From b5c0f6f1e958a4aa0caaca8931d3e30c1154558b Mon Sep 17 00:00:00 2001
From: Nikolay Shopik <shopik@nvcube.net>
Date: Sun, 24 May 2026 14:17:18 +0300
Subject: [PATCH] iperf_sctp: apply default socket buffer to fix slow
 single-CPU throughput

Linux SCTP, unlike TCP, does not auto-tune SO_RCVBUF/SO_SNDBUF (there
is no equivalent of tcp_moderate_rcvbuf).  Without an explicit setting
the per-association receive window collapses to ~40 KB, which on
single-CPU systems gates single-stream loopback throughput to roughly
1 Gbps even after the delayed-SACK fix from #1815: sender and softirq
compete for the same core, so the small rwnd is never reopened in time.

Apply DEFAULT_SCTP_SOCKBUF (512 KB) to SO_RCVBUF and SO_SNDBUF on the
client connect socket, the server listen socket, and the per-
association accept socket whenever --window is not given.  The kernel
silently caps at net.core.{r,w}mem_max where smaller, and the explicit
--window path is unchanged.  Failures on the default path are non-fatal
(best-effort, not user-requested).

Measured on Debian 13 (kernel 6.12.48), loopback, single stream, 30 s
runs, median of 3:

    1-vCPU box
      TCP default          : 25.5 Gbps (unchanged)
      SCTP default before  :  0.94 Gbps
      SCTP default after   : 25.5 Gbps  (~27x)

    2-vCPU box
      SCTP default before/after: ~22 Gbps (within run-to-run variance)

The gap only manifests when sender and softirq share a CPU; on multi-
core systems the kernel already works around the small rwnd and the
patch is a no-op there.
---
 src/iperf_api.h  |  1 +
 src/iperf_sctp.c | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/src/iperf_api.h b/src/iperf_api.h
index 25ac951e0..2246faa98 100644
--- a/src/iperf_api.h
+++ b/src/iperf_api.h
@@ -66,6 +66,7 @@ typedef atomic_uint_fast64_t atomic_iperf_size_t;
 #define DEFAULT_UDP_BLKSIZE 1460 /* default is dynamically set, else this */
 #define DEFAULT_TCP_BLKSIZE (128 * 1024)  /* default read/write block size */
 #define DEFAULT_SCTP_BLKSIZE (64 * 1024)
+#define DEFAULT_SCTP_SOCKBUF (512 * 1024) /* Linux SCTP has no rcvbuf auto-tune */
 #define DEFAULT_PACING_TIMER 1000
 #define DEFAULT_NO_MSG_RCVD_TIMEOUT 120000
 #define MIN_NO_MSG_RCVD_TIMEOUT 100
diff --git a/src/iperf_sctp.c b/src/iperf_sctp.c
index 1e02a8d50..09ef05428 100644
--- a/src/iperf_sctp.c
+++ b/src/iperf_sctp.c
@@ -125,6 +125,23 @@ iperf_sctp_accept(struct iperf_test * test)
         return -1;
     }
 
+    {
+        /*
+         * Apply SO_RCVBUF/SO_SNDBUF on the accepted association.  The
+         * listener-side setting does not reliably open the per-association
+         * receive window on Linux SCTP (see iperf_sctp_listen()), so repeat
+         * here.  Honour --window if the user supplied it, otherwise apply
+         * DEFAULT_SCTP_SOCKBUF.  Best-effort; failures are silently ignored
+         * to match the iperf_sctp_listen() / iperf_sctp_connect() default
+         * branches.
+         */
+        int opt = test->settings->socket_bufsize;
+        if (opt == 0)
+            opt = DEFAULT_SCTP_SOCKBUF;
+        (void) setsockopt(s, SOL_SOCKET, SO_RCVBUF, &opt, sizeof(opt));
+        (void) setsockopt(s, SOL_SOCKET, SO_SNDBUF, &opt, sizeof(opt));
+    }
+
     if (Nread(s, cookie, COOKIE_SIZE, Psctp) < 0) {
         i_errno = IERECVCOOKIE;
         close(s);
@@ -206,6 +223,18 @@ iperf_sctp_listen(struct iperf_test *test)
             i_errno = IESETBUF;
             return -1;
         }
+    } else {
+        /*
+         * Linux SCTP does not auto-tune SO_RCVBUF/SO_SNDBUF the way TCP does
+         * (no equivalent of tcp_moderate_rcvbuf), so without an explicit
+         * setting the per-association rwnd collapses to a few tens of KB and
+         * gates single-stream throughput.  Apply a sensible default; the
+         * kernel will silently cap at net.core.{r,w}mem_max where smaller.
+         * Failures are non-fatal -- this is best-effort, not user-requested.
+         */
+        opt = DEFAULT_SCTP_SOCKBUF;
+        (void) setsockopt(s, SOL_SOCKET, SO_RCVBUF, &opt, sizeof(opt));
+        (void) setsockopt(s, SOL_SOCKET, SO_SNDBUF, &opt, sizeof(opt));
     }
 
     if (test->bind_dev) {
@@ -341,6 +370,11 @@ iperf_sctp_connect(struct iperf_test *test)
             i_errno = IESETBUF;
             return -1;
         }
+    } else {
+        /* see iperf_sctp_listen() for rationale; best-effort, non-fatal. */
+        opt = DEFAULT_SCTP_SOCKBUF;
+        (void) setsockopt(s, SOL_SOCKET, SO_RCVBUF, &opt, sizeof(opt));
+        (void) setsockopt(s, SOL_SOCKET, SO_SNDBUF, &opt, sizeof(opt));
     }
 
     if (test->bind_dev) {