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