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

Dave Taht dave.taht at gmail.com
Thu Jun 21 11:18:07 EDT 2018

On Thu, Jun 21, 2018 at 5:55 AM, Eric Dumazet <eric.dumazet at gmail.com> wrote:
> On 06/21/2018 02:22 AM, Toke Høiland-Jørgensen wrote:
>> Dave Taht <dave.taht at gmail.com> writes:
>>> Nice war story. I'm glad this last problem with the fq_codel wifi code
>>> is solved
>> This wasn't specific to the fq_codel wifi code, but hit all WiFi devices
>> that were running TCP on the local stack. Which would be mostly laptops,
>> I guess...
> Yes.
> Also switching TCP stack to always GSO has been a major gain for wifi in my tests.
> (TSQ budget is based on sk_wmem_alloc, tracking truesize of skbs, and not having
> GSO is considerably inflating the truesize/payload ratio)
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a6b2a1dc2a2105f178255fe495eb914b09cb37a
> tcp: switch to GSO being always on
> I expect SACK compression to also give a nice boost to wifi.
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d9f4262b7ea41ca9981cc790e37cca6e37c789e
> tcp: add SACK compression
> Lastly I am working on adding ACK compression in TCP stack itself.

One thing I've seen repeatedly on mac80211 aircaps is a tendency for
clients to use up two TXOPs rather than one.


1) A tcp burst arrives at the client
2) A single ack migrates down the client stack into the driver, into
the device, which then attempts to compete for airtime on that TXOP
for that single ack, sometimes waiting 10s of msec to get that op
3) a bunch more acks arrive "slightly late"[1], and then get queued
for the next TXOP, waiting, again sometimes 10s of msec

(similar scenario in a client making a quick string of web related requests)

This is a case where inserting a teeny bit more latency to fill up the
queue (ugh!), or a driver having some way to ask the probability of
seeing more data in the
next 10us, or... something like that, could help.


[1] if you need coffee through your nose this morning, regarding usage
of the phrase "slightly late", read


Dave Täht
CEO, TekLibre, LLC
Tel: 1-669-226-2619

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