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Message-ID: <CAKgNAkhTSs-GXPjenoPWx2ndZAZOcq0k-q+V5WHqiJLNba=0NA@mail.gmail.com>
Date: Mon, 20 Jan 2014 17:45:49 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: David Miller <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Neal Cardwell <ncardwell@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: TCP_NOTSENT_LOWAT socket option
Hi Eric,
On Mon, Jul 22, 2013 at 9:36 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Idea of this patch is to add optional limitation of number of
> unsent bytes in TCP sockets, to reduce usage of kernel memory.
>
> TCP receiver might announce a big window, and TCP sender autotuning
> might allow a large amount of bytes in write queue, but this has little
> performance impact if a large part of this buffering is wasted :
>
> Write queue needs to be large only to deal with large BDP, not
> necessarily to cope with scheduling delays (incoming ACKS make room
> for the application to queue more bytes)
>
> For most workloads, using a value of 128 KB or less is OK to give
> applications enough time to react to POLLOUT events in time
> (or being awaken in a blocking sendmsg())
>
> This patch adds two ways to set the limit :
> 1) Per socket option TCP_NOTSENT_LOWAT
>
> 2) A sysctl (/proc/sys/net/ipv4/tcp_notsent_lowat) for sockets
> not using TCP_NOTSENT_LOWAT socket option (or setting a zero value)
> Default value being UINT_MAX (0xFFFFFFFF), meaning this has no effect.
>
>
> This changes poll()/select()/epoll() to report POLLOUT
> only if number of unsent bytes is below tp->nosent_lowat
>
> Note this might increase number of sendmsg() calls when using non
> blocking sockets, and increase number of context switches for
> blocking sockets.
Would you be willing to write a patch to the tcp(7) man page [1] that
describes the user-space API aspects of TCP_NOTSENT_LOWAT /
/proc/sys/net/ipv4/tcp_notsent_lowat and their effect on
poll()/select()? If the *roff markup is too much of a hassle, I'd be
happy enough to get some plain text that I'll then integrate into the
man page.
Cheers,
Michael
[1] https://www.kernel.org/doc/man-pages/download.html
> Tested:
>
> netperf sessions, and watching /proc/net/protocols "memory" column for TCP
>
> Even in the absence of shallow queues, we get a benefit.
>
> With 200 concurrent netperf -t TCP_STREAM sessions, amount of kernel memory
> used by TCP buffers shrinks by ~55 % (20567 pages instead of 45458)
>
> lpq83:~# echo -1 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# (super_netperf 200 -t TCP_STREAM -H remote -l 90 &); sleep 60 ; grep TCP /proc/net/protocols
> TCPv6 1880 2 45458 no 208 yes ipv6 y y y y y y y y y y y y y n y y y y y
> TCP 1696 508 45458 no 208 yes kernel y y y y y y y y y y y y y n y y y y y
>
> lpq83:~# echo 131072 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# (super_netperf 200 -t TCP_STREAM -H remote -l 90 &); sleep 60 ; grep TCP /proc/net/protocols
> TCPv6 1880 2 20567 no 208 yes ipv6 y y y y y y y y y y y y y n y y y y y
> TCP 1696 508 20567 no 208 yes kernel y y y y y y y y y y y y y n y y y y y
>
> Using 128KB has no bad effect on the throughput of a single flow, although
> there is an increase of cpu time as sendmsg() calls trigger more
> context switches. A bonus is that we hold socket lock for a shorter amount
> of time and should improve latencies.
