WLAN latency (was Re: WLAN with Linux SDIO: sneak preview)
Werner Almesberger
werner at openmoko.org
Tue Nov 11 01:26:03 CET 2008
Joerg Reisenweber wrote:
> WLAN-hw automatically entering power-save-mode comes to mind. Leaving this
> fast power-save mode on detection of heavy traffic.
I bow before your intuition :-) Andy Lowe mailed me that he had also
encountered excessive ping latency, and that there is indeed a way to
make power management less aggressive:
wmiconfig -i eth0 --power maxperf
I ran the usual measurement suite. First, the Linux SDIO stack with
the default setting:
---- host => -------- => neo ---------
AVG: 722.95 389 0% 719.43 5888 18%
SDV: 13.05 69 0% 13.38 281 0%
---- neo => --------- => host --------
AVG: 723.84 487 4% 718.53 11601 0%
SDV: 8.10 120 0% 7.61 19 0%
host => neo: min /avg /max stddev (100 samples)
11.80/ 78.96/ 216.00 44.85 ms
neo => host: min /avg /max stddev (100 samples)
10.94/ 77.86/ 311.07 44.70 ms
This is within measurement tolerances of previous results.
And now with --power maxperf:
---- host => -------- => neo ---------
AVG: 718.45 401 0% 715.15 5911 18%
SDV: 16.46 58 0% 16.43 345 0%
---- neo => --------- => host --------
AVG: 707.04 457 3% 702.14 11603 0%
SDV: 9.34 43 0% 9.55 13 0%
host => neo: min /avg /max stddev (100 samples)
5.51/ 12.17/ 76.20 9.10 ms
neo => host: min /avg /max stddev (100 samples)
6.25/ 12.63/ 100.95 12.67 ms
Round-trip time drops dramatically and also the delay variation is
now quite reasonable. There are still a few outliers, but even they
are starting to behave. The results are not as good as with flood
pings, though. So there's still something else going on, but this
is a big step in the right direction.
The improvement comes at a price, though: while the whole systems's
idle battery current is about 166mA (at 4V, USB detached), it jumps
to 289mA with --power maxperf.
Also, throughput seems to have decreased a little in the neo to PC
direction.
Thanks for bringing this idea up, and thanks to Andy Lowe for
letting me know about his experience with --power maxperf !
- Werner
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