path: root/Documentation
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authorEric Dumazet <edumazet@google.com>2013-08-27 05:46:32 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-11-04 04:30:59 -0800
commit5e25ba5003ee5de0ba2be56bfd54d16d4b1b028d (patch)
tree7a953aea4b06398a6f3c66835d6e76b8048acaa4 /Documentation
parent14e9c7db465387ede7f019c42f28c90f99fc2793 (diff)
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>
Diffstat (limited to 'Documentation')
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 3458d6343e01..3994f0bbeeb6 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -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.