Reduced memory allocations in readIndexRange() (#2807)

* Reduced memory allocations in readIndexRange() Signed-off-by: Marco Pracucci <[email protected]> * Updated CHANGELOG Signed-off-by: Marco Pracucci <[email protected]>
thanos-io · Jun 25, 2020 · a9e60f2 · a9e60f2
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commit a9e60f2
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -18,6 +18,7 @@ We use *breaking* word for marking changes that are not backward compatible (rel
 - [#2705](https://github.com/thanos-io/thanos/pull/2705) minio-go: Added support for `af-south-1` and `eu-south-1` regions.
 - [#2728](https://github.com/thanos-io/thanos/pull/2728) Query: Fixed panics when using larger number of replica labels with short series label sets.
 - [#2787](https://github.com/thanos-io/thanos/pull/2787) Update Prometheus mod to pull in prometheus/prometheus#7414.
+- [#2807](https://github.com/thanos-io/thanos/pull/2807) Store: decreased memory allocations while querying block's index.
 
 ### Changed
 

diff --git a/pkg/store/bucket.go b/pkg/store/bucket.go
@@ -1319,11 +1319,15 @@ func (b *bucketBlock) readIndexRange(ctx context.Context, off, length int64) ([]
 	}
 	defer runutil.CloseWithLogOnErr(b.logger, r, "readIndexRange close range reader")
 
-	c, err := ioutil.ReadAll(r)
-	if err != nil {
+	// Preallocate the buffer with the exact size so we don't waste allocations
+	// while progressively growing an initial small buffer. The buffer capacity
+	// is increased by MinRead to avoid extra allocations due to how ReadFrom()
+	// internally works.
+	buf := bytes.NewBuffer(make([]byte, 0, length+bytes.MinRead))
+	if _, err := buf.ReadFrom(r); err != nil {
 		return nil, errors.Wrap(err, "read range")
 	}
-	return c, nil
+	return buf.Bytes(), nil
 }
 
 func (b *bucketBlock) readChunkRange(ctx context.Context, seq int, off, length int64) (*[]byte, error) {