netdev - Re: splice() performance for TCP socket forwarding

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Date:   Thu, 13 Dec 2018 04:49:48 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marek Majkowski <marek@...udflare.com>, netdev@...r.kernel.org
Subject: Re: splice() performance for TCP socket forwarding



On 12/13/2018 03:25 AM, Marek Majkowski wrote:
> Hi!
> 
> I'm basically trying to do TCP splicing in Linux. I'm focusing on
> performance of the simplest case: receive data from one TCP socket,
> write data to another TCP socket. I get poor performance with splice.
> 
> First, the naive code, pretty much:
> 
> while(1){
>  n = read(rs, buf);
>  write(ws, buf, n);
> }
> 
> With GRO enabled, this code does roughly line-rate of 10Gbps, hovering
> ~50% of CPU in application (sys mostly).
> 
> When replaced with splice version:
> 
> pipe(pfd);
> fcntl(pfd[0], F_SETPIPE_SZ, 1024 * 1024);

Why 1 MB ?

splice code will be expensive if less than 1MB is present in receive queue.

> while(1) {
>  n = splice(rd, NULL, pfd[1], NULL, 1024*1024,
>                        SPLICE_F_MOVE);
>   splice(pfd[0], NULL, wd, NULL, n, SPLICE_F_MOVE);
> }
> 
> Full code:
> https://gist.github.com/majek/c58a97b9be7d9217fe3ebd6c1328faaa#file-proxy-splice-c-L59
> 
> I get 100% cpu (sys) and dramatically worse performance (1.5x slower).
> 
> naive run of perf record ./proxy-splice shows:
>    5.73%  [k] queued_spin_lock_slowpath
>    5.23%  [k] ipt_do_table
>    4.72%  [k] __splice_segment.part.59
>    4.72%  [k] do_tcp_sendpages
>    3.47%  [k] _raw_spin_lock_bh
>    3.36%  [k] __x86_indirect_thunk_rax
> 
> (kernel 4.14.71)
> 
> Is it possible to squeeze more from splice? Is it possible to force
> splice() to hang forever and not return quickly (SO_RCVLOWAT doesn't
> work).

I believe it should work on recent linux kernels (4.18 )

03f45c883c6f391ed4fff8292415b35bd1107519 tcp: avoid extra wakeups for SO_RCVLOWAT users
796f82eafcd96629c2f9a0332dbb4f474854aaf8 tcp: fix delayed acks behavior for SO_RCVLOWAT
d1361840f8c519eaee9a78ffe09e4f0a1b586846 tcp: fix SO_RCVLOWAT and RCVBUF autotuning


> 
> Is there another way of doing TCP splicing? I'm aware of TCP ZEROCOPY
> that landed in 4.19.
>

TCP zero copy is only working if your MSS is exactly 4096 bytes (+ TCP options),
so might be tricky, as it also requires NIC driver abilities to perform nice header splitting.