automatic memory ballooning?

automatic memory ballooning?

[Thomas Fjellstrom]

I'm wondering if kvm supports automatic memory ballooning. I've had a kvm 
guest running for a couple days, and the balloon driver was loaded, and I 
could manually change the amount of ram it had allocated in the console, but 
it never seemed to change automatically.

Is there any support for that?

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca

Re: automatic memory ballooning?

[Avi Kivity]

On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
> I'm wondering if kvm supports automatic memory ballooning. I've had a kvm
> guest running for a couple days, and the balloon driver was loaded, and I
> could manually change the amount of ram it had allocated in the console, but
> it never seemed to change automatically.
>
> Is there any support for that?
>    

That would be part of a management application.  qemu only knows about 
the guest it controls, while ballooning needs a global view of the system.

-- 
error compiling committee.c: too many arguments to function

Re: automatic memory ballooning?

[Thomas Fjellstrom]

On Sun August 16 2009, Avi Kivity wrote:
> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
> > I'm wondering if kvm supports automatic memory ballooning. I've had a kvm
> > guest running for a couple days, and the balloon driver was loaded, and I
> > could manually change the amount of ram it had allocated in the console,
> > but it never seemed to change automatically.
> >
> > Is there any support for that?
>
> That would be part of a management application.  qemu only knows about
> the guest it controls, while ballooning needs a global view of the system.

All a single guest needs to do is only use as much ram as it needs at any 
given time (up to the max allocated). So if the guest hasn't used much ram in 
a given time frame, "free" the free ram from the host, and only reallocate 
when needed. It doesn't _need_ a management application, just happens to be 
the way people do it.

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca

Re: automatic memory ballooning?

[Avi Kivity]

On 08/16/2009 05:18 PM, Thomas Fjellstrom wrote:
> On Sun August 16 2009, Avi Kivity wrote:
>    
>> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
>>      
>>> I'm wondering if kvm supports automatic memory ballooning. I've had a kvm
>>> guest running for a couple days, and the balloon driver was loaded, and I
>>> could manually change the amount of ram it had allocated in the console,
>>> but it never seemed to change automatically.
>>>
>>> Is there any support for that?
>>>        
>> That would be part of a management application.  qemu only knows about
>> the guest it controls, while ballooning needs a global view of the system.
>>      
> All a single guest needs to do is only use as much ram as it needs at any
> given time (up to the max allocated). So if the guest hasn't used much ram in
> a given time frame, "free" the free ram from the host, and only reallocate
> when needed. It doesn't _need_ a management application, just happens to be
> the way people do it.
>    

Well, for a single guest you are correct, but this is better solved 
generically for multiple guests.

-- 
error compiling committee.c: too many arguments to function

Re: automatic memory ballooning?

[Dor Laor]

On 08/16/2009 05:18 PM, Thomas Fjellstrom wrote:
> On Sun August 16 2009, Avi Kivity wrote:
>> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
>>> I'm wondering if kvm supports automatic memory ballooning. I've had a kvm
>>> guest running for a couple days, and the balloon driver was loaded, and I
>>> could manually change the amount of ram it had allocated in the console,
>>> but it never seemed to change automatically.
>>>
>>> Is there any support for that?
>>
>> That would be part of a management application.  qemu only knows about
>> the guest it controls, while ballooning needs a global view of the system.
>
> All a single guest needs to do is only use as much ram as it needs at any
> given time (up to the max allocated). So if the guest hasn't used much ram in
> a given time frame, "free" the free ram from the host, and only reallocate
> when needed. It doesn't _need_ a management application, just happens to be
> the way people do it.
>

This is far from being an accurate description of the reality ( ;) )
You cannot just expect the guest to do so. The guest has page cache that 
uses memory, it might run many processes that consume lots of memory, etc.
Even if you could have done it, the translation between the guest-host 
is not 1-1 and the host needs to be aware of the guest memory usage.

This is what ballooning does. A target is determined by host management 
daemon. As a response, the guest balloon driver try to allocate memory 
and pass it as Guest Physical Addresses to the host. Now the host can 
use madvise in order to mark these pages as not needed (and free the mmu 
of pinning them).

The complexity is for the management to dynamically shift memory between 
the host and the guest to reach maximum performance.

Regards,
Dor

Re: automatic memory ballooning?

[Thomas Fjellstrom]

