Date: Fri, 30 Nov 2001 14:00:43 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Nate Williams <nate@yogotech.com> Cc: Alexander Haderer <alexander.haderer@charite.de>, freebsd-hackers@FreeBSD.ORG Subject: Re: Found the problem, w/patch (was Re: FreeBSD performing worse than Linux?) Message-ID: <200111302200.fAUM0hD27448@apollo.backplane.com> References: <20011128153817.T61580@monorchid.lemis.com> <15364.38174.938500.946169@caddis.yogotech.com> <20011128104629.A43642@walton.maths.tcd.ie> <5.1.0.14.1.20011130181236.00a80160@postamt1.charite.de> <200111302047.fAUKlT811090@apollo.backplane.com> <200111302130.fAULUU324648@apollo.backplane.com> <15367.64883.390696.863120@caddis.yogotech.com>
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:Note, my experiences (and John Capos) are showing degraded performance
:when *NOT* on a LAN segment. In other words, when packet loss enters
:the mix, performance tends to fall off rather quickly.
:
:This is with or without newreno (which should theoretically help with
:packet loss). John claims that disabling delayed_ack doesn't seem to
:affect his performance, and I've not been able to verify if delayed_ack
:helps/hurts in my situation, since the testers have been pressed for
:time so I can't get them to iterate through the different settings.
:
:I do however have some packet dumps, although I'm not sure they will
:tell anything. :(
:
:Nate
Packet loss will screw up TCP performance no matter what you do.
NewReno, assuming it is working properly, can improve performance
for that case but it will not completely solve the problem (nothing will).
Remember that our timers are only good to around 20ms by default, so
even the best retransmission case is going to create a serious hicup.
The question here is... is it actually packet loss that is creating
this issue for you and John, or is it something else? The only way
to tell for sure is to run tcpdump on BOTH the client and server
and then observe whether packet loss is occuring by comparing the dumps.
I would guess that turning off delayed-acks will improve performance
in the face of packet loss, since a lost ack packet in that case will
not be as big an issue.
-Matt
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