On 04/28/2020 09:27 AM, Dave Taht wrote:
On Tue, Apr 28, 2020 at 7:56 AM Steve deRosier <derosier@xxxxxxxxx> wrote:
On Mon, Apr 27, 2020 at 7:54 AM <greearb@xxxxxxxxxxxxxxx> wrote:
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
While running tcp upload + download tests with ~200
concurrent TCP streams, 1-2 processes, and 30 station
vdevs, I noticed that the __ieee80211_stop_queue was taking
around 20% of the CPU according to perf-top, which other locking
taking an additional ~15%.
I believe the issue is that the ath10k driver would unlock the
txqueue when a single frame could be transmitted, instead of
waiting for a low water mark.
So, this patch adds a low-water mark that is 1/4 of the total
tx buffers allowed.
This appears to resolve the performance problem that I saw.
Tested with recent wave-1 ath10k-ct firmware.
Hey Ben,
Did you do any testing with this patch around latency? The nature of
the thing that you fixed makes me wonder if it was intentional with
respect to making WiFi fast - ie getting rid of buffers as much as
possible. Obviously the CPU impact is likely to be an unintended
consequence. In any case, I don't know anything for sure, it was just
a thought that went through my head when reading this.
I note that I'd prefer a BQL-like high/low watermark approach in
general... bytes, not packets, or better, perceived
airtime in a revision of AQL...
... but we'll try this patch, thx!
Is there a nice diagram somewhere that shows where the various
buffer-bloat technologies sit in the stack? I suspect such should
be above the txqueue start/stop logic in the driver that I mucked
with, and certainly the old behaviour has no obvious tie-in with
any higher-level algorithms.
I doubt my patch will change much except in pathological cases where
the system is transmitting frames fast enough to fully fill the tx buffers
and also loaded in such a way that it can process just a few tx frames
at a time to keep bouncing to full and not full over and over.
Thanks,
Ben
--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc http://www.candelatech.com
