0 +1,83 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package readfile
+
+import (
+	"bytes"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"math/rand"
+	"testing"
+
+	"golang.org/x/text/encoding"
+)
+
+func BenchmarkEncoderReader(b *testing.B) {
+	const (
+		bufferSize   = 1024
+		lineMaxLimit = 1000000 // never hit by the input data
+	)
+
+	runBench := func(name string, lineMaxLimit int, lines []byte) {
+		b.Run(name, func(b *testing.B) {
+			b.ReportAllocs()
+			for bN := 0; bN < b.N; bN++ {
+				reader, err := NewEncodeReader(ioutil.NopCloser(bytes.NewReader(lines)), Config{encoding.Nop, bufferSize, LineFeed, lineMaxLimit})
+				if err != nil {
+					b.Fatal("failed to initialize reader:", err)
+				}
+				// Read decodec lines and test
+				size := 0
+				for i := 0; ; i++ {
+					msg, err := reader.Next()
+					if err != nil {
+						if err == io.EOF {
+							b.ReportMetric(float64(i), "processed_lines")
+							break
+						} else {
+							b.Fatal("unexpected error:", err)
+						}
+					}
+					size += msg.Bytes
+				}
+				b.ReportMetric(float64(size), "processed_bytes")
+			}
+		})
+	}
+
+	runBench("buffer-sized lines", lineMaxLimit, createBenchmarkLines(100, 1020))
+	runBench("short lines", lineMaxLimit, createBenchmarkLines(100, 10))
+	runBench("long lines", lineMaxLimit, createBenchmarkLines(100, 10_000))
+	// short lineMaxLimit to exercise skipUntilNewLine
+	runBench("skip lines", 1024, createBenchmarkLines(100, 10_000))
+}
+
+func createBenchmarkLines(numLines int, lineLength int) []byte {
+	buf := bytes.NewBuffer(nil)
+	for i := 0; i < numLines; i++ {
+		line := make([]byte, hex.DecodedLen(lineLength))
+		if _, err := rand.Read(line); err != nil {
+			panic(fmt.Sprintf("failed to generate random input: %v", err))
+		}
+		buf.WriteString(hex.EncodeToString(line))
+		buf.WriteRune('\n')
+	}
+	return buf.Bytes()
+}

libbeat/reader/readfile/line.go

@@ -30,12 +30,11 @@ import (
 
 const unlimited = 0
 
-// lineReader reads lines from underlying reader, decoding the input stream
+// LineReader reads lines from underlying reader, decoding the input stream
 // using the configured codec. The reader keeps track of bytes consumed
 // from raw input stream for every decoded line.
 type LineReader struct {
 	reader     io.ReadCloser
-	bufferSize int
 	maxBytes   int // max bytes per line limit to avoid OOM with malformatted files
 	nl         []byte
 	decodedNl  []byte
@@ -44,10 +43,11 @@ type LineReader struct {
 	inOffset   int // input buffer read offset
 	byteCount  int // number of bytes decoded from input buffer into output buffer
 	decoder    transform.Transformer
+	tempBuffer []byte
 	logger     *logp.Logger
 }
 
-// New creates a new reader object
+// NewLineReader creates a new reader object
 func NewLineReader(input io.ReadCloser, config Config) (*LineReader, error) {
 	encoder := config.Codec.NewEncoder()
 
@@ -64,13 +64,13 @@ func NewLineReader(input io.ReadCloser, config Config) (*LineReader, error) {
 
 	return &LineReader{
 		reader:     input,
-		bufferSize: config.BufferSize,
 		maxBytes:   config.MaxBytes,
 		decoder:    config.Codec.NewDecoder(),
 		nl:         nl,
 		decodedNl:  terminator,
 		inBuffer:   streambuf.New(nil),
 		outBuffer:  streambuf.New(nil),
+		tempBuffer: make([]byte, config.BufferSize),
 		logger:     logp.NewLogger("reader_line"),
 	}, nil
 }
@@ -133,18 +133,17 @@ func (r *LineReader) advance() error {
 			r.inOffset = newOffset
 		}
 
-		buf := make([]byte, r.bufferSize)
-
 		// Try to read more bytes into buffer
-		n, err := r.reader.Read(buf)
+		n, err := r.reader.Read(r.tempBuffer)
 
 		if err == io.EOF && n > 0 {
 			// Continue processing the returned bytes. The next call will yield EOF with 0 bytes.
 			err = nil
 		}
 
-		// Appends buffer also in case of err
-		r.inBuffer.Append(buf[:n])
+		// Write to buffer also in case of err
+		r.inBuffer.Write(r.tempBuffer[:n])
+
 		if err != nil {
 			return err
 		}
@@ -170,7 +169,7 @@ func (r *LineReader) advance() error {
 
 			// If newLine is not found and the incoming data buffer exceeded max bytes limit, then skip until the next newLine
 			if idx == -1 && r.inBuffer.Len() > r.maxBytes {
-				skipped, err := r.skipUntilNewLine(buf)
+				skipped, err := r.skipUntilNewLine()
 				if err != nil {
 					r.logger.Error("Error skipping until new line, err:", err)
 					return err
@@ -204,7 +203,7 @@ func (r *LineReader) advance() error {
 	return err
 }
 
-func (r *LineReader) skipUntilNewLine(buf []byte) (int, error) {
+func (r *LineReader) skipUntilNewLine() (int, error) {
 	// The length of the line skipped
 	skipped := r.inBuffer.Len()
 
@@ -221,14 +220,14 @@ func (r *LineReader) skipUntilNewLine(buf []byte) (int, error) {
 
 	// Read until the new line is found
 	for idx := -1; idx == -1; {
-		n, err := r.reader.Read(buf)
+		n, err := r.reader.Read(r.tempBuffer)
 
 		// Check bytes read for newLine
 		if n > 0 {
-			idx = bytes.Index(buf[:n], r.nl)
+			idx = bytes.Index(r.tempBuffer[:n], r.nl)
 
 			if idx != -1 {
-				r.inBuffer.Append(buf[idx+len(r.nl) : n])
+				r.inBuffer.Write(r.tempBuffer[idx+len(r.nl) : n])
 				skipped += idx
 			} else {
 				skipped += n
@@ -249,14 +248,13 @@ func (r *LineReader) skipUntilNewLine(buf []byte) (int, error) {
 
 func (r *LineReader) decode(end int) (int, error) {
 	var err error
-	buffer := make([]byte, 1024)
 	inBytes := r.inBuffer.Bytes()
 	start := 0
 
 	for start < end {
 		var nDst, nSrc int
 
-		nDst, nSrc, err = r.decoder.Transform(buffer, inBytes[start:end], false)
+		nDst, nSrc, err = r.decoder.Transform(r.tempBuffer, inBytes[start:end], false)
 		if err != nil {
 			// Check if error is different from destination buffer too short
 			if err != transform.ErrShortDst {
@@ -270,7 +268,7 @@ func (r *LineReader) decode(end int) (int, error) {
 		}
 
 		start += nSrc
-		r.outBuffer.Write(buffer[:nDst])
+		r.outBuffer.Write(r.tempBuffer[:nDst])
 	}
 
 	r.byteCount += start