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Re: [Qemu-devel] RAM overcomittment


From: The MoonSeeker
Subject: Re: [Qemu-devel] RAM overcomittment
Date: Thu, 28 Sep 2006 00:16:32 +0200

Le 27 sept. 06 à 23:57, James Olsen a écrit :

It seems to me that no change to QEMU is needed for this; it should

already be supported by your host OS. Simply allocate the memory that

you want (256mb, for example) for each virtual machine. Depending on

how many virtual machines you have open at once, as well as the memory

used by the host OS and any other applications you're running, the

host OS will fit the as much as it can in the physical ram, and

automatically swap out the excess allocated memory to disk.


However, swapping (using virtual memory) will be a huge performance

killer. It will affect the performance of your host OS and all

applications, as well as for your QEMU instances.


The problem is the swaping even the VM don't need all of ressource assigned... We lost RAM...

By example you run 4 VM who have 256 MB (assigned with -M option) but just use 56MB in most of time.. 

with qemu we lost 4 X (256-56MB) = 800MB of RAM!

Somme other virtualization solution like OpenVz and VServer let this RAM overcommitment. On a VM when you make the command free you can show all RAM installed on the workstation, and with file configuration you set the guaranteed RAM (in this case 56MB) and the limite (in this case 256MB). The limit can be reach only for a cup of time (when VM need ressource..)!!!

Do you understant what I mean?

I'd like to create an open network simulator based on qemu but this restriction can't let me use qemu...

What do you think? 


 

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