Re: Vegas_cong_avoid patch redux (was Re: TCP Vegas Patch) LONG

Andi Kleen (ak@muc.de)
Wed, 4 Aug 1999 08:52:20 +0200


On Wed, Aug 04, 1999 at 03:30:24AM +0200, paulr wrote:
> > Reality check:
> > Vegas only changes the TCP sending algorithms, nothing at the
> > receiver side (ACK policy is not changed). So for bulk downloads at
> > the receiver side it should make no difference. Did the sender in this
> > case run the Vegas algorithm too? Did it behave differently with
> > other senders not running Vegas?
>
> The web page I referred to in my posting suggested that the reimple-
> mented TCP_V_C algorithm performed better in the presence of *delayed
> ACKS* (ACK/NACK policy??). I take that to mean that the algorithm
> uses a different method of transmit/retransmit control when ACK/NACK
> link signals are time-delayed due to net.congestion.

Yes, that is a sender algorithm.

> Cardwell mentioned in his paper that a part of the improvement resulted
> from the use of usleep() for a more finely-grained timing algorithm.
> Cardwell stated to the effect that a previous kernel implementation
> of Reno-Vegas did not achieve its potential because of the rather
> coarse granularirty of the 10ms (1 jiffy???) "clock tick" that was used....

There is (fortunately) no usleep in kernel.

His patch uses a more finegrained timer to tune some sending decisions,
but this is pure sender side too (if you read the patch you see that
the parts that decide if to send an ACK or not are not touched)

> Isn't Van-Jacobsen a header compression algorithm?
> (I haven't read RFC-1122...)

Van Jacobsen invented lots of algorithms, among them a header compression
algorithm and the standard congestion avoidance in use with TCP nowadays

>
> I'd like to try out your new code. Anything to improve this
> dial-up link would be a gift from heaven ;-)

One thing you can do now is to experiment with shorter queues.

After the PPP interface is established (you need recent net-tools):

/sbin/ifconfig <ppp-interface> txqueuelen <len>

Good values for a PPP connection may be between 3 and 20, you have to
experiment.

-Andi

-- 
This is like TV. I don't like TV.

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