The branch main has been updated by glebius:
URL:
https://cgit.FreeBSD.org/src/commit/?id=f2c2ed7df313f641451ca5a468f658fd350aae52
commit f2c2ed7df313f641451ca5a468f658fd350aae52
Author: Gleb Smirnoff <gleb...@freebsd.org>
AuthorDate: 2025-07-25 20:05:56 +0000
Commit: Gleb Smirnoff <gleb...@freebsd.org>
CommitDate: 2025-07-25 20:05:56 +0000
sendfile: don't hack sb_lowat for sockets that manage the watermark
In the sendfile(2) we carry an old hack (originating from d99b0dd2c5297)
to help dumb benchmarks and applications to achieve higher performance. We
would modify low watermark on the socket send buffer to avoid socket being
reported as writable too early, which would result in lots of small
writes.
Skip that hack for applications that do setsockopt(SO_SNDLOWAT) or that
register the socket in kevent(2) with NOTE_LOWAT feature. First, we don't
want the hack to rewrite the watermark value explicitly specified by the
user. Second, in certain cases that can lead to real performance
regressions. A kevent(2) with NOTE_LOWAT would report socket as writable,
but then sendfile(2) would write 0 bytes and return EAGAIN.
The change also disables the hack for unix(4) sockets, leaving only TCP.
Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D50581
---
sys/kern/kern_sendfile.c | 11 +++++++----
sys/kern/uipc_sockbuf.c | 1 +
sys/kern/uipc_socket.c | 6 +++++-
sys/netinet/tcp_usrreq.c | 2 +-
sys/sys/sockbuf.h | 2 +-
5 files changed, 15 insertions(+), 7 deletions(-)
diff --git a/sys/kern/kern_sendfile.c b/sys/kern/kern_sendfile.c
index 35b258e68701..8438298afc0e 100644
--- a/sys/kern/kern_sendfile.c
+++ b/sys/kern/kern_sendfile.c
@@ -698,10 +698,13 @@ sendfile_wait_generic(struct socket *so, off_t need, int
*space)
*/
error = 0;
SOCK_SENDBUF_LOCK(so);
- if (so->so_snd.sb_lowat < so->so_snd.sb_hiwat / 2)
- so->so_snd.sb_lowat = so->so_snd.sb_hiwat / 2;
- if (so->so_snd.sb_lowat < PAGE_SIZE && so->so_snd.sb_hiwat >= PAGE_SIZE)
- so->so_snd.sb_lowat = PAGE_SIZE;
+ if (so->so_snd.sb_flags & SB_AUTOLOWAT) {
+ if (so->so_snd.sb_lowat < so->so_snd.sb_hiwat / 2)
+ so->so_snd.sb_lowat = so->so_snd.sb_hiwat / 2;
+ if (so->so_snd.sb_lowat < PAGE_SIZE &&
+ so->so_snd.sb_hiwat >= PAGE_SIZE)
+ so->so_snd.sb_lowat = PAGE_SIZE;
+ }
retry_space:
if (so->so_snd.sb_state & SBS_CANTSENDMORE) {
error = EPIPE;
diff --git a/sys/kern/uipc_sockbuf.c b/sys/kern/uipc_sockbuf.c
index ec00878cd9a5..ffaa9b800592 100644
--- a/sys/kern/uipc_sockbuf.c
+++ b/sys/kern/uipc_sockbuf.c
@@ -779,6 +779,7 @@ sbsetopt(struct socket *so, struct sockopt *sopt)
* high-water.
*/
*lowat = (cc > *hiwat) ? *hiwat : cc;
+ *flags &= ~SB_AUTOLOWAT;
break;
}
diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
index 6c9eb7139cd1..4e8c179acee9 100644
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -1211,7 +1211,8 @@ solisten_clone(struct socket *head)
so->so_rcv.sb_timeo = head->sol_sbrcv_timeo;
so->so_snd.sb_timeo = head->sol_sbsnd_timeo;
so->so_rcv.sb_flags = head->sol_sbrcv_flags & SB_AUTOSIZE;
- so->so_snd.sb_flags = head->sol_sbsnd_flags & SB_AUTOSIZE;
+ so->so_snd.sb_flags = head->sol_sbsnd_flags &
+ (SB_AUTOSIZE | SB_AUTOLOWAT);
if ((so->so_proto->pr_flags & PR_SOCKBUF) == 0) {
so->so_snd.sb_mtx = &so->so_snd_mtx;
so->so_rcv.sb_mtx = &so->so_rcv_mtx;
@@ -4514,6 +4515,9 @@ sokqfilter_generic(struct socket *so, struct knote *kn)
SOCK_BUF_LOCK(so, which);
knlist_add(knl, kn, 1);
sb->sb_flags |= SB_KNOTE;
+ if ((kn->kn_sfflags & NOTE_LOWAT) &&
+ (sb->sb_flags & SB_AUTOLOWAT))
+ sb->sb_flags &= ~SB_AUTOLOWAT;
SOCK_BUF_UNLOCK(so, which);
}
SOCK_UNLOCK(so);
diff --git a/sys/netinet/tcp_usrreq.c b/sys/netinet/tcp_usrreq.c
index 70e4c04b79e5..98c934955121 100644
--- a/sys/netinet/tcp_usrreq.c
+++ b/sys/netinet/tcp_usrreq.c
@@ -164,7 +164,7 @@ tcp_usr_attach(struct socket *so, int proto, struct thread
*td)
goto out;
so->so_rcv.sb_flags |= SB_AUTOSIZE;
- so->so_snd.sb_flags |= SB_AUTOSIZE;
+ so->so_snd.sb_flags |= (SB_AUTOLOWAT | SB_AUTOSIZE);
error = in_pcballoc(so, &V_tcbinfo);
if (error)
goto out;
diff --git a/sys/sys/sockbuf.h b/sys/sys/sockbuf.h
index 7f6234ade6f4..2fed44bc9825 100644
--- a/sys/sys/sockbuf.h
+++ b/sys/sys/sockbuf.h
@@ -40,7 +40,7 @@
#define SB_SEL 0x08 /* someone is selecting */
#define SB_ASYNC 0x10 /* ASYNC I/O, need signals */
#define SB_UPCALL 0x20 /* someone wants an upcall */
-/* was SB_NOINTR 0x40 */
+#define SB_AUTOLOWAT 0x40 /* sendfile(2) may autotune
sb_lowat */
#define SB_AIO 0x80 /* AIO operations queued */
#define SB_KNOTE 0x100 /* kernel note attached */
#define SB_NOCOALESCE 0x200 /* don't coalesce new data into
existing mbufs */
