Re: 2.6.38: rt2800usb: high latency (1000ms)?

From: Justin Piszcz
Date: Wed Apr 13 2011 - 16:13:11 EST

On Wed, 13 Apr 2011, Daniel Halperin wrote:

On Wed, Apr 13, 2011 at 10:57 AM, Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx> wrote:

When powersave is enabled, it is very jumpy, I've used satellite comms
and (~600ms-1200ms was more smooth) as it did not jump around as much.  The
application is just a standalone desktop with minimal activity for the
of the time, maybe thats why..

With powersave disabled, I now see 0% packet loss (802.11n) and low ping
times, this looks like the proper solution for the wireless USB device I
am using.  By the way, is it possible/are there wireless USB devices out
that support wake on wireless lan (WOWL?

Your ping command with power off:
1408 bytes from server ( icmp_req=539 ttl=64 time=1.07 ms
1408 bytes from server ( icmp_req=540 ttl=64 time=1.31 ms
1408 bytes from server ( icmp_req=541 ttl=64 time=1.07 ms
1408 bytes from server ( icmp_req=542 ttl=64 time=1.26 ms

Your ping command with power on:
1408 bytes from server ( icmp_req=649 ttl=64 time=1.80 ms
1408 bytes from server ( icmp_req=650 ttl=64 time=1.85 ms
1408 bytes from server ( icmp_req=651 ttl=64 time=2.86 ms
1408 bytes from server ( icmp_req=652 ttl=64 time=1.46 ms

You are correct, if there is a lot of traffic, its good, but if the system
is relatively idle and all that's going on is an SSH session, there is

Gotcha. I might still look around in the network stack and/or driver
and see what the time constants are. For instance:

(1) What is the AP's beacon period and DTIM? Typical values are 100
TUs for beacons (102.4 ms) and 2 for DTIM (2 beacons per power-save
wakeup) which should imply a mean of 100 and max of 200 ms delay even
on pings.
I am using a WNDR3700 with default settings in terms of beacons/etc. No
issues with any device (laptop, computer, etc (in windows)), I have two
wireless USB adapters (bought two) and in Windows, no problems, I don't think
it is the WNDR3700. As far as linux/wpa-supplicant, using default settings.

(2) How long does the client wait after waking up to go back to sleep?
It should be at least a few seconds. For ssh, then, you should see
something like a 100-200 ms delay for the first key and then nothing
at all unless you stop typing for a bit.
It lags with each word I type, it is terrible. If I run something like
dmesg or ps auxww, the entire session freezes for 5-10 seconds before it
comes back.

I'm SSHing over a Wi-Fi link that uses power save right this second,
and have for years. It's not generally an issue, I suspect something
worse is going on.
Maybe the wireless usb adapters do not function well in Linux with power save

I bought them awhile ago, they had the highest reviews, and in Windows,
they did do 10-15MiB/s, in Linux, I see ~4.6MiB/s (but that was with power save on) about the same, 4.5MiB/s.

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