From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: passt.top; dkim=pass (1024-bit key; unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=BjHYlZQh; dkim-atps=neutral Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTPS id 7EA2F5A0272 for ; Fri, 20 Mar 2026 11:32:25 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1774002744; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=7AVH1On7Qp8ANezNEelIUYwrjc8yL01FLDlXpeMqBjc=; b=BjHYlZQh8CWy2jeRCn1nt3OI6Z9qAs+HL3ilK46zzVkpdCn4mE+k3hQ0tdzogPg21sTSme uISaYRau2tqnZERCLywSkxb7XE8xMrKDpmEDBO82A+AgSP4eZiYuHMNVi6bYRR7I+GoVHi XqJyN1hCrftudPMvJJtjBYdLGOoIUBA= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-657-OGPuEGoROdCH5JJ9IAXv3w-1; Fri, 20 Mar 2026 06:32:22 -0400 X-MC-Unique: OGPuEGoROdCH5JJ9IAXv3w-1 X-Mimecast-MFC-AGG-ID: OGPuEGoROdCH5JJ9IAXv3w_1774002742 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D738D195609E; Fri, 20 Mar 2026 10:32:21 +0000 (UTC) Received: from fedora.redhat.com (unknown [10.72.116.110]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id DB5941800351; Fri, 20 Mar 2026 10:32:17 +0000 (UTC) From: Yumei Huang To: passt-dev@passt.top, sbrivio@redhat.com Subject: [RFC PATCH] tcp: Replace send buffer boost with EPOLLOUT monitoring Date: Fri, 20 Mar 2026 18:32:14 +0800 Message-ID: <20260320103214.99320-1-yuhuang@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 4wKp2IlZ1tIm3f2vJOq2xBd5SEt6cUXzFC3Ta96VRK4_1774002742 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true Message-ID-Hash: BGDYHRPVSVP6TQW3DLYVZXA5PYY6MKRX X-Message-ID-Hash: BGDYHRPVSVP6TQW3DLYVZXA5PYY6MKRX X-MailFrom: yuhuang@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: david@gibson.dropbear.id.au, yuhuang@redhat.com X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: Currently we use the SNDBUF boost mechanism to force TCP auto-tuning. However, it doesn't always work, and sometimes causes a lot of retransmissions. As a result, the throughput suffers. This patch replaces it with monitoring EPOLLOUT when sendmsg() failure (with EAGAIN and EWOULDBLOCK) and partial sends occur. Tested with iperf3 inside pasta: throughput is now comparable to running iperf3 directly on the host without pasta. However, retransmissions can still be elevated when RTT >= 50ms. For example, when RTT is between 200ms and 500ms, retransmission count varies from 30 to 120 in roughly 80% of test runs. Link: https://bugs.passt.top/show_bug.cgi?id=138 Suggested-by: Stefano Brivio Signed-off-by: Yumei Huang --- tcp.c | 57 +++++++++++++++++---------------------------------------- 1 file changed, 17 insertions(+), 40 deletions(-) diff --git a/tcp.c b/tcp.c index 9d91c3c..f7e3932 100644 --- a/tcp.c +++ b/tcp.c @@ -353,13 +353,6 @@ enum { #define LOW_RTT_TABLE_SIZE 8 #define LOW_RTT_THRESHOLD 10 /* us */ -/* Parameters to temporarily exceed sending buffer to force TCP auto-tuning */ -#define SNDBUF_BOOST_BYTES_RTT_LO 2500 /* B * s: no boost until here */ -/* ...examples: 5 MB sent * 500 ns RTT, 250 kB * 10 ms, 8 kB * 300 ms */ -#define SNDBUF_BOOST_FACTOR 150 /* % */ -#define SNDBUF_BOOST_BYTES_RTT_HI 6000 /* apply full boost factor */ -/* 12 MB sent * 500 ns RTT, 600 kB * 10 ms, 20 kB * 300 ms */ - /* Ratio of buffer to bandwidth * delay product implying interactive traffic */ #define SNDBUF_TO_BW_DELAY_INTERACTIVE /* > */ 20 /* (i.e. < 5% of buffer) */ @@ -1023,35 +1016,6 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, return MAX(l3len + sizeof(struct ethhdr), ETH_ZLEN); } -/** - * tcp_sndbuf_boost() - Calculate limit of sending buffer to force auto-tuning - * @conn: Connection pointer - * @tinfo: tcp_info from kernel, must be pre-fetched - * - * Return: increased sending buffer to use as a limit for advertised window - */ -static unsigned long tcp_sndbuf_boost(const struct tcp_tap_conn *conn, - const struct tcp_info_linux *tinfo) -{ - unsigned long bytes_rtt_product; - - if (!bytes_acked_cap) - return SNDBUF_GET(conn); - - /* This is *not* a bandwidth-delay product, but it's somewhat related: - * as we send more data (usually at the beginning of a connection), we - * try to make the sending buffer progressively grow, with the RTT as a - * factor (longer delay, bigger buffer needed). - */ - bytes_rtt_product = (long long)tinfo->tcpi_bytes_acked * - tinfo->tcpi_rtt / 1000 / 1000; - - return clamped_scale(SNDBUF_GET(conn), bytes_rtt_product, - SNDBUF_BOOST_BYTES_RTT_LO, - SNDBUF_BOOST_BYTES_RTT_HI, - SNDBUF_BOOST_FACTOR); -} - /** * tcp_update_seqack_wnd() - Update ACK sequence and window to guest/tap * @c: Execution context @@ -1174,8 +1138,6 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, if ((int)sendq > SNDBUF_GET(conn)) /* Due to memory pressure? */ limit = 0; - else if ((int)tinfo->tcpi_snd_wnd > SNDBUF_GET(conn)) - limit = tcp_sndbuf_boost(conn, tinfo) - (int)sendq; else limit = SNDBUF_GET(conn) - (int)sendq; @@ -2050,14 +2012,28 @@ eintr: if (errno == EAGAIN || errno == EWOULDBLOCK) { tcp_send_flag(c, conn, ACK | DUP_ACK); + uint32_t events = tcp_conn_epoll_events(conn->events, + conn->flags); + events |= EPOLLOUT; + if (flow_epoll_set(&conn->f, EPOLL_CTL_MOD, events, + conn->sock, !TAPSIDE(conn)) < 0) + debug("Failed to add EPOLLOUT"); return p->count - idx; - } return -1; } - if (n < (int)(seq_from_tap - conn->seq_from_tap)) + if (n < (int)(seq_from_tap - conn->seq_from_tap)) { partial_send = 1; + uint32_t events = tcp_conn_epoll_events(conn->events, + conn->flags); + events |= EPOLLOUT; + if (flow_epoll_set(&conn->f, EPOLL_CTL_MOD, events, conn->sock, + !TAPSIDE(conn)) < 0) + debug("Failed to add EPOLLOUT"); + } else { + tcp_epoll_ctl(conn); + } conn->seq_from_tap += n; @@ -2661,6 +2637,7 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref, tcp_data_from_sock(c, conn); if (events & EPOLLOUT) { + tcp_epoll_ctl(conn); if (tcp_update_seqack_wnd(c, conn, false, NULL)) tcp_send_flag(c, conn, ACK); } -- 2.53.0