perf: parallel range-GET S3 downloads for large objects

internal/cache/s3.go

@@ -263,13 +263,13 @@ func (s *S3) Open(ctx context.Context, key Key) (io.ReadCloser, http.Header, err
 		}
 	}
 
-	// Get object
-	obj, err := s.client.GetObject(ctx, s.config.Bucket, objectName, minio.GetObjectOptions{})
+	// Download object using parallel range-GET for large objects.
+	reader, err := s.parallelGetReader(ctx, s.config.Bucket, objectName, objInfo.Size)
 	if err != nil {
-		return nil, nil, errors.Errorf("failed to get object: %w", err)
+		return nil, nil, err
 	}
 
-	return &s3Reader{obj: obj}, headers, nil
+	return reader, headers, nil
 }
 
 // refreshExpiration updates the Expires-At metadata on an S3 object using

internal/cache/s3_parallel_get.go

@@ -0,0 +1,116 @@
+package cache
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"github.com/alecthomas/errors"
+	"github.com/minio/minio-go/v7"
+)
+
+const (
+	// s3DownloadChunkSize is the size of each parallel range-GET request.
+	// 32 MiB matches the gradle-cache-tool's benchmarked default.
+	s3DownloadChunkSize = 32 << 20
+	// s3DownloadWorkers is the number of concurrent range-GET requests.
+	// Benchmarking showed no throughput difference from 4 to 128 workers
+	// (extraction IOPS is the bottleneck), so 8 keeps connection count low.
+	s3DownloadWorkers = 8
+)
+
+// parallelGetReader returns an io.ReadCloser that downloads the S3 object
+// using parallel range-GET requests and reassembles chunks in order.
+// For objects smaller than one chunk, it falls back to a single GetObject.
+func (s *S3) parallelGetReader(ctx context.Context, bucket, objectName string, size int64) (io.ReadCloser, error) {
+	if size <= s3DownloadChunkSize {
+		// Small object: single stream.
+		obj, err := s.client.GetObject(ctx, bucket, objectName, minio.GetObjectOptions{})
+		if err != nil {
+			return nil, errors.Errorf("failed to get object: %w", err)
+		}
+		return &s3Reader{obj: obj}, nil
+	}
+
+	// Large object: parallel range requests reassembled in order via io.Pipe.
+	pr, pw := io.Pipe()
+	go func() {
+		pw.CloseWithError(s.parallelGet(ctx, bucket, objectName, size, pw))
+	}()
+	return pr, nil
+}
+
+// parallelGet downloads an S3 object in parallel chunks and writes them in
+// order to w. Each worker downloads its chunk into memory so the TCP
+// connection stays active at full speed. Peak memory: numWorkers × chunkSize.
+func (s *S3) parallelGet(ctx context.Context, bucket, objectName string, size int64, w io.Writer) error {
+	numChunks := int((size + s3DownloadChunkSize - 1) / s3DownloadChunkSize)
+	numWorkers := min(s3DownloadWorkers, numChunks)
+
+	type chunkResult struct {
+		data []byte
+		err  error
+	}
+
+	// One buffered channel per chunk so workers never block after reading.
+	results := make([]chan chunkResult, numChunks)
+	for i := range results {
+		results[i] = make(chan chunkResult, 1)
+	}
+
+	// Work queue of chunk indices.
+	work := make(chan int, numChunks)
+	for i := range numChunks {
+		work <- i
+	}
+	close(work)
+
+	var wg sync.WaitGroup
+	for range numWorkers {
+		wg.Go(func() {
+			for seq := range work {
+				start := int64(seq) * s3DownloadChunkSize
+				end := min(start+s3DownloadChunkSize-1, size-1)
+
+				opts := minio.GetObjectOptions{}
+				if err := opts.SetRange(start, end); err != nil {
+					results[seq] <- chunkResult{err: errors.Errorf("set range %d-%d: %w", start, end, err)}
+					continue
+				}
+
+				obj, err := s.client.GetObject(ctx, bucket, objectName, opts)
+				if err != nil {
+					results[seq] <- chunkResult{err: errors.Errorf("get range %d-%d: %w", start, end, err)}
+					continue
+				}
+
+				// Drain the body immediately so the TCP connection stays at
+				// full speed. All workers do this concurrently, saturating
+				// the available S3 bandwidth.
+				data, readErr := io.ReadAll(obj)
+				obj.Close() //nolint:errcheck,gosec
+				results[seq] <- chunkResult{data: data, err: readErr}
+			}
+		})
+	}
+
+	// Write chunks in order. Each receive blocks until that chunk's worker
+	// finishes, while other workers continue downloading concurrently.
+	var writeErr error
+	for _, ch := range results {
+		r := <-ch
+		if writeErr != nil {
+			continue // drain remaining channels so goroutines can exit
+		}
+		if r.err != nil {
+			writeErr = r.err
+			continue
+		}
+		if _, err := w.Write(r.data); err != nil {
+			writeErr = err
+		}
+	}
+
+	wg.Wait()
+	return writeErr
+}