The TCP window advertised to the guest/container must balance two
competing needs: large enough to trigger kernel socket buffer
auto-tuning, but not so large that sendmsg() partially fails, causing
retransmissions.
The current approach uses the difference (SNDBUF_GET() - SIOCOUTQ), but
SNDBUF_GET() returns a scaled value that only roughly accounts for
per-skb overhead. The clamped_scale approximation doesn't accurately
track the actual per-segment overhead, which can lead to both excessive
retransmissions and reduced throughput.
We now introduce the use of SO_MEMINFO to obtain SK_MEMINFO_SNDBUF and
SK_MEMINFO_WMEM_QUEUED from the kernel. The latter is presented in the
kernel's own accounting units, i.e. including the sk_buff overhead,
and matches exactly what the kernel's own sk_stream_memory_free()
function is using.
When data is queued and the overhead ratio is observable, we calculate
the per-segment overhead as (wmem_queued - sendq) / num_segments, then
determine how many additional segments should fit in the remaining
buffer space, considering the calculated per-mss overhead. This approach
treats segments as discrete quantities, and produces a more accurate
estimate of available buffer space than a linear scaling factor does.
When the ratio cannot be observed, e.g. because the queue is empty or
we are in a transient state, we fall back to the existing clamped_scale
calculation (scaling between 100% and 75% of buffer capacity).
When SO_MEMINFO succeeds, we also use SK_MEMINFO_SNDBUF directly to
set SNDBUF, avoiding a separate SO_SNDBUF getsockopt() call.
If SO_MEMINFO is unavailable, we fall back to the pre-existing
SNDBUF_GET() - SIOCOUTQ calculation.
Link: https://bugs.passt.top/show_bug.cgi?id=138
Signed-off-by: Jon Maloy
---
v2: Updated according to feedback from Stefano. My own measurements
indicate that this approach largely solves both the retransmission
and throughput issues observed with the previous version.
---
tcp.c | 42 ++++++++++++++++++++++++++++++++++--------
tcp_conn.h | 2 +-
2 files changed, 35 insertions(+), 9 deletions(-)
diff --git a/tcp.c b/tcp.c
index 43b8fdb..2ba08fd 100644
--- a/tcp.c
+++ b/tcp.c
@@ -295,6 +295,7 @@
#include
#include
+#include
#include "checksum.h"
#include "util.h"
@@ -1128,19 +1129,44 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
new_wnd_to_tap = tinfo->tcpi_snd_wnd;
} else {
unsigned rtt_ms_ceiling = DIV_ROUND_UP(tinfo->tcpi_rtt, 1000);
+ uint32_t mem[SK_MEMINFO_VARS];
+ socklen_t mem_sl = sizeof(mem);
+ int mss = MSS_GET(conn);
uint32_t sendq;
- int limit;
+ uint32_t limit;
if (ioctl(s, SIOCOUTQ, &sendq)) {
debug_perror("SIOCOUTQ on socket %i, assuming 0", s);
sendq = 0;
}
- tcp_get_sndbuf(conn);
- if ((int)sendq > SNDBUF_GET(conn)) /* Due to memory pressure? */
- limit = 0;
- else
- limit = SNDBUF_GET(conn) - (int)sendq;
+ if (getsockopt(s, SOL_SOCKET, SO_MEMINFO, &mem, &mem_sl)) {
+ tcp_get_sndbuf(conn);
+ if (sendq > SNDBUF_GET(conn))
+ limit = 0;
+ else
+ limit = SNDBUF_GET(conn) - sendq;
+ } else {
+ uint32_t sb = mem[SK_MEMINFO_SNDBUF];
+ uint32_t wq = mem[SK_MEMINFO_WMEM_QUEUED];
+ uint32_t cs = clamped_scale(sb, sb, SNDBUF_SMALL,
+ SNDBUF_BIG, 75);
+
+ SNDBUF_SET(conn, MIN(INT_MAX, cs));
+
+ if (wq > sb) {
+ limit = 0;
+ } else if (!sendq || wq <= sendq || !mss) {
+ limit = SNDBUF_GET(conn) - sendq;
+ } else {
+ uint32_t nsegs = MAX(sendq / mss, 1);
+ uint32_t overhead = (wq - sendq) / nsegs;
+ uint32_t remaining = sb - wq;
+
+ nsegs = remaining / (mss + overhead);
+ limit = nsegs * mss;
+ }
+ }
/* If the sender uses mechanisms to prevent Silly Window
* Syndrome (SWS, described in RFC 813 Section 3) it's critical
@@ -1168,11 +1194,11 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
* but we won't send enough to fill one because we're stuck
* with pending data in the outbound queue
*/
- if (limit < MSS_GET(conn) && sendq &&
+ if (limit < (uint32_t)MSS_GET(conn) && sendq &&
tinfo->tcpi_last_data_sent < rtt_ms_ceiling * 10)
limit = 0;
- new_wnd_to_tap = MIN((int)tinfo->tcpi_snd_wnd, limit);
+ new_wnd_to_tap = MIN(tinfo->tcpi_snd_wnd, limit);
}
new_wnd_to_tap = MIN(new_wnd_to_tap, MAX_WINDOW);
diff --git a/tcp_conn.h b/tcp_conn.h
index 6985426..9f5bee0 100644
--- a/tcp_conn.h
+++ b/tcp_conn.h
@@ -98,7 +98,7 @@ struct tcp_tap_conn {
#define SNDBUF_BITS 24
unsigned int sndbuf :SNDBUF_BITS;
#define SNDBUF_SET(conn, bytes) (conn->sndbuf = ((bytes) >> (32 - SNDBUF_BITS)))
-#define SNDBUF_GET(conn) (conn->sndbuf << (32 - SNDBUF_BITS))
+#define SNDBUF_GET(conn) ((uint32_t)(conn->sndbuf << (32 - SNDBUF_BITS)))
uint8_t seq_dup_ack_approx;
--
2.52.0