Brad Fitzpatrick (Gerrit)

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net/http/internal/http2: don't pin frame-sized write buffers on idle conns

A Framer's wbuf grows to the size of the largest frame written
(typically a full DATA frame) and was retained for the life of the
connection, pinning several kB per idle connection.

The frame is fully written out by the end of endWrite, so release
large write buffers to a sync.Pool there and have startWrite adopt a
pooled buffer for the next frame. Buffers up to 1 kB stay attached to
their Framer, so connections writing only small control frames don't
pay for the pooling.

Measured with 1000 idle HTTP/2 connections on separate TCP flows
after POST exchanges with 4 kB responses (heap bytes per connection,
runtime.MemProfileRate=1, HPACK dynamic tables disabled, and with the
corresponding read buffer fix already applied):

	HeapAlloc/conn:  15265 B  ->  10377 B
	HeapInuse/conn:  20357 B  ->  15114 B

http2 benchmarks are unchanged within noise. The largest remaining
consumers are the serverConn struct (~1.8 kB), TLS cipher state
(~1.8 kB), and the tls.Conn struct (~0.9 kB).

Updates #80735

Change-Id: I7c1e08e2f6df944d34a2ef7a5f5091b1d3648b95

Change diff

diff --git a/src/net/http/internal/http2/frame.go b/src/net/http/internal/http2/frame.go
index 4632259..5567c2a 100644
--- a/src/net/http/internal/http2/frame.go
+++ b/src/net/http/internal/http2/frame.go
@@ -310,8 +310,16 @@
 
 	maxWriteSize uint32 // zero means unlimited; TODO: implement
 
-	w    io.Writer
+	w io.Writer
+	// wbuf is the frame currently being written, from startWrite's
+	// header through endWrite backfilling the length and writing it
+	// out. endWrite releases large buffers to writeBufPool so that
+	// idle connections don't pin a frame-sized buffer.
 	wbuf []byte
+	// wbufP, if non-nil, is the pooled container holding wbuf's array,
+	// to be returned to writeBufPool by endWrite. It is nil when wbuf
+	// is small (under maxIdleWriteBufCap) and not worth pooling.
+	wbufP *[]byte
 
 	// AllowIllegalWrites permits the Framer's Write methods to
 	// write frames that do not conform to the HTTP/2 spec. This
@@ -372,7 +380,27 @@
 	return fr.MaxHeaderValueCount
 }
 
+// writeBufPool pools the frame-sized buffers that back Framer.wbuf.
+// endWrite returns the buffer to the pool once the frame has been
+// written out, so that idle connections don't each pin a buffer sized
+// by the last frame they wrote.
+var writeBufPool sync.Pool
+
+// maxIdleWriteBufCap is the largest wbuf capacity that a Framer keeps
+// between frame writes. It is large enough for control frames and
+// small HEADERS frames, so that only connections writing larger frames
+// pay for the buffer pooling.
+const maxIdleWriteBufCap = 1024
+
 func (f *Framer) startWrite(ftype FrameType, flags Flags, streamID uint32) {
+	if f.wbuf == nil {
+		// endWrite released the previous buffer; start with a pooled
+		// one if available. Appends below grow it as needed.
+		if bp, _ := writeBufPool.Get().(*[]byte); bp != nil {
+			f.wbufP = bp
+			f.wbuf = (*bp)[:0]
+		}
+	}
 	// Write the FrameHeader.
 	f.wbuf = append(f.wbuf[:0],
 		0, // 3 bytes of length, filled in endWrite
@@ -405,9 +433,31 @@
 	if err == nil && n != len(f.wbuf) {
 		err = io.ErrShortWrite
 	}
+	f.releaseWriteBuf()
 	return err
 }
 
