[Bloat] lwn.net's tcp small queues vs wifi aggregation solved

[Toke Høiland-Jørgensen]

Michael Richardson <mcr at sandelman.ca> writes:

> Jonathan Morton <chromatix99 at gmail.com> wrote:
>     >>> I would instead frame the problem as "how can we get hardware to
>     >>> incorporate extra packets, which arrive between the request and grant
>     >>> phases of the MAC, into the same TXOP?"  Then we no longer need to
>     >>> think probabilistically, or induce unnecessary delay in the case that
>     >>> no further packets arrive.
>     >>
>     >> I've never looked at the ring/buffer/descriptor structure of the ath9k, but
>     >> with most ethernet devices, they would just continue reading descriptors
>     >> until it was empty.   Is there some reason that something similar can not
>     >> occur?
>     >>
>     >> Or is the problem at a higher level?
>     >> Or is that we don't want to enqueue packets so early, because it's a source
>     >> of bloat?
>
>     > The question is of when the aggregate frame is constructed and
>     > "frozen", using only the packets in the queue at that instant.  When
>     > the MAC grant occurs, transmission must begin immediately, so most
>     > hardware prepares the frame in advance of that moment - but how far in
>     > advance?
>
> Oh, I understand now.  The aggregate frame has to be constructed, and it's
> this frame that is actually in the xmit queue.  I'm guessing that it's in the
> hardware, because if it was in the driver, then we could perhaps do
> something?

No, it's in the driver for ath9k. So it would be possible to delay it
slightly to try to build a larger one. The timing constraints are too
tight to do it reactively when the request is granted, though; so
delaying would result in idleness if there are no other flows to queue
before then...

Even for devices that build aggregates in firmware or hardware (as all
AC chipsets do), it might be possible to throttle the queues at higher
levels to try to get better batching. It's just not obvious that there's
an algorithm that can do this in a way that will "do no harm" for other
types of traffic, for instance...

-Toke