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,
    hotplug_method: HotplugMethod,
    hotplug_size: Option<u64>,
    hotplugged_size: Option<u64>,
    shared: bool,
    hugepages: bool,
    hugepage_size: Option<u64>,
    prefault: bool,
    thp: bool
    zones: Option<Vec<MemoryZoneConfig>>,
}

--memory <memory>	Memory parameters "size=<guest_memory_size>,mergeable=on|off,shared=on|off,hugepages=on|off,hugepage_size=<hugepage_size>,hotplug_method=acpi|virtio-mem,hotplug_size=<hotpluggable_memory_size>,hotplugged_size=<hotplugged_memory_size>,prefault=on|off,thp=on|off" [default: size=512M,thp=on]

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

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 is invalid.

Example

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

hotplugged_size

Amount of memory that will be dynamically added to the VM at boot. This option allows for starting a VM with a certain amount of memory that can be reduced during runtime.

This is only valid when the hotplug_method is virtio-mem as it does not make sense for the acpi use case. When using ACPI, the memory can't be resized after it has been extended.

This option is only valid when hotplug_size is specified, and its value can't exceed the value of hotplug_size.

Value is an unsigned integer of 64 bits. A value of 0 is invalid.

Example

--memory size=1G,hotplug_method=virtio-mem,hotplug_size=1G,hotplugged_size=512M

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.

If hugepages=on then the value of this field is ignored as huge pages always requires MAP_SHARED.

Example

--memory size=1G,shared=on

hugepages and hugepage_size

Specifies if the memory must be created and mmap(2) with MAP_HUGETLB and size flags. This performs a memory mapping relying on the specified huge page size. If no huge page size is supplied the system's default huge page size is used.

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.

The user is responsible for ensuring there are sufficient huge pages of the specified size for the VMM to use. Failure to do so may result in strange VMM behaviour, e.g. error with ReadKernelImage is common. If there is a strange error with hugepages enabled, just disable it or check whether there are enough huge pages.

If hugepages=on then the value of shared is ignored as huge pages always requires MAP_SHARED.

By default this option is turned off.

Example

--memory size=1G,hugepages=on,hugepage_size=2M

prefault

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

By triggering prefault, one can allocate all required physical memory and create its page tables while calling mmap. With physical memory allocated, the number of page faults will decrease during running, and performance will also improve.

Note that boot of VM will be slower with prefault enabled because of allocating physical memory and creating page tables in advance, and physical memory of the specified size will be consumed quickly.

This option only takes effect at boot of VM. There is also a prefault option in restore and its choice will overwrite prefault in memory.

By default this option is turned off.

Example

--memory size=1G,prefault=on

thp

Specifies if private anonymous memory for the guest (i.e. shared=off and no backing file) should be labelled MADV_HUGEPAGE with madvise(2) indicating to the kernel that this memory may be backed with huge pages transparently.

The use of transparent huge pages can improve the performance of the guest as there will fewer virtualisation related page faults. Unlike using hugepages=on a specific number of huge pages do not need to be allocated by the kernel.

By default this option is turned on.

Example

--memory size=1G,thp=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 {
    id: String,
    size: u64,
    file: Option<PathBuf>,
    shared: bool,
    hugepages: bool,
    hugepage_size: Option<u64>,
    host_numa_node: Option<u32>,
    hotplug_size: Option<u64>,
    hotplugged_size: Option<u64>,
    prefault: bool,
}

--memory-zone <memory-zone>	User defined memory zone parameters "size=<guest_memory_region_size>,file=<backing_file>,shared=on|off,hugepages=on|off,hugepage_size=<hugepage_size>,host_numa_node=<node_id>,id=<zone_identifier>,hotplug_size=<hotpluggable_memory_size>,hotplugged_size=<hotplugged_memory_size>,prefault=on|off"

This parameter expects one or more occurrences, 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.

id

Memory zone identifier. This identifier must be unique, otherwise an error will be returned.

This option is useful when referring to a memory zone previously created. In particular, the --numa parameter can associate a memory zone to a specific NUMA node based on the memory zone identifier.

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

Value is a string.

Example

--memory size=0
--memory-zone id=mem0,size=1G

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 id=mem0,size=1G

file

Path to the file backing the memory zone. The file will be opened and 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 id=mem0,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.

If hugepages=on then the value of this field is ignored as huge pages always requires MAP_SHARED.

