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

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Date:   Thu, 13 Dec 2018 14:17:20 +0100
From:   Marek Majkowski <marek@...udflare.com>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: splice() performance for TCP socket forwarding

Eric,

On Thu, Dec 13, 2018 at 1:49 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.

I'm not sure what you are suggesting. I'm just shuffling data between
two sockets. Is there a better buffer size value? Is it possible to
keep splice() blocked until it succeeds to forward N bytes of data? (I
tried this unsuccessfully with SO_RCVLOWAT).

Here is a snippet from strace:

splice(4, NULL, 11, NULL, 1048576, 0) = 373760 <0.000048>
splice(10, NULL, 5, NULL, 373760, 0) = 373760 <0.000108>
splice(4, NULL, 11, NULL, 1048576, 0) = 335800 <0.000065>
splice(10, NULL, 5, NULL, 335800, 0) = 335800 <0.000202>
splice(4, NULL, 11, NULL, 1048576, 0) = 227760 <0.000029>
splice(10, NULL, 5, NULL, 227760, 0) = 227760 <0.000106>
splice(4, NULL, 11, NULL, 1048576, 0) = 16060 <0.000019>
splice(10, NULL, 5, NULL, 16060, 0) = 16060 <0.000028>
splice(4, NULL, 11, NULL, 1048576, 0) = 7300 <0.000013>
splice(10, NULL, 5, NULL, 7300, 0) = 7300 <0.000021>

> > 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

I can confirm this. On 4.19 indeed splice program goes down to
expected ~50% cpu and performance comparable to naive read/write
version.

> >
> > 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.

Oh, that's a pity.

Thanks for help.
Marek