>
> lpq83:~# echo -1 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# perf stat -e context-switches ./netperf -H lpq84 -t omni -l 20 -Cc
> OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpq84 () port 0 AF_INET
> Local Remote Local Elapsed Throughput Throughput Local Local Remote Remote Local Remote Service
> Send Socket Recv Socket Send Time Units CPU CPU CPU CPU Service Service Demand
> Size Size Size (sec) Util Util Util Util Demand Demand Units
> Final Final % Method % Method
> 2097152 6000000 16384 20.00 16509.68 10^6bits/s 3.05 S 4.50 S 0.363 0.536 usec/KB
>
> Performance counter stats for './netperf -H lpq84 -t omni -l 20 -Cc':
>
> 30,141 context-switches
>
> 20.006308407 seconds time elapsed
>
> lpq83:~# echo 131072 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# perf stat -e context-switches ./netperf -H lpq84 -t omni -l 20 -Cc
> OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpq84 () port 0 AF_INET
> Local Remote Local Elapsed Throughput Throughput Local Local Remote Remote Local Remote Service
> Send Socket Recv Socket Send Time Units CPU CPU CPU CPU Service Service Demand
> Size Size Size (sec) Util Util Util Util Demand Demand Units
> Final Final % Method % Method
> 1911888 6000000 16384 20.00 17412.51 10^6bits/s 3.94 S 4.39 S 0.444 0.496 usec/KB
>
> Performance counter stats for './netperf -H lpq84 -t omni -l 20 -Cc':
>
> 284,669 context-switches
>
> 20.005294656 seconds time elapsed
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> ---
> v2: title/changelog fix (TCP_NOSENT_LOWAT -> TCP_NOTSENT_LOWAT)
>
> Documentation/networking/ip-sysctl.txt | 13 +++++++++++++
> include/linux/tcp.h | 1 +
> include/net/sock.h | 15 ++++++++++-----
> include/net/tcp.h | 14 ++++++++++++++
> include/uapi/linux/tcp.h | 1 +
> net/ipv4/sysctl_net_ipv4.c | 7 +++++++
> net/ipv4/tcp.c | 12 ++++++++++--
> net/ipv4/tcp_ipv4.c | 1 +
> net/ipv4/tcp_output.c | 3 +++
> net/ipv6/tcp_ipv6.c | 1 +
> 10 files changed, 61 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index 1074290..53cea9b 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -516,6 +516,19 @@ tcp_wmem - vector of 3 INTEGERs: min, default, max
> this value is ignored.
> Default: between 64K and 4MB, depending on RAM size.
>
> +tcp_notsent_lowat - UNSIGNED INTEGER
> + A TCP socket can control the amount of unsent bytes in its write queue,
> + thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
> + reports POLLOUT events if the amount of unsent bytes is below a per
> + socket value, and if the write queue is not full. sendmsg() will
> + also not add new buffers if the limit is hit.
> +
> + This global variable controls the amount of unsent data for
> + sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
> + to the global variable has immediate effect.
> +
> + Default: UINT_MAX (0xFFFFFFFF)
> +
> tcp_workaround_signed_windows - BOOLEAN
> If set, assume no receipt of a window scaling option means the
> remote TCP is broken and treats the window as a signed quantity.
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 472120b..9640803 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -238,6 +238,7 @@ struct tcp_sock {
>
> u32 rcv_wnd; /* Current receiver window */
> u32 write_seq; /* Tail(+1) of data held in tcp send buffer */
> + u32 notsent_lowat; /* TCP_NOTSENT_LOWAT */
> u32 pushed_seq; /* Last pushed seq, required to talk to windows */
> u32 lost_out; /* Lost packets */
> u32 sacked_out; /* SACK'd packets */
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 95a5a2c..7be0b22 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -746,11 +746,6 @@ static inline int sk_stream_wspace(const struct sock *sk)
>
> extern void sk_stream_write_space(struct sock *sk);
>
> -static inline bool sk_stream_memory_free(const struct sock *sk)
> -{
> - return sk->sk_wmem_queued < sk->sk_sndbuf;
> -}
> -
> /* OOB backlog add */
> static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
> {
> @@ -950,6 +945,7 @@ struct proto {
> unsigned int inuse_idx;
> #endif
>
> + bool (*stream_memory_free)(const struct sock *sk);
> /* Memory pressure */
> void (*enter_memory_pressure)(struct sock *sk);
> atomic_long_t *memory_allocated; /* Current allocated memory. */
> @@ -1089,6 +1085,15 @@ static inline struct cg_proto *parent_cg_proto(struct proto *proto,
> #endif
>
>
> +static inline bool sk_stream_memory_free(const struct sock *sk)
> +{
> + if (sk->sk_wmem_queued >= sk->sk_sndbuf)
> + return false;
> +
> + return sk->sk_prot->stream_memory_free ?
