Messages in this thread | | | Subject | Re: Weird tcp performance differences with 2.0 and 2.2 kernels | From | (Harvey J. Stein) | Date | 10 Feb 1999 21:19:07 +0200 |
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Andi Kleen <ak@muc.de> writes:
> In article <m2679aio4w.fsf@blinky.bfr.co.il>, > hjstein@bfr.co.il (Harvey J. Stein) writes: > > "Mark R. Boyns" <boyns@contigo.com> writes: > >> We're trying to figure out some weird performance differences between > >> 2.0.x / 2.2.x kernels and the interface being used. > > > I noticed some weird behavior last night. I was dialed into the > > office from home. I was uploading some files via ftp. The modem send > > light started off blinking rapidly (basically flickering) indicating > > high traffic. Then it suddenly backed off, maybe sending about 1 > > packet/second. It stayed like this for maybe 30-60 seconds & then > > sped up again, but not to as fast as it was initially. Final > > throughput was a little on the low side. > > Congratulations. You just discovered the slow start TCP algorithm. Please > read RFC2001 for more information.
It's not explained by slow start. Ping times are <200ms, so even with the congestion window reduced to size 1 my machine still should have only been waiting about 100ms for an ACK before transmitting the next packet.
It could be congestion avoidance, but I fail to see why this should be triggered when I'm uploading over a 33.6kbps ppp link to a machine on an ethernet. It's transfers in the other direction that should trigger congestion avoidance.
In fact, what seems to be happening is that eventually it's settling on a congestion window size of 1, waiting for each ack before sending another packet. This basically agrees with what I observed after it sped up again, but still doesn't explain the period of time where it only was sending about 1 packet per second. It also doesn't explain why it didn't increase the congestion window during this.
BTW, I only brought it up because I haven't observed this behavior with 2.0.x.
-- Harvey J. Stein BFM Financial Research hjstein@bfr.co.il
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