It seems that recent (and maybe not so recent) linux kernels have a TCP
problem that causes them to acknowledge almost every segment. While,
strictly speaking, this is not against the spec, any sane TCP only acks
every second segment in steady state. The statistics appended below
illustrate the problem.
Why do I care? Because I'm connected through a cable network with a
severe bandwidth asymmetry; the upstream is rate limited to 256 kbps,
while the downstream can theoretically yield 10 Mbps (assuming a quiet
period). Right now, the excessive ack rate seems to be a limiting factor
on peak performance.
I've already disabled quickacks, replaced the receive MSS estimate with
advertised MSS in the ack sending policy (two places), and removed one
dubious "immediate ack" condition from send_delay_ack(). The annoying
thing is that none of this seem to make any real difference. I must be
missing something huge that's right in front of my nose, but I'm
starting to run out of steam.
Any thoughts on this?
Regards,
MikaL
Some stats from a unidirectional 6MB transfer:
c->d: d->c:
total packets: 3643 total packets: 4498
ack pkts sent: 3642 ack pkts sent: 4498
pure acks sent: 3640 pure acks sent: 2
unique bytes sent: 108 unique bytes sent: 6161570
actual data pkts: 1 actual data pkts: 4494
actual data bytes: 108 actual data bytes: 6161570
rexmt data pkts: 0 rexmt data pkts: 0
rexmt data bytes: 0 rexmt data bytes: 0
outoforder pkts: 0 outoforder pkts: 10
pushed data pkts: 1 pushed data pkts: 3043
SYN/FIN pkts sent: 1/1 SYN/FIN pkts sent: 1/1
req 1323 ws/ts: Y/Y req 1323 ws/ts: Y/Y
adv wind scale: 0 adv wind scale: 0
req sack: Y req sack: Y
sacks sent: 28 sacks sent: 0
mss requested: 1460 bytes mss requested: 1460
bytes
max segm size: 108 bytes max segm size: 1448
bytes
min segm size: 108 bytes min segm size: 92
bytes
avg segm size: 107 bytes avg segm size: 1371
bytes
max win adv: 63712 bytes max win adv: 32120
bytes
min win adv: 5840 bytes min win adv: 32120
bytes
zero win adv: 0 times zero win adv: 0
times
avg win adv: 63515 bytes avg win adv: 32120
bytes
initial window: 108 bytes initial window: 489
bytes
initial window: 1 pkts initial window: 1 pkts
ttl stream length: 108 bytes ttl stream length: 6161570
bytes
missed data: 0 bytes missed data: 0
bytes
truncated data: 46 bytes truncated data: 5882942
bytes
truncated packets: 1 pkts truncated packets: 4494 pkts
data xmit time: 0.000 secs data xmit time: 10.663 secs
idletime max: 98.1 ms idletime max: 98.0 ms
throughput: 10 Bps throughput: 572079 Bps
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