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sendfile: don't hack sb_lowat for sockets that manage the watermark

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In the sendfile(2) we carry an old hack (originating from d99b0dd2c5)
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
This commit is contained in:
Gleb Smirnoff 2025-07-25 13:05:56 -07:00
parent ee110941e3
commit f2c2ed7df3
5 changed files with 15 additions and 7 deletions
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11
sys/kern/kern_sendfile.c
View file

@ -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;

1
sys/kern/uipc_sockbuf.c
View file

@ -779,6 +779,7 @@ sbsetopt(struct socket *so, struct sockopt *sopt)
* high-water.
*/
*lowat = (cc > *hiwat) ? *hiwat : cc;
*flags &= ~SB_AUTOLOWAT;
break;
}

6
sys/kern/uipc_socket.c
View file

@ -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);

2
sys/netinet/tcp_usrreq.c
View file

@ -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;

2
sys/sys/sockbuf.h
View file

@ -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 */