Re: Modifying RAM during runtime on guest

[Daniel Bareiro <daniel-listas@gmx.net>]

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Hi Brian.

On Tuesday, 08 September 2009 16:18:09 -0500,
Brian Jackson wrote:

> > I'm trying to modify the amount of RAM that has some of guests. Host
> > has 2.6.30 kernel with KVM-88.
> > 
> > In one of guest I didn't have problems when decreasing the amount of
> > memory from 3584 MIB to 1024 MiB. This guest has 2.6.26-2-686 stock
> > kernel. Also I was trying to decrease the amount RAM of another guest
> > from 3584 MiB to 2048 MiB, but it didn't work. This other guest has
> > 2.6.24-etchnhalf.1-686-bigmem stock kernel. Does Ballooning in guest
> > require 2.6.25 or superior?

> I don't know, if that kernel has a virtio-balloon driver, I'd think that
> was all you need to balloon memory.

Then can be that it is related to kernel that is using the guest:

# uname -a
Linux aprender01 2.6.24-etchnhalf.1-686-bigmem #1 SMP Mon Jul 27 03:56:49 UTC 2009 i686 GNU/Linux

# cat config-2.6.24-etchnhalf.1-686-bigmem | grep -i virtio
#

> > Thinking that it could be an impediment related to the kernel version
> > of guest, I tried to increase the memory of another one guest with
> > 2.6.26-2-686 from 512 MIB to 1024 MIB, but this didn't work either.

> You can only grow memory up to the amount you specified on the command
> line if you've already ballooned down.

Good. Thanks for clarifying to me this detail.

> So if you specify "-m 1024M" on the command line, then shrink it to 512,
> you could then balloon it back up to a max of 1024.

According to the tests that I was doing in guest with kernel with support
for virtio, shrinking works, but when trying to return to the amount of
initial memory, seems that it fails and I lose connectivity by serial
console and ssh.

In the guest:

# uname -a
Linux central 2.6.26-2-amd64 #1 SMP Sun Jul 26 20:35:48 UTC 2009 x86_64 GNU/Linux

# cat config-2.6.26-2-amd64 | grep -i virtio
CONFIG_NET_9P_VIRTIO=m
CONFIG_VIRTIO_BLK=m
CONFIG_VIRTIO_NET=m
CONFIG_HW_RANDOM_VIRTIO=m
CONFIG_VIRTIO=m
CONFIG_VIRTIO_RING=m
CONFIG_VIRTIO_PCI=m
CONFIG_VIRTIO_BALLOON=m

In the host:

# telnet localhost 4045
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
QEMU 0.10.50 monitor - type 'help' for more information
(qemu)
(qemu) info balloon
balloon: actual=512
(qemu) balloon 256
(qemu) info balloon
balloon: actual=256
(qemu)
(qemu) balloon 512
(qemu) info balloon
balloon: actual=36

At this moment I'm unable to connect by serial console to the guest and the
ssh sessions are freeze nevertheless kvm process of the guest is running.
Using a VNC client I see several messages of this type in the guest:

Out of memory: kill process xxxx (<name>) score xx or a child
Killed process xxxx (<name>)

Finally:

Kernel panic - not syncing - Out of memory and no killable processes...



Thanks for your reply.

Regards,
Daniel
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