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

[Zhi Yong Wu]

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.
>
>> I checked the qemu code, and found that in qemu_memalign() function,
>> posix_memalign() failed to allocate requested memory. Perhaps due to
>> No enough memory.
>>
>> But when i tried with "-m 3000", it can work.
>>
>> So i have one question about this. How to determine the supported max
>> memory size of one VM on one specifc host?
>
> Since QEMU may use host virtual memory for guest RAM the limit isn't a
> hard limit that is easy to describe.  If the host OS performs memory
> overcommit by default (i.e. will allow processes to map memory without
> dedicating that amount of swap), then you can theoretically allocate
> up to your address space limit (32-bit or 64-bit) and won't get error
Since the host is 64 bits, it should not fail with "-m 4000". I am
very surprised at why posix_memalign failed.

> 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?
> betting on KSM to deduplicate guest memory.
>
> Stefan



-- 
Regards,

Zhi Yong Wu