The bug is quite easy to replicate, of you have access to the right
type of network.
You need a path with a RTT > 150 ms, capable of at least 500 Mbps
throughput.
Then set both the send and receive buffer to 20 MB, and watch what
happens.
I might be able to provide access to such a network if necessary.
I'll check out oprofile and see what I find.
Hopefully folks will read the paper to see how the combination of
web100 and NetLogger make a great TCP analysis tool.
Cheers.
On Monday, January 27, 2003, at 12:21 PM, Pekka Pietikainen wrote:
On Mon, Jan 27, 2003 at 10:31:00AM -0800, Brian Tierney wrote:
Hi Pekka
I thought you might find this paper interesting. Please forward to the
appropriate Linux TCP folks.
Thanks.
http://www-didc.lbl.gov/papers/PFDL.tierney.pdf
Hi
It was certainly an interesting read. I'll Cc: this reply
netdev@xxxxxxxxxxx, which has relevant people on it. One idea that
might help in pinpointing the problem is using oprofile to see where
all that
CPU is going (http://oprofile.sourceforge.net) when the bug occurs.
What it does is lets you profile applications/the kernel quite
transparently
and see where all your CPU is going when the errors start happening.
Even if it's not useful in finding this problem, it's certainly a very
cool tool you should look at ;)
To find out the problem, they'll of course need a description of the
environment (kernel versions, network between them, tcpdump logs etc.)
I do remember seeing a similar problem on local GigE too when the
zerocopy
patches first came out. That did get fixed (or maybe just made
impossible
to trigger on GigE). Can't remember the details, what happened was
that the
cwnd and ssthresh dropped when there was an error and never recovered
(resulting in something like a 80 -> 50-60MB/s performance drop, which
lasted until the route cache was flushed).
An evil hack to try is
/sbin/ip route add 192.168.9.2 ssthresh <largenumber> dev eth0 ,
which might make it "work" (but it's not the right solution, it just
makes the tcp stack very rude in finding the proper speed to send
after an error :) )
--
Pekka Pietikainen
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Brian L. Tierney, Lawrence Berkeley National Laboratory (LBNL)
1 Cyclotron Rd. MS: 50B-2239, Berkeley, CA 94720
tel: 510-486-7381 fax: 510-495-2998 efax: 240-332-4065
bltierney@xxxxxxx http://www-didc.lbl.gov/~tierney
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