cloud-hypervisor/docs/memory.md

Memory

Cloud-Hypervisor has many ways to expose memory to the guest VM. This document aims to explain what Cloud-Hypervisor is capable of and how it can be used to meet the needs of very different use cases.

Basic Parameters

MemoryConfig or what is known as --memory from the CLI perspective is the easiest way to get started with Cloud-Hypervisor.

struct MemoryConfig {
    size: u64,
    mergeable: bool,
    shared: bool,
    hugepages: bool,
    hotplug_method: HotplugMethod,
    hotplug_size: Option<u64>,
    balloon: bool,
    balloon_size: u64,
    zones: Option<Vec<MemoryZoneConfig>>,
}

--memory <memory>	Memory parameters "size=<guest_memory_size>,mergeable=on|off,shared=on|off,hugepages=on|off,hotplug_method=acpi|virtio-mem,hotplug_size=<hotpluggable_memory_size>,balloon=on|off"

size

Size of the RAM in the guest VM.

This option is mandatory when using the --memory parameter.

Value is an unsigned integer of 64 bits.

Example

--memory size=1G

mergeable

Specifies if the pages from the guest RAM must be marked as mergeable. In case this option is true or on, the pages will be marked with madvise(2) to let the host kernel know which pages are eligible for being merged by the KSM daemon.

This option can be used when trying to reach a higher density of VMs running on a single host, as it will reduce the amount of memory consumed by each VM.

By default this option is turned off.

Example

--memory size=1G,mergeable=on

shared

Specifies if the memory must be mmap(2) with MAP_SHARED flag.

By sharing a memory mapping, one can share the guest RAM with other processes running on the host. One can use this option when running vhost-user devices as part of the VM device model, as they will be driven by standalone daemons needing access to the guest RAM content.

By default this option is turned off, which results in performing mmap(2) with MAP_PRIVATE flag.

Example

--memory size=1G,shared=on

hugepages

Specifies if the memory must be mmap(2) with MAP_HUGETLB and MAP_HUGE_2MB flags. This performs a memory mapping relying on 2MiB pages instead of the default 4kiB pages.

By using hugepages, one can improve the overall performance of the VM, assuming the guest will allocate hugepages as well. Another interesting use case is VFIO as it speeds up the VM's boot time since the amount of IOMMU mappings are reduced.

By default this option is turned off.

Example

--memory size=1G,hugepages=on

hotplug_method

Selects the way of adding and/or removing memory to/from a booted VM.

Possible values are acpi and virtio-mem. Default value is acpi.

Example

--memory size=1G,hotplug_method=acpi

hotplug_size

Amount of memory that can be dynamically added to the VM.

Value is an unsigned integer of 64 bits. A value of 0 simply means that no memory can be added to the VM.

Example

--memory size=1G,hotplug_size=1G

balloon

Specifies if the virtio-balloon device must be activated. This creates a dedicated virtio device for managing the balloon in the guest, which allows guest to access more or less memory depending on the balloon size.

By default this option is turned off.

Example

--memory size=1G,balloon=on

Advanced Parameters

MemoryZoneConfig or what is known as --memory-zone from the CLI perspective is a power user parameter. It allows for a full description of the guest RAM, describing how every memory region is backed and exposed to the guest.

struct MemoryZoneConfig {
    size: u64,
    file: Option<PathBuf>,
    shared: bool,
    hugepages: bool,
    host_numa_node: Option<u32>,
    guest_numa_node: Option<u32>,
}

--memory-zone <memory-zone>	User defined memory zone parameters "size=<guest_memory_region_size>,file=<backing_file>,shared=on|off,hugepages=on|off,host_numa_node=<node_id>,guest_numa_node=<node_id>"

This parameter expects one or more occurences, allowing for a list of memory zones to be defined. It must be used with --memory size=0, clearly indicating that the memory will be described through advanced parameters.

Each zone is given a list of options which we detail through the following sections.

size

Size of the memory zone.

This option is mandatory when using the --memory-zone parameter.

Value is an unsigned integer of 64 bits.

Example

--memory size=0
--memory-zone size=1G

file

Path to the file backing the memory zone. This can be either a file or a directory. In case of a file, it will be opened and used as the backing file for the mmap(2) operation. In case of a directory, a temporary file with no hard link on the filesystem will be created. This file will be used as the backing file for the mmap(2) operation.

This option can be particularly useful when trying to back a part of the guest RAM with a well known file. In the context of the snapshot/restore feature, and if the provided path is a file, the snapshot operation will not perform any copy of the guest RAM content for this specific memory zone since the user has access to it and it would duplicate data already stored on the current filesystem.

Value is a string.

Example

--memory size=0
--memory-zone size=1G,file=/foo/bar

shared

Specifies if the memory zone must be mmap(2) with MAP_SHARED flag.

