netdev - Re: [PATCH net-next] tcp: adjust TSO packet sizes based on min

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Date:   Wed, 9 Mar 2022 11:25:28 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>, Kevin Yang <yyd@...gle.com>
Subject: Re: [PATCH net-next] tcp: adjust TSO packet sizes based on min_rtt

On Tue, Mar 8, 2022 at 8:58 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> Back when tcp_tso_autosize() and TCP pacing were introduced,
> our focus was really to reduce burst sizes for long distance
> flows.
>
> The simple heuristic of using sk_pacing_rate/1024 has worked
> well, but can lead to too small packets for hosts in the same
> rack/cluster, when thousands of flows compete for the bottleneck.
>
> Neal Cardwell had the idea of making the TSO burst size
> a function of both sk_pacing_rate and tcp_min_rtt()
>
> Indeed, for local flows, sending bigger bursts is better
> to reduce cpu costs, as occasional losses can be repaired
> quite fast.
>
> This patch is based on Neal Cardwell implementation
> done more than two years ago.
> bbr is adjusting max_pacing_rate based on measured bandwidth,
> while cubic would over estimate max_pacing_rate.
>
> /proc/sys/net/ipv4/tcp_tso_rtt_log can be used to tune or disable
> this new feature, in logarithmic steps.
>
> Tested:
>
> 100Gbit NIC, two hosts in the same rack, 4K MTU.
> 600 flows rate-limited to 20000000 bytes per second.
>
> Before patch: (TSO sizes would be limited to 20000000/1024/4096 -> 4 segments per TSO)
>
> ~# echo 0 >/proc/sys/net/ipv4/tcp_tso_rtt_log
> ~# nstat -n;perf stat ./super_netperf 600 -H otrv6 -l 20 -- -K dctcp -q 20000000;nstat|egrep "TcpInSegs|TcpOutSegs|TcpRetransSegs|Delivered"
>   96005
>
>  Performance counter stats for './super_netperf 600 -H otrv6 -l 20 -- -K dctcp -q 20000000':
>
>          65,945.29 msec task-clock                #    2.845 CPUs utilized
>          1,314,632      context-switches          # 19935.279 M/sec
>              5,292      cpu-migrations            #   80.249 M/sec
>            940,641      page-faults               # 14264.023 M/sec
>    201,117,030,926      cycles                    # 3049769.216 GHz                   (83.45%)
>     17,699,435,405      stalled-cycles-frontend   #    8.80% frontend cycles idle     (83.48%)
>    136,584,015,071      stalled-cycles-backend    #   67.91% backend cycles idle      (83.44%)
>     53,809,530,436      instructions              #    0.27  insn per cycle
>                                                   #    2.54  stalled cycles per insn  (83.36%)
>      9,062,315,523      branches                  # 137422329.563 M/sec               (83.22%)
>        153,008,621      branch-misses             #    1.69% of all branches          (83.32%)
>
>       23.182970846 seconds time elapsed
>
> TcpInSegs                       15648792           0.0
> TcpOutSegs                      58659110           0.0  # Average of 3.7 4K segments per TSO packet
> TcpExtTCPDelivered              58654791           0.0
> TcpExtTCPDeliveredCE            19                 0.0
>
> After patch:
>
> ~# echo 9 >/proc/sys/net/ipv4/tcp_tso_rtt_log
> ~# nstat -n;perf stat ./super_netperf 600 -H otrv6 -l 20 -- -K dctcp -q 20000000;nstat|egrep "TcpInSegs|TcpOutSegs|TcpRetransSegs|Delivered"
>   96046
>
>  Performance counter stats for './super_netperf 600 -H otrv6 -l 20 -- -K dctcp -q 20000000':
>
>          48,982.58 msec task-clock                #    2.104 CPUs utilized
>            186,014      context-switches          # 3797.599 M/sec
>              3,109      cpu-migrations            #   63.472 M/sec
>            941,180      page-faults               # 19214.814 M/sec
>    153,459,763,868      cycles                    # 3132982.807 GHz                   (83.56%)
>     12,069,861,356      stalled-cycles-frontend   #    7.87% frontend cycles idle     (83.32%)
>    120,485,917,953      stalled-cycles-backend    #   78.51% backend cycles idle      (83.24%)
>     36,803,672,106      instructions              #    0.24  insn per cycle
>                                                   #    3.27  stalled cycles per insn  (83.18%)
>      5,947,266,275      branches                  # 121417383.427 M/sec               (83.64%)
>         87,984,616      branch-misses             #    1.48% of all branches          (83.43%)
>
>       23.281200256 seconds time elapsed
>
> TcpInSegs                       1434706            0.0
> TcpOutSegs                      58883378           0.0  # Average of 41 4K segments per TSO packet
> TcpExtTCPDelivered              58878971           0.0
> TcpExtTCPDeliveredCE            9664               0.0
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---

Thanks, Eric!

Reviewed-by: Neal Cardwell <ncardwell@...gle.com>

neal