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Re: network queue lengths

On Thu, Mar 19, 2009 at 12:47:01PM -0500, David Young wrote:
> This is getting a bit long, so I am going to hastily draw some
> conclusions.  Please tell me if I am way off base:

Let me say up front that I don't much care about ALTQ, and I care
more about not harming performance on machines with very fast networks
on both sides (or their only "side") than about reducing latency on
machines with dramatically mismatched networks.

> 1 in order for ALTQ to be really effective at controlling latency for
>   delay-sensitive traffic, it has to feed a very short Tx ring

So, this one I don't much care about, personally.

> 2 maximum queue/ring lengths in NetBSD are tuned for very high-speed
>   networks, now; the maximums should adapt to hold down the expected delay
>   while absorbing momentary overflows.

Can we trust the "baudrate" reported by an interface?  If we can, it should
be possible to algorithmically tune this, but knowledge of the system's
network topology and characteristic packet flows -- which can probably
be inferred at runtime -- may be required in order to get it right.

Constant, parametric tuning like "grow the queue until latency hits _X_"
seems highly likely to interact in very negative ways with TCP.

Thor Lancelot Simon                               
    "Even experienced UNIX users occasionally enter rm *.* at the UNIX
     prompt only to realize too late that they have removed the wrong
     segment of the directory structure." - Microsoft WSS whitepaper

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