Re: [PATCH v3 net-next 2/2] tcp: TCP_NOTSENT_LOWAT socket option

[Neal Cardwell <ncardwell@google.com>]

On Mon, Jul 22, 2013 at 11:27 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.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()/sendfile() calls
> when using non blocking sockets,
> and increase number of context switches for blocking sockets.
>
> Note this is not related to SO_SNDLOWAT (as SO_SNDLOWAT is
> defined as :
>  Specify the minimum number of bytes in the buffer until
>  the socket layer will pass the data to the protocol)
>
> Tested:
>
> netperf sessions, and watching /proc/net/protocols "memory" column for TCP
>
> 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 or cpu usage
> of a single flow, although there is an increase of context switches.
>
> A bonus is that we hold socket lock for a shorter amount
> of time and should improve latencies of ACK processing.
>
> lpq83:~# echo -1 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# perf stat -e context-switches ./netperf -H 7.7.7.84 -t omni -l 20 -c -i10,3
> OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.7.84 () port 0 AF_INET : +/-2.500% @ 99% conf.
> 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
> 1651584     6291456     16384  20.00   17447.90   10^6bits/s  3.13  S      -1.00  U      0.353   -1.000  usec/KB
>
>  Performance counter stats for './netperf -H 7.7.7.84 -t omni -l 20 -c -i10,3':
>
>            412,514 context-switches
>
>      200.034645535 seconds time elapsed
>
> lpq83:~# echo 131072 >/proc/sys/net/ipv4/tcp_notsent_lowat
> lpq83:~# perf stat -e context-switches ./netperf -H 7.7.7.84 -t omni -l 20 -c -i10,3
> OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.7.84 () port 0 AF_INET : +/-2.500% @ 99% conf.
> 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
> 1593240     6291456     16384  20.00   17321.16   10^6bits/s  3.35  S      -1.00  U      0.381   -1.000  usec/KB
>
>  Performance counter stats for './netperf -H 7.7.7.84 -t omni -l 20 -c -i10,3':
>
>          2,675,818 context-switches
>
>      200.029651391 seconds time elapsed
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>
> Cc: Yuchung Cheng <ycheng@google.com>


Acked-by: Neal Cardwell <ncardwell@google.com>


[apologies for the dup, patchworks didn't seem to like my last email,
I think perhaps due to a missing final newline...]