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  1. #1

    Question Real VNC client's remote desktop hangs?


    I am using a quad core Xeon workstation with 16GB RAM as VNC server for 20 remote desktop connections for each user using an EDA tool. There are two wireless routers connected to the server on different RF channel for a dedicated WLAN to support the 20 users which connect via 802.11n.

    Although it works fine, some users complain slow connection and/or their desktop hangs indefinitely. When I check the load, memory usage is always less than 50%, CPUs usage is momentarily high during simulations and Ethernet traffic is never 100%. What could be the reason and what is the solution?

    BTW I am using Red Hat Enterprise Linux 5.

  2. #2
    Linux Guru Rubberman's Avatar
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    1. Update to RHEL or CentOS 6.7
    2. It may be that your WiFi is overloaded. 10 people per access point / router doing graphics intensive work is a big load. Have you used something like wireshark or netstat to see what your network traffic looks like?
    3. Do both of your routers / AP's use the same channel? If so, change that so that each is on a different one, preferably not adjacent to the other. IE, one at 3, and the other at 8 would be reasonable.

    When you say that ethernet traffic is never at 100%, what are the maximums? How are you monitoring the system performance? The sar tool is good, but you will need to change the default capture time period from 10 minutes to about 1 minute (in the /etc/cron.d/sysstat file). This is an area I am somewhat an "expert" at - I was principal performance engineer at Nokia Mobile Phones for 2 1/2 years.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  3. #3
    Linux Guru Rubberman's Avatar
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    FWIW, anything over 80% ethernet load (and remember that WiFi doesn't have the bandwidth of wired enet) will cause serious timeouts/retries/delays. TCP/IP is great in that it will persist until packets get through, but that doesn't mean that it is "healthy". Try to keep traffic to a maximum of 60-70% of available bandwidth. Also, make sure each access point / router is connected to a different NIC on the server.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

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  5. #4

    Question

    I guess the Ethernet traffic is less than 100% but both RF channels are well over occupied. But each wireless-N broadband router, Linksys by Cisco should support 10 to 15 users easily for a total of 20 users.

    How come cooperate organizations have so many thin client users with laptops for each division/group and have no such issues? Is it because of 5GHz WiFi instead of 2.4GHz?

  6. #5
    MRTG (Multi Router Traffic Grapher) used for monitoring the traffic load on network links. It allows the user to see traffic load on a network over time in graphical form. MRTG displays graphs for 24 hrs, weekly, monthly and previous month usage. In most cases your IDC should provide the same. If not you can install MRTG on your own server by following these instructions. Wireshark, a network analysis tool formerly known as Ethereal, captures packets in real time and display them in human-readable format. Wireshark includes filters, color-coding and other features that let you dig deep into network traffic and inspect individual packets. You can download Wireshark for RHEL/CentOS/Fedora from its official website. If you’re using Linux or another UNIX-like system, you’ll probably find Wireshark in its package repositories.

  7. #6
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    Quote Originally Posted by emacsx View Post
    Although it works fine, some users complain slow connection and/or their desktop hangs indefinitely. When I check the load, memory usage is always less than 50%, CPUs usage is momentarily high during simulations and Ethernet traffic is never 100%. What could be the reason and what is the solution?
    You should check your and some of your client WiFi statistics data.

    Let me explain a bit how WiFi bandwidth works.

    802.11n with his 65-150 Mbit/s is a marketing mostly if you didn't know that already. When 20 clients connected to the WiFi simultaniously they're sharing the same 65-150 Mbits/s between all of them actually! So each of them will have about 3 Mbits/s, and half of it will be reserved to upload and the rest for download traffic. Not so much, don't you think?

    If you're using 2 WiFi routers with the different networks and channels it will be easier but not too much.

    And if you can see 50% load on your ethernet port you can be sure that WiFi is already overloaded.

    So what happened is that some of your clients just haven't enough bandwidth available. Probably it's the clients with the worst WiFi signal.

    You will need either to add one more WiFi AP or move some of them to the ethernet connection I suppose.

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