Re: [Qemu-devel] Memory: how to determine the max memory size of one VM?

[Stefan Hajnoczi]

On Fri, Feb 10, 2012 at 11:00 AM, Zhi Yong Wu <address@hidden> wrote:
> On Fri, Feb 10, 2012 at 6:35 PM, Stefan Hajnoczi <address@hidden> wrote:
>> On Fri, Feb 10, 2012 at 9:47 AM, Zhi Yong Wu <address@hidden> wrote:
>>> Today i tried to create one VM with the option "-m 4000", and found it
>>> failed with the following errors:
>>>
>>> Failed to allocate 4194304000 B: Cannot allocate memory
>>> Aborted (core dumped)
>>
>> Did you run on a 32-bit host?
> No, it is one x86_64 host.

That is weird.  Have you tried strace(1) to find out which system call
is failing and why?

>> until you begin dirtying more host pages than the host has resources
>> for.
>>
>> But normally for KVM you want sum(vms) < host_ram, unless you are
> here host_ram is the free ram of host?

I meant that for predictable, good performance it is normal to size
hosts so they have more physical RAM than the sum of guest RAM.

For example, 2 GB VM and 4 GB VM => make sure you have more than 6 GB
RAM.  If you go under that threshold you will hit swap when the guests
utilize their memory.

Stefan