[Make-wifi-fast] On the ath9k performance regression with FQ and crypto

[Kevin Hayes]

>And the
crypto itself should be offloaded to hardware (shouldn't it? we do see a
marked drop in overall throughput from just enabling crypto)

Seems like you need to deterministically determine if the hw crypto is
enabled, and actually happening.  If not, then SW crypto could be
consuming, um, more CPU than you want.  I assume you are working on this.

Beyond that, yes, the use of crypto will add about 16 B of overhead to each
MPDU, so maybe 1%.  Maybe the reported MTU might be less than 1500B so
something must fragment??

K++

On Tue, Aug 16, 2016 at 1:47 PM, Eric Dumazet <eric.dumazet at gmail.com>
wrote:

>
> Do you have tcpdumps of
>
> 1) sample with crypto
>
> 2) sample without crypto.
>
> Looks like some TCP Small queue interaction with skb->truesize, if GSO
> is involved, or encapsulation adding overhead.
>
>
> On Tue, 2016-08-16 at 22:41 +0200, Toke Høiland-Jørgensen wrote:
> > So Dave and I have been spending the last couple of days trying to
> > narrow down why there's a performance regression in some cases on ath9k
> > with the softq-FQ patches. Felix first noticed this regression, and LEDE
> > currently carries a patch [1] to disable the FQ portion of the softq
> > patches to avoid it.
> >
> > While we have been able to narrow it down a little bit, no solution has
> > been forthcoming, so this is an attempt to describe the bug in the hope
> > that someone else will have an idea about what could be causing it.
> >
> > What we're seeing is the following (when the access point is running
> > ath9k with the softq patches):
> >
> > When running two or more flows to a station, their combined throughput
> > will be roughly 20-30% lower than the throughput of a single flow to the
> > same station. This happens:
> >
> > - for both TCP and UDP traffic.
> > - independent of the base rate (i.e. signal quality).
> > - but only with crypto enabled (WPA2 CCMP in this case).
> >
> > However, the regression completely disappears if either of the
> > following is true:
> >
> > - no crypto is enabled.
> > - the FQ part of mac80211 is disabled (as in [1]).
> >
> > We have been able to reproduce this behaviour on two different ath9k
> > hardware chips and two different architectures.
> >
> > The cause of the regression seems to be that the aggregates are smaller
> > when there are two flows than when there is only one. Adding debug
> > statements to the aggregate forming code indicates that this is because
> > no more packets are available when the aggregates are built (i.e.
> > ieee80211_tx_dequeue() returns NULL).
> >
> > We have not been able to determine why the queues run empty when this
> > combination of circumstances arise. Since we easily get upwards of 120
> > Mbps of TCP throughput without crypto but with full FQ, it's clearly not
> > the hashing overhead in itself that does it (and the hashing also
> > happens with just one flow, so the overhead is still there). And the
> > crypto itself should be offloaded to hardware (shouldn't it? we do see a
> > marked drop in overall throughput from just enabling crypto), so how
> > would the queueing (say, mixing of packets from different flows)
> > influence that?
> >
> > Does anyone have any ideas? We are stumped...
> >
> > -Toke
> >
> > [1] https://git.lede-project.org/?p=lede/nbd/staging.git;a=blob;
> f=package/kernel/mac80211/patches/220-fq_disable_hack.patch;h=
> 7f420beea56335d5043de6fd71b5febae3e9bd79;hb=HEAD
> > _______________________________________________
> > Make-wifi-fast mailing list
> > Make-wifi-fast at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/make-wifi-fast
>
>
>


-- 
Kevin Hayes
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/make-wifi-fast/attachments/20160816/1c5649b7/attachment-0001.html>