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