H.K. Jerry Chu wrote:
Hello,
I'm testing 2.6.18 over an emulated long haul network (using netem
with 100ms delay).
For some reason if I just rely on the autotuning ffeature of the stack
(i.e., no -w to
iperf) over a 1GbE link, I can only get ~820Mbps throughput. All the
other sysctl
tunables (e.g., tcp_rmem, tcp_wmem) have been set to sufficiently large
(4096 8388608 20971520). In order to saturate the link I had to
specify SO_SND/RCVBUF
sizes to, e.g, 13MB using the -w option to iperf, effectively
disabling autotuning.
I also discovered that the autotuning deficiency seems to reside only
on the xmit side,
i.e., if i specify -w 13m on the xmit side only I can attain 945Mbps
throughput.
Also if i increase the delay to 150ms I can only get up to ~615Mbps
from autotuning.
Am I bumping into some known bugs?
Are you getting any packet losses? Sometimes autotuning allows
congestion control to operate as it should, which does not always fully
utilize a link. Careful hand tuning can give you a window that is
capped just below congestion so you avoid packet losses altogether and
get perfect utilization. This situation isn't a bug, just a limitation
of TCP congestion control. I'm not sure this is what's happening to
you. Can you check?
-John
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