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This is my question. I have one Xen server with 8 CPU's and 6 virtual machine running, each virtual hard disk are running in diferent physical hardisk. Everything worked fine but sometimes one virtual machine get almost whole CPU, if the Domain-0 is 90% that is normal, the virtual machine is 500% usage of CPU. I have improved that it is not depends who are working with the VM even when nobody are working with the server this still happens. I dont know what happen. Anyone have any idea?¿ or anyone have happened the same?¿

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Sorry If I write about something you already know, here are my 2cents:

1) Xen Virtual CPUs given to DomU are not same as physical machines CPUs ... virtual machine with 4 virtual CPUs can have all those 4 mapped to 1 physical CPU during some moment - for example - that way you can get 400% usage within DomU and 100% on Dom0 - but if you have enough cores these moments does not happen , xen is not that dumb. you should check "xm vcpu-list" to find out and "xm vcpu-pin" to play a little

2) Then there is a credit scheduling - Xen measures how many CPU resources is each domU spending, but there is no cap for that in default. I should maybe try setting that cap and observe whether that maybe will slow down for real - whether the services from that machine will start lagging or something slows down a lot.

3) Did you logged into this machine during these moments? Is it heavy on IO? is it swapping?

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Sorry for the delay... too much work!! Yeah sometimes i can log in the machines but sometimes not, its depend. I have installed a munin system to monitor the xens. And when it happened the IO is really heavy. I'm starting to think that its one process which does what hasnt to do. –  ki0 Jan 13 '11 at 12:35

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