[Tmrg] Round table: Buffer sizes

[Lachlan Andrew]

Greetings John,

On 25/09/2007, John Heffner <jheffner at psc.edu> wrote:
> Lachlan Andrew wrote:
> >
> > suitable buffer sizes for this core set of tests?
>
>
> Given the relatively wide range of buffer sizes in hardware out there at
> any given speed, and the significant effect buffer size can have on
> congestion control, it seems like this should be an extra dimension
> rather than fixed per link type.

Absolutely.  As Sally suggests, it would be good to have some tests
specifically along this dimension.  That leaves open what value(s?) to
use for this parameter for tests along other dimensions.

> Maybe select a couple different sizes based on a set of drain times?
> Say, {1 ms, 10 ms, 100 ms, 1 sec}?  Convert this to bytes or packets by
> dividing by the link byte or MTU packet rate.

Sounds good.  As I pointed out, converting delay to packets isn't as
simple as dividing by MTU packet rate, because we have reverse ACKs
which aren't full sized.  If we have an equal number of forward and
backward flows every second ACK is delayed, we need a buffer of
roughly  (3/2)(drain_time/MTU_rate)  packets.  Without delayed ACKs,
it becomes  rougly  2(drain_time/MTU_rate).


In my limited experience, buffer sizes are typically powers of 2.  I
propose that all "core" tests use buffers with a limit (in packets)
equal to a power of 2.  Do others agree?

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
Phone: +1 (626) 395-8820    Fax: +1 (626) 568-3603