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PostPosted: Sun Feb 19, 2012 4:42 am 
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Joined: Tue Mar 22, 2005 9:18 pm
Posts: 1422
Location: Brisbane, Queensland, Australia
I acquired an old Lenovo lappie and I have just put 7.2 on it as a FE to my 6.04 system. Whilst I haven't played too much I did find that when I watched TV it was a bit jerky.

I checked top and the processor was not being taxed much at all, it is an Intel Dual Core t2400 (1.83GB) with 1GB RAM and an ATI graphics card.

I am running it over Wifi so I am wondering is there a way to monitor to the Wifi usage to see if that is the issue or could someone suggest any place that I could look for the issue?

Thanks

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PostPosted: Sun Feb 19, 2012 12:09 pm 
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Joined: Thu Mar 25, 2004 11:00 am
Posts: 9551
Location: Arlington, MA
Look at your network throughput, errors, and packet latency. Looking at a recent one hour HD recording it's 7393920244 bytes for 64 minutes:

7,393,920,244 / (64*60) == 1,925,500 bytes/second

That means you would need a constant and consistent 2Mbytes/s or 20Mbits/s throughput to stream the file. Increasing buffering may help but it can only smooth over so much.

You want to see a throughput graph that looks relatively flat or like a fairly regular pattern without big gaps or spikes. Every error or dropped packet will cause a stall where the system has to notify the server to retransmit then wait for the packet to arrive so it can be processed in order. This in turn may cause network buffers to overflow and other packets to get dropped...

Using netstat -s or the RRD network traffic graphs is one place to start. You can also run ping tests between the client and server using flood pings and large packets. For example I used ping commands like this to investigate connection problems:
Code:
sudo ping -c2000 -s1500 -A YOURMACHINE

You need to run as root for the -A or -f options have maximum effect. (Note that if you compute the throughput from your ping results it will be artificially low because the next packet isn't sent until the previous one is acked and real stream connections use a bigger packet window.)

My next diagnostic would be to copy large files between the machines to determine the bandwidth limits and look for stalls. Beware that if your network is fast enough the disk I/O or bus speed can become the limiting factor, so anchor your results with copies to/from the bit bucket devices. Using NFS and cat on that 7Gb file from the media box gave me roughly 50Mbytes/second over a wired gigabit link which isn't bad, but the old box topped out at 30Mb/s because of the hd and the PCI bus. Also try things like scp or rsync which will let you monitor the progress and watch for stalls.


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PostPosted: Tue Feb 21, 2012 6:52 pm 
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Joined: Tue Mar 22, 2005 9:18 pm
Posts: 1422
Location: Brisbane, Queensland, Australia
Here are the result of a ping test from the BE to the FE:

Code:
2000 packets transmitted, 2000 received, 0% packet loss, time 3326ms
rtt min/avg/max/mdev = 1.151/1.467/8.623/0.617 ms, ipg/ewma 1.664/1.676 ms


Some other ineteresting things I have found playing with this Latop, when I play Recordings or Videos the video image plays really fast, whilst the audio plays in real time.

Anyhow I don't know how to monitor the wireless connection or check the buffer, I am not even sure how to see what speed the connection is at. If you could point me in the direction of some commands I would be most grateful.

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PostPosted: Tue Feb 21, 2012 9:12 pm 
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Joined: Thu Mar 25, 2004 11:00 am
Posts: 9551
Location: Arlington, MA
Checking for networking problems is pretty simple, understanding the information you get is harder. Run netstat like this on the both machines:
Code:
netstat -s

Look for errors, fails, timeouts, retransmits, resets, ... some are normal/inevitable but a lot can mean problems. For best results do this after a reboot (to reset the counters) and trying to play a recording on the slave. If a few minutes of playback produces piles of errors we may have a clue. Also try:
Code:
ifconfig eth0

That provides less detail but maybe less overwhelming.

For throughput monitoring, all the RRD network traffic graphing does is periodically run ifconfig, grab the RX/TX numbers, and compute the deltas. Start with a simple loop like this:
Code:
while true; do ifconfig eth0 | grep '[RT]X' ; echo ; sleep 5 ; done

From there you can get as fancy as you want. Some simple awk or python code could massage that raw output into something you could graph.


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