linux-kernel - [v2.6.34-stable 039/213] tcp: allow splice() to build full TSO packets

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Message-ID: <1391630568-49251-40-git-send-email-paul.gortmaker@windriver.com>
Date:	Wed, 5 Feb 2014 14:59:54 -0500
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	<stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Mahesh Bandewar <maheshb@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	"David S. Miller" <davem@...emloft.net>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [v2.6.34-stable 039/213] tcp: allow splice() to build full TSO packets

From: Eric Dumazet <eric.dumazet@...il.com>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit 2f53384424251c06038ae612e56231b96ab610ee upstream.

vmsplice()/splice(pipe, socket) call do_tcp_sendpages() one page at a
time, adding at most 4096 bytes to an skb. (assuming PAGE_SIZE=4096)

The call to tcp_push() at the end of do_tcp_sendpages() forces an
immediate xmit when pipe is not already filled, and tso_fragment() try
to split these skb to MSS multiples.

4096 bytes are usually split in a skb with 2 MSS, and a remaining
sub-mss skb (assuming MTU=1500)

This makes slow start suboptimal because many small frames are sent to
qdisc/driver layers instead of big ones (constrained by cwnd and packets
in flight of course)

In fact, applications using sendmsg() (adding an additional memory copy)
instead of vmsplice()/splice()/sendfile() are a bit faster because of
this anomaly, especially if serving small files in environments with
large initial [c]wnd.

Call tcp_push() only if MSG_MORE is not set in the flags parameter.

This bit is automatically provided by splice() internals but for the
last page, or on all pages if user specified SPLICE_F_MORE splice()
flag.

In some workloads, this can reduce number of sent logical packets by an
order of magnitude, making zero-copy TCP actually faster than
one-copy :)

Reported-by: Tom Herbert <therbert@...gle.com>
Cc: Nandita Dukkipati <nanditad@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Tom Herbert <therbert@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: H.K. Jerry Chu <hkchu@...gle.com>
Cc: Maciej Żenczykowski <maze@...gle.com>
Cc: Mahesh Bandewar <maheshb@...gle.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Signed-off-by: Eric Dumazet <eric.dumazet@...il>com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 3a8cbf72b06e..cea0a9223c5d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -849,7 +849,7 @@ wait_for_memory:
 	}
 
 out:
-	if (copied)
+	if (copied && !(flags & MSG_MORE))
 		tcp_push(sk, flags, mss_now, tp->nonagle);
 	return copied;
 
-- 
1.8.5.2

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