tcp: try to send bigger TSO packets
While investigating TCP performance, I found that TCP would
sometimes send big skbs followed by a single MSS skb,
in a 'locked' pattern.
For instance, BIG TCP is enabled, MSS is set to have 4096 bytes
of payload per segment. gso_max_size is set to 181000.
This means that an optimal TCP packet size should contain
44 * 4096 = 180224 bytes of payload,
However, I was seeing packets sizes interleaved in this pattern:
172032, 8192, 172032, 8192, 172032, 8192, <repeat>
tcp_tso_should_defer() heuristic is defeated, because after a split of
a packet in write queue for whatever reason (this might be a too small
CWND or a small enough pacing_rate),
the leftover packet in the queue is smaller than the optimal size.
It is time to try to make 'leftover packets' bigger so that
tcp_tso_should_defer() can give its full potential.
After this patch, we can see the following output:
14:13:34.009273 IP6 sender > receiver: Flags [P.], seq
4048380:
4098360, ack 1, win 256, options [nop,nop,TS val
3425678144 ecr
1561784500], length 49980
14:13:34.010272 IP6 sender > receiver: Flags [P.], seq
4098360:
4148340, ack 1, win 256, options [nop,nop,TS val
3425678145 ecr
1561784501], length 49980
14:13:34.011271 IP6 sender > receiver: Flags [P.], seq
4148340:
4198320, ack 1, win 256, options [nop,nop,TS val
3425678146 ecr
1561784502], length 49980
14:13:34.012271 IP6 sender > receiver: Flags [P.], seq
4198320:
4248300, ack 1, win 256, options [nop,nop,TS val
3425678147 ecr
1561784503], length 49980
14:13:34.013272 IP6 sender > receiver: Flags [P.], seq
4248300:
4298280, ack 1, win 256, options [nop,nop,TS val
3425678148 ecr
1561784504], length 49980
14:13:34.014271 IP6 sender > receiver: Flags [P.], seq
4298280:
4348260, ack 1, win 256, options [nop,nop,TS val
3425678149 ecr
1561784505], length 49980
14:13:34.015272 IP6 sender > receiver: Flags [P.], seq
4348260:
4398240, ack 1, win 256, options [nop,nop,TS val
3425678150 ecr
1561784506], length 49980
14:13:34.016270 IP6 sender > receiver: Flags [P.], seq
4398240:
4448220, ack 1, win 256, options [nop,nop,TS val
3425678151 ecr
1561784507], length 49980
14:13:34.017269 IP6 sender > receiver: Flags [P.], seq
4448220:
4498200, ack 1, win 256, options [nop,nop,TS val
3425678152 ecr
1561784508], length 49980
14:13:34.018276 IP6 sender > receiver: Flags [P.], seq
4498200:
4548180, ack 1, win 256, options [nop,nop,TS val
3425678153 ecr
1561784509], length 49980
14:13:34.019259 IP6 sender > receiver: Flags [P.], seq
4548180:
4598160, ack 1, win 256, options [nop,nop,TS val
3425678154 ecr
1561784510], length 49980
With 200 concurrent flows on a 100Gbit NIC, we can see a reduction
of TSO packets (and ACK packets) of about 30 %.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240418214600.1291486-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>