netdev - Re: Stable regression with 'tcp: allow splice() to build full TSO packets'

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Date:	Thu, 17 May 2012 17:43:00 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willy Tarreau <w@....eu>
Cc:	netdev@...r.kernel.org
Subject: Re: Stable regression with 'tcp: allow splice() to build full TSO
 packets'

On Thu, 2012-05-17 at 17:01 +0200, Willy Tarreau wrote:
> Hi Eric,
> 
> On Thu, May 17, 2012 at 02:18:00PM +0200, Willy Tarreau wrote:
> > Hi Eric,
> > 
> > I'm facing a regression in stable 3.2.17 and 3.0.31 which is
> > exhibited by your patch 'tcp: allow splice() to build full TSO
> > packets' which unfortunately I am very interested in !
> > 
> > What I'm observing is that TCP transmits using splice() stall
> > quite quickly if I'm using pipes larger than 64kB (even 65537
> > is enough to reliably observe the stall).
> (...)
> 
> I managed to fix the issue and I really think that the fix makes sense.
> I'm appending the patch, please could you review it and if approved,
> push it for inclusion ?
> 
> BTW, your patch significantly improves performance here. On this
> machine I was reaching max 515 Mbps of proxied traffic, and with
> the patch I reach 665 Mbps with the same test ! I think you managed
> to fix what caused splice() to always be slightly slower than
> recv/send() for a long time in my tests with a number of NICs !
> 

What NIC do you use exactly ?

With commit 1d0c0b328a6 in net-next
(net: makes skb_splice_bits() aware of skb->head_frag)
You'll be able to get even more speed, if NIC uses frag to hold frame.

> Thanks,
> Willy
> 
> -------
> 
> From 6da6a21798d0156e647a993c31782eec739fa5df Mon Sep 17 00:00:00 2001
> From: Willy Tarreau <w@....eu>
> Date: Thu, 17 May 2012 16:48:56 +0200
> Subject: [PATCH] tcp: force push data out when buffers are missing
> 
> Commit 2f533844242 (tcp: allow splice() to build full TSO packets)
> significantly improved splice() performance for some workloads but
> caused stalls when pipe buffers were larger than socket buffers.
> 

But... This seems bug was already there ? Only having more data in pipe
trigger it more often ?

> The issue seems to happen when no data can be copied at all due to
> lack of buffers, which results in pending data never being pushed.
> 
> This change checks if all pending data has been pushed or not and
> pushes them when waiting for send buffers.
> ---
>  net/ipv4/tcp.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 80b988f..f6d9e5f 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -850,7 +850,7 @@ new_segment:
>  wait_for_sndbuf:
>  		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
>  wait_for_memory:
> -		if (copied)
> +		if (tcp_sk(sk)->pushed_seq != tcp_sk(sk)->write_seq)
>  			tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);
>  
>  		if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)

Can you check you have commit 35f9c09fe9c72eb8ca2b8e89a593e1c151f28fc2 
( tcp: tcp_sendpages() should call tcp_push() once )


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