Messages in this thread | | | Date | Fri, 21 May 1999 10:12:54 -0400 | From | Josip Loncaric <> | Subject | Re: [patch] TCP/IP delacks disabled with MPI |
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Andrea Arcangeli wrote: > > [...] > > One of the thing I noticed from the tcp traces sent to me from Josip, is > that currently we send the ack only if ATO expires or we received more > than 2*MSS data but we don't care about the exact _number_ of not acked > packets we have in the receive queue.
My understanding is that Linux TCP incorrectly handles the slow start algorithm because it does not count the _number_ of packets that each ACK represents. This may be at the root of the problem we see, and perhaps explains why immediately ACKing every packet fixes the problem. Unfortunately, I cannot test 2.2.9 w/Andrea's patch because our system is now in use, and we've got to stick with 2.0.36 until at least September (for a variety of reasons).
BTW, rfc2414 suggests that the initial congestion window can be 3 or even 4, and that this is a good minimum value to use after being idle for a while. By default, TCP resets congestion window to 1 after being idle for an entire RTO. For our internal use only when TCP_NODELAY is set, we use exponential backoff to a minimum cong_window of 3, and various timeout constants are up to ten times lower (for faster recovery). Normal TCP operation is maintained for sockets without TCP_NODELAY.
Our Beowulf-class cluster has demonstrated that Linux TCP is simply not tuned for highly interactive interprocess communication. We need Fast Ethernet latencies well under 100 microseconds, 90+Mbit/s bandwidth, and responsiveness generally 10-1000 times better than what standard TCP uses need. Congestion on our switched fast ethernet network is very rare, and the sockets mostly behave as if direct full duplex crossover cables were used. Apart from the computational cost imposed by ACKing every packet, our modified TCP comes close to what we need. I hope that a better fix will become available, but in the near future we'd also like to try M-VIA which promises latencies about three times better than standard TCP.
Sincerely, Josip
-- Dr. Josip Loncaric, Senior Staff Scientist mailto:josip@icase.edu ICASE, Mail Stop 132C http://www.icase.edu/~josip/ NASA Langley Research Center mailto:j.loncaric@larc.nasa.gov Hampton, VA 23681-2199, USA Tel. +1 757 864-2192 Fax +1 757 864-6134
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