tcp: TSO packets automatic sizing

[ Upstream commits 6d36824e730f247b602c90e8715a792003e3c5a7,
  02cf4ebd82ff0ac7254b88e466820a290ed8289a, and parts of
  7eec4174ff29cd42f2acfae8112f51c228545d40 ]

After hearing many people over past years complaining against TSO being
bursty or even buggy, we are proud to present automatic sizing of TSO
packets.

One part of the problem is that tcp_tso_should_defer() uses an heuristic
relying on upcoming ACKS instead of a timer, but more generally, having
big TSO packets makes little sense for low rates, as it tends to create
micro bursts on the network, and general consensus is to reduce the
buffering amount.

This patch introduces a per socket sk_pacing_rate, that approximates
the current sending rate, and allows us to size the TSO packets so
that we try to send one packet every ms.

This field could be set by other transports.

Patch has no impact for high speed flows, where having large TSO packets
makes sense to reach line rate.

For other flows, this helps better packet scheduling and ACK clocking.

This patch increases performance of TCP flows in lossy environments.

A new sysctl (tcp_min_tso_segs) is added, to specify the
minimal size of a TSO packet (default being 2).

A follow-up patch will provide a new packet scheduler (FQ), using
sk_pacing_rate as an input to perform optional per flow pacing.

This explains why we chose to set sk_pacing_rate to twice the current
rate, allowing 'slow start' ramp up.

sk_pacing_rate = 2 * cwnd * mss / srtt

v2: Neal Cardwell reported a suspect deferring of last two segments on
initial write of 10 MSS, I had to change tcp_tso_should_defer() to take
into account tp->xmit_size_goal_segs

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Van Jacobson <vanj@google.com>
Cc: Tom Herbert <therbert@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

This commit is contained in:

Eric Dumazet

2013-08-27 05:46:32 -07:00

committed by

Greg Kroah-Hartman

parent 14e9c7db46

commit 5e25ba5003

8 changed files with 80 additions and 7 deletions

Documentation/networking/ip-sysctl.txt

View File

@@ -478,6 +478,15 @@ tcp_syn_retries - INTEGER
 tcp_timestamps - BOOLEAN
 	Enable timestamps as defined in RFC1323.
 tcp_min_tso_segs - INTEGER
 	Minimal number of segments per TSO frame.
 	Since linux-3.12, TCP does an automatic sizing of TSO frames,
 	depending on flow rate, instead of filling 64Kbytes packets.
 	For specific usages, it's possible to force TCP to build big
 	TSO frames. Note that TCP stack might split too big TSO packets
 	if available window is too small.
 	Default: 2
 tcp_tso_win_divisor - INTEGER
 	This allows control over what percentage of the congestion window
 	can be consumed by a single TSO frame.