[Qemu-devel] Virtio-Balloon : config_set_size

[Qemu-devel] Virtio-Balloon : config_set_size

[Saptarshi Sen]

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Hi all,

    I am experimenting with the Virtio- balloon driver in qemu.

    When I set the balloon size to a arbitrary low value. I see
     the actual value of the balloon set is not what I intended
    but to a level probably decided by the  system.

    I am not able to explain this part who decides on the final
    size of the balloon.

    Another observation each time I do a qmp request to deflate the balloon
    the in the virtio-balloon.c config_set_size function is called. I do not
    understand who calls it and the method of activation

    Thanks

Regards
Saptarshi

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Re: [Qemu-devel] Virtio-Balloon : config_set_size

[Luiz Capitulino]

On Thu, 20 Jun 2013 12:49:17 +0800
Saptarshi Sen <saptarshi.mrg@gmail.com> wrote:

> Hi all,
> 
>     I am experimenting with the Virtio- balloon driver in qemu.
> 
>     When I set the balloon size to a arbitrary low value. I see
>      the actual value of the balloon set is not what I intended
>     but to a level probably decided by the  system.

A few things might be happening there. Maybe the guest is just slow
and is still inflating the balloon when you type 'info balloon'. Or
the guest may be running out of memory and is temporarily unable to
keep inflating the balloon. Finally, if the guest runs out of memory
(because you inflated too much) it may OOPs and then you won't see
any balloon activity anymore.

>     I am not able to explain this part who decides on the final
>     size of the balloon.
> 
>     Another observation each time I do a qmp request to deflate the balloon
>     the in the virtio-balloon.c config_set_size function is called. I do not
>     understand who calls it and the method of activation

There's no such function in virtio-balloon.c, at least not in latest
git HEAD. Are you referring to virtio_balloon_set_config()? This function
is called when the virtio balloon driver in the guest wants to update
the balloon size.