By sharing a memory zone mapping, one can share part of the guest RAM with other processes running on the host. One can use this option when running vhost-user devices as part of the VM device model, as they will be driven by standalone daemons needing access to the guest RAM content.

By default this option is turned off, which result in performing mmap(2) with MAP_PRIVATE flag.

Example

--memory size=0
--memory-zone size=1G,shared=on

hugepages

Specifies if the memory zone must be mmap(2) with MAP_HUGETLB and MAP_HUGE_2MB flags. This performs a memory zone mapping relying on 2MiB pages instead of the default 4kiB pages.

By using hugepages, one can improve the overall performance of the VM, assuming the guest will allocate hugepages as well. Another interesting use case is VFIO as it speeds up the VM's boot time since the amount of IOMMU mappings are reduced.

By default this option is turned off.

Example

--memory size=0
--memory-zone size=1G,hugepages=on

host_numa_node

Node identifier of a node present on the host. This option will let the user pick a specific NUMA node from which the memory must be allocated. After the memory zone is mmap(2), the NUMA policy for this memory mapping will be applied through mbind(2), relying on the provided node identifier. If the node does not exist on the host, the call to mbind(2) will fail.

This option is useful when trying to back a VM memory with a specific type of memory from the host. Assuming a host has two types of memory, with one slower than the other, each related to a distinct NUMA node, one could create a VM with slower memory accesses by backing the entire guest RAM from the furthest NUMA node on the host.

This option also gives the opportunity to create a VM with non uniform memory accesses as one could define a first memory zone backed by fast memory, and a second memory zone backed by slow memory.

Value is an unsigned integer of 32 bits.

Example

--memory size=0
--memory-zone size=1G,host_numa_node=0

guest_numa_node

Node identifier of a node that must be created in the guest. This option gives the user a way to create NUMA nodes in the guest and associate them with memory zones.

This option can be very useful and powerful when combined with host_numa_node as it allows for creating a VM with non uniform memory accesses, and let the guest know about it. It allows for exposing memory zones through different NUMA nodes, which can help the guest workload run more efficiently.

Value is an unsigned integer of 32 bits.

Example

--memory size=0
--memory-zone size=1G,guest_numa_node=0

NUMA settings

Along with the guest NUMA nodes created through the --memory-zone parameter, NumaConfig or what is known as --numa from the CLI perspective has been introduced to define additional settings related to each NUMA node.

struct NumaConfig {
    id: u32,
    cpus: Option<Vec<u8>>,
    distances: Option<Vec<NumaDistance>>,
}

--numa <numa>	Settings related to a given NUMA node "id=<node_id>,cpus=<cpus_id>,distances=<list_of_distances_to_destination_nodes>"

id

Node identifier of a guest NUMA node. The node referred by this identifier has been created through the guest_numa_node option from the --memory-zone parameter.

This option is mandatory when using the --numa parameter.

Value is an unsigned integer of 32 bits.

Example

--memory size=0
--memory-zone size=1G,guest_numa_node=0
--numa id=0

cpus

List of virtual CPUs attached to the guest NUMA node identified by the id option. This allows for describing a list of CPUs which must be seen by the guest as belonging to the NUMA node id.

One can use this option for a fine grained description of the NUMA topology regarding the CPUs associated with it, which might help the guest run more efficiently.

Multiple values can be provided to define the list. Each value is an unsigned integer of 8 bits.

For instance, if one needs to attach all CPUs from 0 to 4 to a specific node, the syntax using - will help define a contiguous range with cpus=0-4. The same example could also be described with cpus=0:1:2:3:4.

A combination of both - and : separators is useful when one might need to describe a list containing all CPUs from 0 to 99 and the CPU 255, as it could simply be described with cpus=0-99:255.

Example

--cpus boot=8
--memory size=0
--memory-zone size=1G,guest_numa_node=0
--memory-zone size=1G,guest_numa_node=1
--numa id=0,cpus=1-3:7
--numa id=1,cpus=0:4-6

distances

List of distances between the current NUMA node referred by id and the destination NUMA nodes listed along with distances. This option let the user choose the distances between guest NUMA nodes. This is important to provide an accurate description of the way non uniform memory accesses will perform in the guest.

One or more tuple of two values must be provided through this option. The first value is an unsigned integer of 32 bits as it represents the destination NUMA node. The second value is an unsigned integer of 8 bits as it represents the distance between the current NUMA node and the destination NUMA node. The two values are separated by @ (value1@value2), meaning the destination NUMA node value1 is located at a distance of value2. Each tuple is separated from the others with : separator.

For instance, if one wants to define 3 NUMA nodes, with each node located at different distances, it can be described with the following example.

Example

--memory size=0
--memory-zone size=1G,guest_numa_node=0
--memory-zone size=1G,guest_numa_node=1
--memory-zone size=1G,guest_numa_node=2
--numa id=0,distances=1@15:2@25
--numa id=1,distances=0@15:2@20
--numa id=2,distances=0@25:1@20