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Date: Wed, 13 Apr 2011 16:13:02 -0400 (EDT) From: Justin Piszcz <jpiszcz@...idpixels.com> To: Daniel Halperin <dhalperi@...washington.edu> cc: Ivo Van Doorn <ivdoorn@...il.com>, linux-usb@...r.kernel.org, linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: 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@...idpixels.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 100+0 records out 104857600 bytes (105 MB) copied, 23.2803 s, 4.5 MB/s Justin.
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