io, net, http: sendfile support

Speeds up static fileserver, avoiding kernel/userspace copies.

Numbers: downloading 14 MB AppEngine Go SDK with ab (Apache Bench)
with 5 threads:

Before/after numbers:

CPU:
user    0m3.910s
sys     0m23.650s
->
user    0m0.720s
sys     0m4.890s

Time taken for tests:   8.906 seconds
->
Time taken for tests:   8.545 seconds

Percentage of the requests served within a certain time (ms)
50%     44
66%     45
75%     46
80%     46
90%     48
95%     51
98%     59
99%     71
100     74 (longest request)
->
50%     42
66%     43
75%     43
80%     44
90%     46
95%     57
98%     62
99%     63
100%    64 (longest request)

R=iant, gary.burd, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/4543071

1 files changed, 21 insertions, 0 deletions

diff --git a/src/pkg/http/server.go b/src/pkg/http/server.go
index eb5a3a365e..ffeac034ef 100644
--- a/src/pkg/http/server.go
+++ b/src/pkg/http/server.go
@@ -119,6 +119,27 @@ type response struct {
 	closeAfterReply bool
 }
 
+type writerOnly struct {
+	io.Writer
+}
+
+func (r *response) ReadFrom(src io.Reader) (n int64, err os.Error) {
+	// Flush before checking r.chunking, as Flush will call
+	// WriteHeader if it hasn't been called yet, and WriteHeader
+	// is what sets r.chunking.
+	r.Flush()
+	if !r.chunking {
+		if rf, ok := r.conn.rwc.(io.ReaderFrom); ok {
+			n, err = rf.ReadFrom(src)
+			r.written += n
+			return
+		}
+	}
+	// Fall back to default io.Copy implementation.
+	// Use wrapper to hide r.ReadFrom from io.Copy.
+	return io.Copy(writerOnly{r}, src)
+}
+
 // Create new connection from rwc.
 func newConn(rwc net.Conn, handler Handler) (c *conn, err os.Error) {
 	c = new(conn)