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Thread: Virtualisation and CPU Resources

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    Virtualisation and CPU Resources

    After experiencing some odd performance issues on one of our virtualised servers at work I was wondering how CPU resources are allocated between virtual servers on the same box? Most other resources are very simple to split between the servers such as RAM and HDD space etc, although a CPU (single core, non-hyperthreading for simplicity) can only run 1 thread at a time.

    The main issue is can very high load on one of the servers affect the performance of another? I'd imagine there's some kind of management done by the 'root' OS so that one server can't take all the resources, sharing the CPU time between the 2 servers. Although I this seems like it could cause response time issues while processes wait for a CPU time slice, since having the time slices too small would lead to time wasted loading data in/out of cache.

    So, can anyone with some more knowledge/experience than me on how well CPU resources are handled between virtualised servers fill me in? Also, I'm not aware of what else is running on the box, since they don't let the devs touch the low level system setup

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    Splash
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    Re: Virtualisation and CPU Resources

    What virtualisation software are you using?

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    Re: Virtualisation and CPU Resources

    if its based on vmware ESX and you only have a single core for the host , then whoever designed the environment needs to be taken outside for a bit of a kicking

    when there is contention for a resource ESX uses the "shares" feature for resource allocation. If all guests have the same ammount of shares then it will simply allocate threads in a round robin fashion. If the shares values are not the same on all hosts then it allocated CPU time by share amount.

    heres some more info for you.

    http://decipherinfosys.wordpress.com...tual-machines/


    also memory allocation is more complex than you'd imagine as you can overcommit memory ( ie allocate more than you have )
    my Virtualisation Blog http://jfvi.co.uk Virtualisation Podcast http://vsoup.net

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    Re: Virtualisation and CPU Resources

    Quote Originally Posted by Splash View Post
    What virtualisation software are you using?
    Quote Originally Posted by Moby-Dick View Post
    if its based on vmware ESX and you only have a single core for the host , then whoever designed the environment needs to be taken outside for a bit of a kicking
    Well, I've got no idea of the setup/spec of the machine. I mentioned single core to make the examples simpler, as you could have a box with 4 cores and 6 virtual machines, I wanted to understand how a core is shared between the servers, and the possible impact of extreme load on one of the servers.

    Thanks for the link Moby, I'll give that a read when I get some more time. Memorys are coming back of modeling CPU time scheduling algorithms at uni

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