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Re: DP performance

From: Marko Zec <zec@xxxxxxxx>
Date: Thu, 1 Dec 2005 23:47:47 +0100

On Thursday 01 December 2005 23:13, Matthew Dillon wrote:
> :...
> :
> :> of latency occuring every once in a while would not have any
> :> adverse effect.
> :
> :A few milliseconds of latency / jitter can sometimes completely kill
> : TCP throughput at gigabit speeds.  A few microseconds won't matter,
> : though.
> :
> :Cheers,
> :
> :Marko
> Not any more, not with scaled TCP windows and SACK.  A few
> milliseconds doesn't matter.  The only effect is that you need a
> larger transmit buffer to hold the data until the round-trap ack
> arrives.  so, e.g. a 1 Megabyte buffer would allow you to have
> 10mS of round-trip latency.  That's an edge case, of course, so to be
> safe one would want to cut it in half and say 5 mS with a 1 megabyte
> buffer.

Mostly true, but having TCP window sizes as large as a megabyte doesn't 
come at no cost, just as you described later in your note (there may be 
other problems as well).  But I don't think that today's gigabit cards 
ever delay interrupts for more than a few dozens of microseconds 
(unless explicitly misconfigured ;), so probably we have a non-issue 



> TCP isn't really the problem, anyway, because it can tolerate any
> amount of latency without 'losing' packets.  So if you have a TCP
> link and you suffer, say, 15 ms of delay once every few seconds,
> the aggregate bandwidth is still pretty much maintained.
> The real problem with TCP is packet backlogs appearing at choke
> points. For example, if you have a GigE LAN and a 45 MBit WAN, an
> incomming TCP stream from a host with an aweful TCP stack (such as a
> windows server) might build up a megabyte worth of packets on your
> network provider's border router all trying to squeeze down into 45
> MBits. NewReno, RED, and other algorithms try to deal with it but the
> best solution is for the server to not try to push out so much data
> in the first place if the target's *PHYSICAL* infrastructure doesn't
> have the bandwidth.  But that's another issue.
> 					-Matt
> 					Matthew Dillon
> 					<dillon@xxxxxxxxxxxxx>

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