Re: NetBSD 5.1 TCP performance issue (lots of ACK)

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Mon, Oct 24, 2011 at 11:44:10AM +0200, Havard Eidnes wrote:
> > To diagnose this, I recommend making a tcpdump of the traffic with
> >
> > tcpdump -w TRACE tcp
> >
> > and then using  tcpdump2plot in graphics/xplot-devel.
> >
> > I am not clear if graphics/xplot-devel (or /xplot) has caught up to
> > minor changes in NetBSD tcp
> 
> I'm more familiar with the graphs which tcptrace can produce.  It can
> make time/sequence plots (ref. the -S option) which are well
> understood by graphics/xplot, at least last I looked.
> 
> > From the stats, it looks like there is loss in the data stream causing
> > SACK.  But I don't see how there would be 237850 pure acks for ~98K data
> > packets; there should be more like 49K.
> 
> I agree, the traditional behaviour is that NetBSD TCP acks every
> second TCP segment.
> 
> There doesn't appear to be all that much retransmission, though; only
> 54 data packets have been retransmitted in the "connection 5" example,
> although the retransmission rate *is* non-zero, so some packets may
> indeed go missing in the network somewhere, which is never a good
> recipe for high TCP performance.

I've also trace where there's no retransmissions at all, but this
doesn't change much the data rate.

> 
> Generally, the congestion control algorithm used by default by Linux
> these days is a bit more agressive when ramping up the congestion
> window ("how much data can we send before having to wait for an ack")
> after an idle period or after a packet loss event, it *may* be that
> the effect of this is what you're seeing.
> 
> When you say that "NetBSD with Linux performs well", which end in this
> setup is the sender of the majority of data?  If it's the Linux end,
> that could be a further hint that it's the TCP congestion control
> algorithm variants which plays a role.

It doesn't matter which end linux is. A NetBSD client is performing
better agaist a Linux server than against NetBSD server, and a linux
client performs better than a NetBSD client against the same NetBSD server.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--