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77 lines
2.1 KiB
Markdown
77 lines
2.1 KiB
Markdown
# Balloon
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Cloud Hypervisor implements a balloon device based on the VIRTIO specification.
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Its main purpose is to provide the host a way to reclaim memory by controlling
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the amount of memory visible to the guest. But it also provides some interesting
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features related to guest memory management.
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## Parameters
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`BalloonConfig` (known as `--balloon` from the CLI perspective) contains the
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list of parameters available for the balloon device.
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```rust
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struct BalloonConfig {
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pub size: u64,
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pub deflate_on_oom: bool,
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pub free_page_reporting: bool,
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}
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```
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```
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--balloon <balloon> Balloon parameters "size=<balloon_size>,deflate_on_oom=on|off,free_page_reporting=on|off"
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```
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### `size`
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Size of the balloon device. It is subtracted from the VM's total size. For
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instance, if creating a VM with 4GiB of RAM, along with a balloon of 1GiB, the
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guest will be able to use 3GiB of accessible memory. The guest sees all the RAM
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and unless it is balloon enlightened is entitled to all of it.
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This parameter is mandatory.
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Value is an unsigned integer of 64 bits corresponding to the balloon size in
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bytes.
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_Example_
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```
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--balloon size=1G
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```
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### `deflate_on_oom`
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Allow the guest to deflate the balloon if running Out Of Memory (OOM). Assuming
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the balloon size is greater than 0, this means the guest is allowed to reduce
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the balloon size all the way down to 0 if this can help recover from the OOM
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event.
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This parameter is optional.
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Value is a boolean set to `off` by default.
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_Example_
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```
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--balloon size=2G,deflate_on_oom=on
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```
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### `free_page_reporting`
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Allow the guest to report lists of free pages. This feature doesn't require the
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balloon to be of any specific size as it doesn't impact the balloon size. The
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guest can let the VMM know about pages that are free after they have been used.
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Based on this information, the VMM can advise the host that it doesn't need
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these pages anymore.
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This parameter is optional.
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Value is a boolean set to `off` by default.
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_Example_
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```
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--balloon size=0,free_page_reporting=on
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```
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