+// releaseWriteBuf returns a large wbuf, whose frame has been written
+// out, to writeBufPool, so that a connection idle between writes does
+// not pin a frame-sized buffer. Small buffers stay attached to the
+// Framer.
+func (f *Framer) releaseWriteBuf() {
+	if cap(f.wbuf) <= maxIdleWriteBufCap {
+		return
+	}
+	bp := f.wbufP
+	if bp == nil {
+		bp = new([]byte)
+	}
+	// If appends outgrew a pooled array, the container is recycled
+	// with the new larger array and the old one is dropped, so the
+	// pool converges toward large buffers.
+	*bp = f.wbuf[:0]
+	writeBufPool.Put(bp)
+	f.wbufP = nil
+	f.wbuf = nil
+}
+
 func (f *Framer) logWrite() {
 	if f.debugFramer == nil {
 		f.debugFramerBuf = new(bytes.Buffer)
diff --git a/src/net/http/internal/http2/frame_test.go b/src/net/http/internal/http2/frame_test.go
index 009e0ad..2d3318b 100644
--- a/src/net/http/internal/http2/frame_test.go
+++ b/src/net/http/internal/http2/frame_test.go
@@ -1723,3 +1723,45 @@
 		t.Error("accessing previous frame's Data after failed ReadFrame did not panic")
 	}
 }
+
+func TestFramerWriteBufReleased(t *testing.T) {
+	fr, buf := testFramer()
+	if err := fr.WriteData(1, false, make([]byte, 2*maxIdleWriteBufCap)); err != nil {
+		t.Fatal(err)
+	}
+	if fr.wbuf != nil || fr.wbufP != nil {
+		t.Errorf("after large write: wbuf = %v (cap %d), wbufP = %v; want both released",
+			fr.wbuf != nil, cap(fr.wbuf), fr.wbufP != nil)
+	}
+
+	// Subsequent writes still produce well-formed frames.
+	if err := fr.WritePing(false, [8]byte{1, 2, 3, 4, 5, 6, 7, 8}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := fr.ReadFrame(); err != nil { // the DATA frame
+		t.Fatal(err)
+	}
+	f, err := fr.ReadFrame()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := f.(*PingFrame); !ok {
+		t.Errorf("second frame = %T, want *PingFrame", f)
+	}
+	_ = buf
+}
+
+func TestFramerWriteBufSmallKept(t *testing.T) {
+	fr, _ := testFramer()
+	fr.wbuf = make([]byte, 0, 64) // small buffer stays attached across writes
+	if err := fr.WritePing(false, [8]byte{}); err != nil {
+		t.Fatal(err)
+	}
+	if fr.wbuf == nil || cap(fr.wbuf) != 64 {
+		t.Errorf("after small write: cap(wbuf) = %d (nil=%v), want the seeded 64-byte buffer kept",
+			cap(fr.wbuf), fr.wbuf == nil)
+	}
+	if fr.wbufP != nil {
+		t.Error("after small write: wbufP != nil, want nil")
+	}
+}

Change information

Files:

M src/net/http/internal/http2/frame.go
M src/net/http/internal/http2/frame_test.go

Change size: M

Delta: 2 files changed, 93 insertions(+), 1 deletion(-)

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Brad Fitzpatrick (Gerrit)

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Sep 5, 2026, 6:45:36 PMSep 5

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File src/net/http/internal/http2/frame.go

Line 313, Patchset 1 (Latest): w io.Writer

Damien Neil . unresolved

We've got a []byte write buffer here, but this is always a *bufio.Writer (or a *bufferedWriter which contains a *bufio.Writer).

Too many buffers!

Anyway, this change seems like an improvement over what we have, but I'm pretty sure we could make this all much better.

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Brad Fitzpatrick (Gerrit)

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File src/net/http/internal/http2/frame.go

Line 313, Patchset 1 (Latest): w io.Writer

Damien Neil . resolved

We've got a []byte write buffer here, but this is always a *bufio.Writer (or a *bufferedWriter which contains a *bufio.Writer).
Too many buffers!
Anyway, this change seems like an improvement over what we have, but I'm pretty sure we could make this all much better.

Brad Fitzpatrick

Agreed. I wasn't sure how much appetite you had for churn here. :)

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Brad Fitzpatrick submitted the change

Change information

Commit message:

net/http/internal/http2: don't pin frame-sized write buffers on idle conns

A Framer's wbuf grows to the size of the largest frame written
(typically a full DATA frame) and was retained for the life of the
connection, pinning several kB per idle connection.

The frame is fully written out by the end of endWrite, so release
large write buffers to a sync.Pool there and have startWrite adopt a
pooled buffer for the next frame. Buffers up to 1 kB stay attached to
their Framer, so connections writing only small control frames don't
pay for the pooling.

Measured with 1000 idle HTTP/2 connections on separate TCP flows
after POST exchanges with 4 kB responses (heap bytes per connection,
runtime.MemProfileRate=1, HPACK dynamic tables disabled, and with the
corresponding read buffer fix already applied):

	HeapAlloc/conn:  15265 B  ->  10377 B
	HeapInuse/conn:  20357 B  ->  15114 B

http2 benchmarks are unchanged within noise. The largest remaining
consumers are the serverConn struct (~1.8 kB), TLS cipher state
(~1.8 kB), and the tls.Conn struct (~0.9 kB).

Updates #80735

Change-Id: I7c1e08e2f6df944d34a2ef7a5f5091b1d3648b95

Reviewed-on: https://go-review.googlesource.com/c/go/+/828206

LUCI-TryBot-Result: golang...@luci-project-accounts.iam.gserviceaccount.com <golang...@luci-project-accounts.iam.gserviceaccount.com>

Reviewed-by: Damien Neil <dn...@google.com>

Reviewed-by: Nicholas Husin <hu...@google.com>

Reviewed-by: Nicholas Husin <n...@golang.org>

Files:

M src/net/http/internal/http2/frame.go
M src/net/http/internal/http2/frame_test.go

Change size: M

Delta: 2 files changed, 93 insertions(+), 1 deletion(-)

Branch: refs/heads/master

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[go] net/http/internal/http2: don't pin frame-sized write buffers on idle conns

Brad Fitzpatrick (Gerrit)

Brad Fitzpatrick has uploaded the change for review

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Brad Fitzpatrick (Gerrit)

Brad Fitzpatrick voted Commit-Queue+1

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Brad Fitzpatrick (Gerrit)

Brad Fitzpatrick added 1 comment

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Nicholas Husin voted Code-Review+2

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Nicholas Husin voted Code-Review+1

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Brad Fitzpatrick (Gerrit)

Brad Fitzpatrick submitted the change

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