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


    On Wed, 13 Apr 2011, Daniel Halperin wrote:

    > On Wed, Apr 13, 2011 at 10:57 AM, Justin Piszcz <jpiszcz@lucidpixels.com> wrote:
    >> Hi,
    >>
    >> When powersave is enabled, it is very jumpy, I've used satellite comms
    >> before
    >> 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
    >> majority
    >> 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
    >> there
    >> that support wake on wireless lan (WOWL?
    >>
    >> Your ping command with power off:
    >> 1408 bytes from server (192.168.1.2): icmp_req=539 ttl=64 time=1.07 ms
    >> 1408 bytes from server (192.168.1.2): icmp_req=540 ttl=64 time=1.31 ms
    >> 1408 bytes from server (192.168.1.2): icmp_req=541 ttl=64 time=1.07 ms
    >> 1408 bytes from server (192.168.1.2): icmp_req=542 ttl=64 time=1.26 ms
    >>
    >> Your ping command with power on:
    >> 1408 bytes from server (192.168.1.2): icmp_req=649 ttl=64 time=1.80 ms
    >> 1408 bytes from server (192.168.1.2): icmp_req=650 ttl=64 time=1.85 ms
    >> 1408 bytes from server (192.168.1.2): icmp_req=651 ttl=64 time=2.86 ms
    >> 1408 bytes from server (192.168.1.2): 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
    >> horrible
    >> latency.
    >
    > 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
    on.

    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.
    http://www.amazon.com/Medialink-Wireless-Adapter-802-11n-Compatible/dp/B002RM08RE

    100+0 records in
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    104857600 bytes (105 MB) copied, 23.2803 s, 4.5 MB/s

    Justin.
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