hi,
i am experimenting with LFN's using "netem".
without adding any delay (using netem), with default (LAN-ish) rtt,
i am able to almost fill a 100Mbps link between two servers. both
nuttcp and iperf report ~94Mbps. thats nice.
then i added 10ms delay using netem on both nodes. making the
rtt as 20ms.
$ tc -s qdisc show dev eth0
qdisc netem 8005: limit 1000 delay 10.0ms
Sent 1199824317 bytes 148119 pkt (dropped 0, overlimits 0 requeues 7)
rate 0bit 0pps backlog 0b 1p requeues 7
i also increased the socket buffers to large values on both nodes:
$ sysctl net | grep mem
net.ipv4.tcp_rmem = 400000 400000 37500000
net.ipv4.tcp_wmem = 400000 400000 37500000
net.ipv4.tcp_mem = 196608 262144 393216
..
net.core.rmem_default = 126976
net.core.wmem_default = 126976
net.core.rmem_max = 37500000
net.core.wmem_max = 37500000
(yes i know those values are really really large. but i finally want
to experiment with 300ms rtts so changed the values to large ones.
both servers have 16GB ram and are dual-core xeons).
and my sles10 kernel looks to supports receiver side autotuning too:
$ uname -a
Linux Node1 2.6.16.21-0.8-smp #1 SMP Mon Jul 3 18:25:39 UTC 2006
x86_64 x86_64 x86_64 GNU/Linux
$ syctl net | grep rcvbuf
net.ipv4.tcp_moderate_rcvbuf = 1
given all this is set (and looks sufficient to me), ttcp or iperf
gives me only 30Mbps.
i tried setting socket buf sizes explicitly too (and turn off
autotuning) via cmdline
options in ttcp/iperf -- but that made no difference.
i expected that i will be able to fill the whole pipe (given i've
tuned all buffers
sufficiently) and get around 90Mbps similar to earlier test (where rtt
was really
small).
am i missing something here? or mis-understanding something?
i can provide the tcpdump trace. but that too looks ok. i.e. window
scale is set to
healthy "10" and the receiver does endup opening large window. but i
may be missing
something.
if you need any other information please do ask.
i am bit puzzled at the moment :-)
thanks in advance,
s.
ps: i did subscribe to linux-net, but have not got any confirmation
yet (waited for 2hrs).
please Cc: me any replies so i dont loose any valuable inputs.
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