> + sk->sk_prot->stream_memory_free(sk) : true;
> +}
> +
> static inline bool sk_has_memory_pressure(const struct sock *sk)
> {
> return sk->sk_prot->memory_pressure != NULL;
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index d198005..ff58714 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -284,6 +284,7 @@ extern int sysctl_tcp_thin_dupack;
> extern int sysctl_tcp_early_retrans;
> extern int sysctl_tcp_limit_output_bytes;
> extern int sysctl_tcp_challenge_ack_limit;
> +extern unsigned int sysctl_tcp_notsent_lowat;
>
> extern atomic_long_t tcp_memory_allocated;
> extern struct percpu_counter tcp_sockets_allocated;
> @@ -1549,6 +1550,19 @@ extern int tcp_gro_complete(struct sk_buff *skb);
> extern void __tcp_v4_send_check(struct sk_buff *skb, __be32 saddr,
> __be32 daddr);
>
> +static inline u32 tcp_notsent_lowat(const struct tcp_sock *tp)
> +{
> + return tp->notsent_lowat ?: sysctl_tcp_notsent_lowat;
> +}
> +
> +static inline bool tcp_stream_memory_free(const struct sock *sk)
> +{
> + const struct tcp_sock *tp = tcp_sk(sk);
> + u32 notsent_bytes = tp->write_seq - tp->snd_nxt;
> +
> + return notsent_bytes < tcp_notsent_lowat(tp);
> +}
> +
> #ifdef CONFIG_PROC_FS
> extern int tcp4_proc_init(void);
> extern void tcp4_proc_exit(void);
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index 8d776eb..377f1e5 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -111,6 +111,7 @@ enum {
> #define TCP_REPAIR_OPTIONS 22
> #define TCP_FASTOPEN 23 /* Enable FastOpen on listeners */
> #define TCP_TIMESTAMP 24
> +#define TCP_NOTSENT_LOWAT 25 /* limit number of unsent bytes in write queue */
>
> struct tcp_repair_opt {
> __u32 opt_code;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index b2c123c..69ed203 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -555,6 +555,13 @@ static struct ctl_table ipv4_table[] = {
> .extra1 = &one,
> },
> {
> + .procname = "tcp_notsent_lowat",
> + .data = &sysctl_tcp_notsent_lowat,
> + .maxlen = sizeof(sysctl_tcp_notsent_lowat),
> + .mode = 0644,
> + .proc_handler = proc_dointvec,
> + },
> + {
> .procname = "tcp_rmem",
> .data = &sysctl_tcp_rmem,
> .maxlen = sizeof(sysctl_tcp_rmem),
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 5423223..5792302 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -499,7 +499,8 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
> mask |= POLLIN | POLLRDNORM;
>
> if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
> - if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk)) {
> + if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk) &&
> + tcp_stream_memory_free(sk)) {
> mask |= POLLOUT | POLLWRNORM;
> } else { /* send SIGIO later */
> set_bit(SOCK_ASYNC_NOSPACE,
> @@ -510,7 +511,8 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
> * wspace test but before the flags are set,
> * IO signal will be lost.
> */
> - if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk))
> + if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk) &&
> + tcp_stream_memory_free(sk))
> mask |= POLLOUT | POLLWRNORM;
> }
> } else
> @@ -2631,6 +2633,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
> else
> tp->tsoffset = val - tcp_time_stamp;
> break;
> + case TCP_NOTSENT_LOWAT:
> + tp->notsent_lowat = val;
> + break;
> default:
> err = -ENOPROTOOPT;
> break;
> @@ -2847,6 +2852,9 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
> case TCP_TIMESTAMP:
> val = tcp_time_stamp + tp->tsoffset;
> break;
> + case TCP_NOTSENT_LOWAT:
> + val = tp->notsent_lowat;
> + break;
> default:
> return -ENOPROTOOPT;
> }
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index b74628e..8390bff 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -2806,6 +2806,7 @@ struct proto tcp_prot = {
> .unhash = inet_unhash,
> .get_port = inet_csk_get_port,
> .enter_memory_pressure = tcp_enter_memory_pressure,
> + .stream_memory_free = tcp_stream_memory_free,
> .sockets_allocated = &tcp_sockets_allocated,
> .orphan_count = &tcp_orphan_count,
> .memory_allocated = &tcp_memory_allocated,
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 92fde8d..884efff 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -65,6 +65,9 @@ int sysctl_tcp_base_mss __read_mostly = TCP_BASE_MSS;
> /* By default, RFC2861 behavior. */
> int sysctl_tcp_slow_start_after_idle __read_mostly = 1;
>
> +unsigned int sysctl_tcp_notsent_lowat __read_mostly = UINT_MAX;
> +EXPORT_SYMBOL(sysctl_tcp_notsent_lowat);
> +
> static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
> int push_one, gfp_t gfp);
>
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index f0d6363..0030cfd 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1927,6 +1927,7 @@ struct proto tcpv6_prot = {
> .unhash = inet_unhash,
> .get_port = inet_csk_get_port,
> .enter_memory_pressure = tcp_enter_memory_pressure,
> + .stream_memory_free = tcp_stream_memory_free,
> .sockets_allocated = &tcp_sockets_allocated,
> .memory_allocated = &tcp_memory_allocated,
> .memory_pressure = &tcp_memory_pressure,
>
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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