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Subjectnetwork problem
Hello anybody,

I try to make some research on the network slowdown problem, which was
reported from some people. In some case the shutoff of tcp_timestamps
was helping, but in other cases not. I don't saw this problem on my
machine up to now, but during some test I saw other strange things.
I the moment I run 2.2.13pre7 (+ ISDN from 2.2.13pre8) but the
same was under plain 2.2.12.
Here a short description of my "test bench":

I make uploads with scp (a ~400K .gz file) over a 64k rawip ISDN line
rate is about 6.6K/s (not optimum, but reliable). The strange things are
the observation on begin and end of the transmit, which I don't understand.

the statistics from scp shows following:

1-2 sec after start 140-160K are transfered
after ~35 sec the transfer is 100%
after ~60 sec the transfer is finished (I got the prompt back).

a netstat -c -t shows allways 40k-60K in Send-Q

the 40k-60K in Send-Q came from a (too big I know) dev->tx_queue_len
value in the ISDN network devive, but where are the other 100K ?

If I try to change tx_queue_len with ifconfig, it don't change anything,
only the new value is displayed in ifconfig.
If I change it in source code the new value is used and here are only
3-6K in Send-Q (with tx_queue_len =5), but here also 140K in buffers after
few seconds.
/proc/sys/net/core/wmem_max is 64K, if I change this with echo -n "8192"...
same behaviour.

This is no problem for me, because the allover rate is OK here, but I like
to understand where and why so much data is buffered somewhere, because
I believe that this may have something to do with the other problem
(network slowdown). With this problem one people saw following:
Networking over an sync ppp ISDN line slows down, a ping freezed.
He started tcpdump on the interface and saw that no packets are going out.
netstat shows data in Send-Q. The ISDN is open and idle.
He was starting a second ping and now pakets are going out, but also from the
old ping every second, so I suspect somewhere a queue or buffer is blocked.
Unfortunatly this case isn't stable, it occours in the moment ones a week
and so he can't make more tests.


Karsten Keil

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