Re: [Cerowrt-devel] [Bloat] heisenbug: dslreports 16 flow test vs cablemodems

[Simon Barber <simon@superduper.net>]

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One question about TCP small queues (which I don't think is a good solution 
to the problem). For 802.11 to be able to perform well it needs to form 
maximum size aggregates. This means that it needs to maintain a minimum 
queue size of at least 64 packets, and sometimes more. Will TCP small 
queues prevent this?

Simon

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On May 15, 2015 6:44:21 AM Jim Gettys <jg@freedesktop.org> wrote:

> On Fri, May 15, 2015 at 9:09 AM, Bill Ver Steeg (versteb) <versteb@cisco.com
> > wrote:
>
> > Lars-
> >
> > You make some good points. It boils down to the fact that there are
> > several things that you can measure, and they mean different things.
> >
> > Bvs
> >
> >
> > -----Original Message-----
> > From: Eggert, Lars [mailto:lars@netapp.com]
> > Sent: Friday, May 15, 2015 8:44 AM
> > To: Bill Ver Steeg (versteb)
> > Cc: Aaron Wood; cake@lists.bufferbloat.net; Klatsky, Carl;
> > cerowrt-devel@lists.bufferbloat.net; bloat
> > Subject: Re: [Bloat] [Cerowrt-devel] heisenbug: dslreports 16 flow test vs
> > cablemodems
> >
> >
> > I disagree. You can use them to establish a lower bound on the delay an
> > application over TCP will see, but not get an accurate estimate of that
> > (because socket buffers are not included in the measurement.) And you rely
> > on the network to not prioritize ICMP/UDP but otherwise leave it in the
> > same queues.
> >
>
> ​On recent versions of  Linux and Mac, you can get most of the socket
> buffers to "go away".  I forget the socket option offhand.​
>
> ​And TCP small queues in Linux means that Linux no longer gratuitously
> generates packets just to dump them into the queue discipline system where
> they will rot.
>
> How accurate this now can be is still an interesting question: but has
> clearly improved the situation a lot over 3-4 years ago.​
>
>
> > > If you can instrument TCP in the kernel to make instantaneous RTT
> > available to the application, that might work. I am not sure how you would
> > roll that out in a timely manner, though.
> >
> > ​Well, the sooner one starts, the sooner it gets deployed.​
>
> Jim
>
>
>
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