On Sun August 16 2009, Dor Laor wrote:
> On 08/16/2009 05:18 PM, Thomas Fjellstrom wrote:
> > On Sun August 16 2009, Avi Kivity wrote:
> >> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
> >>> I'm wondering if kvm supports automatic memory ballooning. I've had a
> >>> kvm guest running for a couple days, and the balloon driver was loaded,
> >>> and I could manually change the amount of ram it had allocated in the
> >>> console, but it never seemed to change automatically.
> >>>
> >>> Is there any support for that?
> >>
> >> That would be part of a management application.  qemu only knows about
> >> the guest it controls, while ballooning needs a global view of the
> >> system.
> >
> > All a single guest needs to do is only use as much ram as it needs at any
> > given time (up to the max allocated). So if the guest hasn't used much
> > ram in a given time frame, "free" the free ram from the host, and only
> > reallocate when needed. It doesn't _need_ a management application, just
> > happens to be the way people do it.
>
> This is far from being an accurate description of the reality ( ;) )
> You cannot just expect the guest to do so. The guest has page cache that
> uses memory, it might run many processes that consume lots of memory, etc.
> Even if you could have done it, the translation between the guest-host
> is not 1-1 and the host needs to be aware of the guest memory usage.
>
> This is what ballooning does. A target is determined by host management
> daemon. As a response, the guest balloon driver try to allocate memory
> and pass it as Guest Physical Addresses to the host. Now the host can
> use madvise in order to mark these pages as not needed (and free the mmu
> of pinning them).
>
> The complexity is for the management to dynamically shift memory between
> the host and the guest to reach maximum performance.
>
> Regards,
> Dor
> --
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One thing I found odd about kvm's ballooning is that it actually seems to 
change how much ram the guest has. I really didn't expect "free -m" to report 
that the guest only had 64M ram after I manually ballooned the ram. I was 
however expecting it just to free ram it wasn't using in the host. To me, it 
just doesn't seem to be the same thing. now it'll start swapping at 64M ram 
instead of just reallocating the ram it used to have.

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca

Re: automatic memory ballooning?

[Avi Kivity]

On 08/17/2009 01:49 PM, Thomas Fjellstrom wrote:
> One thing I found odd about kvm's ballooning is that it actually seems to
> change how much ram the guest has. I really didn't expect "free -m" to report
> that the guest only had 64M ram after I manually ballooned the ram. I was
> however expecting it just to free ram it wasn't using in the host. To me, it
> just doesn't seem to be the same thing. now it'll start swapping at 64M ram
> instead of just reallocating the ram it used to have.
>    

You expectations aren't realistic.  kvm never allocates the ram the 
guest doesn't use in the first place.   Ballooning just the "free" 
memory is pointless since it's usually a very small amount.

It may be worthwhile for the guest to give up that memory voluntarily 
though.

-- 
error compiling committee.c: too many arguments to function

Re: automatic memory ballooning?

[Thomas Fjellstrom]

On Mon August 17 2009, Avi Kivity wrote:
> On 08/17/2009 01:49 PM, Thomas Fjellstrom wrote:
> > One thing I found odd about kvm's ballooning is that it actually seems to
> > change how much ram the guest has. I really didn't expect "free -m" to
> > report that the guest only had 64M ram after I manually ballooned the
> > ram. I was however expecting it just to free ram it wasn't using in the
> > host. To me, it just doesn't seem to be the same thing. now it'll start
> > swapping at 64M ram instead of just reallocating the ram it used to have.
>
> You expectations aren't realistic.  kvm never allocates the ram the
> guest doesn't use in the first place.

Really? So htop is lying to me then? I gave 1G ram to a kvm linux guest using 
virtio (disk, net, ballooning), and RES clearly said 1G, and VIRT actually 
said somewhere around 1.3 to 1.6G. It stayed that way for over a day, and the 
guest did nothing the entire time.

I know the kernel lies a little bit about ram usage, but it seems at least 
with kvm, the ram is in use when it says it is, while with vmware, it usually 
isn't.

> Ballooning just the "free"
> memory is pointless since it's usually a very small amount.

> It may be worthwhile for the guest to give up that memory voluntarily
> though.

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca

Re: automatic memory ballooning?

[Thomas Fjellstrom]

On Sun August 16 2009, Avi Kivity wrote:
> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote:
> > I'm wondering if kvm supports automatic memory ballooning. I've had a kvm
> > guest running for a couple days, and the balloon driver was loaded, and I
> > could manually change the amount of ram it had allocated in the console,
> > but it never seemed to change automatically.
> >
> > Is there any support for that?
>
> That would be part of a management application.  qemu only knows about
> the guest it controls, while ballooning needs a global view of the system.

Where can I find such a management application?

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca

Re: automatic memory ballooning?

[Avi Kivity]

On 08/17/2009 03:32 PM, Thomas Fjellstrom wrote:
> On Mon August 17 2009, Avi Kivity wrote:
>    
>> On 08/17/2009 01:49 PM, Thomas Fjellstrom wrote:
>>      
>>> One thing I found odd about kvm's ballooning is that it actually seems to
>>> change how much ram the guest has. I really didn't expect "free -m" to
>>> report that the guest only had 64M ram after I manually ballooned the
>>> ram. I was however expecting it just to free ram it wasn't using in the
>>> host. To me, it just doesn't seem to be the same thing. now it'll start
>>> swapping at 64M ram instead of just reallocating the ram it used to have.
>>>        
>> You expectations aren't realistic.  kvm never allocates the ram the
>> guest doesn't use in the first place.
>>      
> Really? So htop is lying to me then?

Of course not.

> I gave 1G ram to a kvm linux guest using
> virtio (disk, net, ballooning), and RES clearly said 1G, and VIRT actually
> said somewhere around 1.3 to 1.6G. It stayed that way for over a day, and the
> guest did nothing the entire time.
>    

VIRT size is pointless, it includes tons of shared libraries which don't 
really affect memory usage.  Try cat /proc/$pid/maps (or pmap $pid) on a 
qemu instance and see.

-- 
error compiling committee.c: too many arguments to function