perf(http): use 64 KiB connection read buffers (#1588)

internal/protocol/http/fetcher.go

@@ -39,6 +39,11 @@ const (
 	stealMinChunkSize     = 512 * 1024 // Min steal size: 512KB (avoid tiny chunks)
 )
 
+// connReadBufferSize is the read buffer each connection allocates once and
+// reuses for its lifetime. 64 KiB keeps per-read overhead low on fast links;
+// at 256 connections it is 16 MiB per task.
+const connReadBufferSize = 64 * 1024
+
 var (
 	errRangeRequestIgnored  = errors.New("server ignored HTTP range request")
 	errInvalidRangeResponse = errors.New("invalid HTTP range response")
@@ -1136,7 +1141,7 @@ func (f *Fetcher) runConnection(conn *connection) {
 
 	// Use fast-fail client for quick retry during download phase
 	client := f.buildFastFailClient()
-	buf := make([]byte, 8192)
+	buf := make([]byte, connReadBufferSize)
 
 	retries := 0
 	f.connMu.Lock()
@@ -1798,7 +1803,7 @@ func (f *Fetcher) runConnectionWithResolveResp(conn *connection) {
 	conn.State = connConnecting
 	f.connMu.Unlock()
 
-	buf := make([]byte, 8192)
+	buf := make([]byte, connReadBufferSize)
 
 	// Get the resolve response
 	f.resolveRespLock.Lock()
@@ -1892,7 +1897,7 @@ func (f *Fetcher) runConnectionWithResolveResp(conn *connection) {
 func (f *Fetcher) runConnectionFallback(conn *connection) {
 	// Use fast-fail client for quick retry during download phase
 	client := f.buildFastFailClient()
-	buf := make([]byte, 8192)
+	buf := make([]byte, connReadBufferSize)
 
 	retries := 0
 	countedRetries := 0