Re: pasta slow at HTTP upload

[Stefano Brivio <sbrivio@redhat.com>]

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On Wed, 12 Nov 2025 07:55:48 +0100
Stefano Brivio <sbrivio@redhat.com> wrote:

> Hi Max,
> 
> On Tue, 11 Nov 2025 23:11:17 -0700
> Max Chernoff <git@maxchernoff.ca> wrote:
> 
> > Hi,
> > 
> > For the past few months, I've noticed that my HTTP uploads from
> > containers are really slow. Reproduction:
> > 
> > [...]
> >
> > - The host that I ran this from is a VM in a datacenter, supposedly with
> >   a symmetrical 1Gb/s connection. Pinging github.com shows an RTT of
> >   2ms, and pinging www.ctan.org shows an RTT of 100ms.  
> 
> Thanks for reporting this. Coincidentally, I'm currently debugging an
> issue that looks similar to this, and I should have some patches for
> you to test in a couple of days, I'll keep you posted.

Hmm, actually, I have a hack that's not quite correct (we should make
ACK_INTERVAL adaptive instead, which is one of the other bits I'm
working on), but if it fixes the issue for you, it should at least mean
that we're talking about the same issue.

Patch attached. Can you give that a try?

-- 
Stefano

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diff --git a/tcp.c b/tcp.c
index e91c0cf..697f80d 100644
--- a/tcp.c
+++ b/tcp.c
@@ -339,7 +339,7 @@ enum {
 #define MSS_DEFAULT			536
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
-#define ACK_INTERVAL			10		/* ms */
+#define ACK_INTERVAL			1		/* ms */
 #define SYN_TIMEOUT			10		/* s */
 #define ACK_TIMEOUT			2
 #define FIN_TIMEOUT			60
@@ -1097,9 +1097,22 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
 	if ((conn->flags & LOCAL) || tcp_rtt_dst_low(conn)) {
 		new_wnd_to_tap = tinfo->tcpi_snd_wnd;
 	} else {
+		uint32_t sendq;
+		int limit;
+
+		ioctl(s, SIOCOUTQ, &sendq);
 		tcp_get_sndbuf(conn);
+
+		if ((int)sendq > SNDBUF_GET(conn))
+			limit = 0;
+		else
+			limit = SNDBUF_GET(conn) - (int)sendq;
+
+		if (limit < MSS_GET(conn))
+			limit = 0;
+
 		new_wnd_to_tap = MIN((int)tinfo->tcpi_snd_wnd,
-				     SNDBUF_GET(conn));
+				     limit);
 	}
 
 	new_wnd_to_tap = MIN(new_wnd_to_tap, MAX_WINDOW);
@@ -1240,7 +1253,8 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
 	th->fin = !!(flags & FIN);
 
 	if (th->ack) {
-		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
+		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
+		    conn->wnd_to_tap > MSS_GET(conn))
 			conn_flag(c, conn, ~ACK_TO_TAP_DUE);
 		else
 			conn_flag(c, conn, ACK_TO_TAP_DUE);