How much physical memory can be used to run domains in a KVM machine?

How much physical memory can be used to run domains in a KVM machine?

[Zhang Qian]

Hi,

I have a KVM box which has 4GB physical memory totally, I'd like to
know how much I can use to run my domains, and how much will be
reserved by hypervisor(KVM) itself?
Thanks!


Regards,
Qian

Re: How much physical memory can be used to run domains in a KVM machine?

[Dor Laor]

On 07/17/2009 08:50 AM, Zhang Qian wrote:
> Hi,
>
> I have a KVM box which has 4GB physical memory totally, I'd like to
> know how much I can use to run my domains, and how much will be
> reserved by hypervisor(KVM) itself?
> Thanks!
>

KVM and the Linux host use relatively low amount of memory.
Unlike other hypervisors you know, kvm does not reserve memory and also 
is able to swap the guest memory, so you can even use more than 4G for 
your guest. (Just note swapping will be slow)


>
> Regards,
> Qian
> --
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> the body of a message to majordomo@vger.kernel.org
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Re: How much physical memory can be used to run domains in a KVM machine?

[sudhir kumar]

On Fri, Jul 17, 2009 at 12:47 PM, Dor Laor<dlaor@redhat.com> wrote:
> On 07/17/2009 08:50 AM, Zhang Qian wrote:
>>
>> Hi,
>>
>> I have a KVM box which has 4GB physical memory totally, I'd like to
>> know how much I can use to run my domains, and how much will be
>> reserved by hypervisor(KVM) itself?
>> Thanks!
>>
>
> KVM and the Linux host use relatively low amount of memory.
> Unlike other hypervisors you know, kvm does not reserve memory and also is
> able to swap the guest memory, so you can even use more than 4G for your
Is that true? I think we can not allocate memory more than the
physical RAM. Or does upstream kvm supports it? My kvm version is not
that old but memory allocation failed for me when I tried to give the
whole memory on my host to the guest.

> guest. (Just note swapping will be slow)
>
>
>>
>> Regards,
>> Qian
>> --
>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
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>



-- 
Sudhir Kumar

Re: How much physical memory can be used to run domains in a KVM machine?

[Amit Shah]

On (Fri) Jul 17 2009 [15:26:49], sudhir kumar wrote:
> On Fri, Jul 17, 2009 at 12:47 PM, Dor Laor<dlaor@redhat.com> wrote:
> > On 07/17/2009 08:50 AM, Zhang Qian wrote:
> >>
> >> Hi,
> >>
> >> I have a KVM box which has 4GB physical memory totally, I'd like to
> >> know how much I can use to run my domains, and how much will be
> >> reserved by hypervisor(KVM) itself?
> >> Thanks!
> >>
> >
> > KVM and the Linux host use relatively low amount of memory.
> > Unlike other hypervisors you know, kvm does not reserve memory and also is
> > able to swap the guest memory, so you can even use more than 4G for your
> Is that true? I think we can not allocate memory more than the
> physical RAM. Or does upstream kvm supports it? My kvm version is not
> that old but memory allocation failed for me when I tried to give the
> whole memory on my host to the guest.

What was the error? Also kvm only allocates memory for the guest when
the guest needs it so even if you give all your host memory to the
guest, it doesn't mean the guest will start using it immediately.

		Amit

Re: How much physical memory can be used to run domains in a KVM machine?

[Sheng Yang]

On Friday 17 July 2009 17:56:49 sudhir kumar wrote:
> On Fri, Jul 17, 2009 at 12:47 PM, Dor Laor<dlaor@redhat.com> wrote:
> > On 07/17/2009 08:50 AM, Zhang Qian wrote:
> >> Hi,
> >>
> >> I have a KVM box which has 4GB physical memory totally, I'd like to
> >> know how much I can use to run my domains, and how much will be
> >> reserved by hypervisor(KVM) itself?
> >> Thanks!
> >
> > KVM and the Linux host use relatively low amount of memory.
> > Unlike other hypervisors you know, kvm does not reserve memory and also
> > is able to swap the guest memory, so you can even use more than 4G for
> > your
>
> Is that true? I think we can not allocate memory more than the
> physical RAM. Or does upstream kvm supports it? My kvm version is not
> that old but memory allocation failed for me when I tried to give the
> whole memory on my host to the guest.

It's not specific to KVM. QEmu use malloc()/mmap() to allocate memory for KVM, 
which is controlled by Linux kernel memory overcommit policy. Linux kernel 
didn't support memory overcommit by default. You can refer to 
Document/vm/overcommit-accounting and other related document to enable it. 

-- 
regards
Yang, Sheng

>
> > guest. (Just note swapping will be slow)
> >
> >> Regards,
> >> Qian
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe kvm" in
> >> the body of a message to majordomo@vger.kernel.org
> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe kvm" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html