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

Example

--memory size=0
--memory-zone id=mem0,size=1G,shared=on

hugepages and hugepage_size

Specifies if the memory must be created and mmap(2) with MAP_HUGETLB and size flags. This performs a memory mapping relying on the specified huge page size. If no huge page size is supplied the system's default huge page size is used.

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.

The user is responsible for ensuring there are sufficient huge pages of the specified size for the VMM to use. Failure to do so may result in strange VMM behaviour, e.g. error with ReadKernelImage is common. If there is a strange error with hugepages enabled, just disable it or check whether there are enough huge pages.

If hugepages=on then the value of shared is ignored as huge pages always requires MAP_SHARED.

By default this option is turned off.

Example

--memory size=0
--memory-zone id=mem0,size=1G,hugepages=on,hugepage_size=2M

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 id=mem0,size=1G,host_numa_node=0

hotplug_size

Amount of memory that can be dynamically added to the memory zone. Since virtio-mem is the only way of resizing a memory zone, one must specify the hotplug_method=virtio-mem to the --memory parameter.

Value is an unsigned integer of 64 bits. A value of 0 is invalid.

Example

--memory size=0,hotplug_method=virtio-mem
--memory-zone id=mem0,size=1G,hotplug_size=1G

hotplugged_size

Amount of memory that will be dynamically added to a memory zone at VM's boot. This option allows for starting a VM with a certain amount of memory that can be reduced during runtime.

This is only valid when the hotplug_method is virtio-mem as it does not make sense for the acpi use case. When using ACPI, the memory can't be resized after it has been extended.

This option is only valid when hotplug_size is specified, and its value can't exceed the value of hotplug_size.

Value is an unsigned integer of 64 bits. A value of 0 is invalid.

Example

--memory size=0,hotplug_method=virtio-mem
--memory-zone id=mem0,size=1G,hotplug_size=1G,hotplugged_size=512M

prefault

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

By triggering prefault, one can allocate all required physical memory and create its page tables while calling mmap. With physical memory allocated, the number of page faults will decrease during running, and performance will also improve.

Note that boot of VM will be slower with prefault enabled because of allocating physical memory and creating page tables in advance, and physical memory of the specified size will be consumed quickly.

This option only takes effect at boot of VM. There is also a prefault option in restore and its choice will overwrite prefault in memory.

By default this option is turned off.

Example

--memory size=0
--memory-zone id=mem0,size=1G,prefault=on

NUMA settings

NumaConfig or what is known as --numa from the CLI perspective has been introduced to define a guest NUMA topology. It allows for a fine description about the CPUs and memory ranges associated with each NUMA node. Additionally it allows for specifying the distance between each NUMA node.

struct NumaConfig {
    guest_numa_id: u32,
    cpus: Option<Vec<u8>>,
    distances: Option<Vec<NumaDistance>>,
    memory_zones: Option<Vec<String>>,
    sgx_epc_sections: Option<Vec<String>>,
}

--numa <numa>	Settings related to a given NUMA node "guest_numa_id=<node_id>,cpus=<cpus_id>,distances=<list_of_distances_to_destination_nodes>,memory_zones=<list_of_memory_zones>,sgx_epc_sections=<list_of_sgx_epc_sections>"

guest_numa_id

Node identifier of a guest NUMA node. This identifier must be unique, otherwise an error will be returned.

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

Value is an unsigned integer of 32 bits.

Example

--numa guest_numa_id=0

cpus

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

As soon as one tries to describe a list of values, [ and ] must be used to demarcate the list.

Example

--cpus boot=8
--numa guest_numa_id=0,cpus=[1-3,7] guest_numa_id=1,cpus=[0,4-6]

distances

List of distances between the current NUMA node referred by guest_numa_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.

As soon as one tries to describe a list of values, [ and ] must be used to demarcate the list.

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

--numa guest_numa_id=0,distances=[1@15,2@25] --numa guest_numa_id=1,distances=[0@15,2@20] guest_numa_id=2,distances=[0@25,1@20]

memory_zones

List of memory zones attached to the guest NUMA node identified by the guest_numa_id option. This allows for describing a list of memory ranges which must be seen by the guest as belonging to the NUMA node guest_numa_id.

This option can be very useful and powerful when combined with host_numa_node option from --memory-zone parameter 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.

Multiple values can be provided to define the list. Each value is a string referring to an existing memory zone identifier. Values are separated from each other with the , separator.

As soon as one tries to describe a list of values, [ and ] must be used to demarcate the list.

Note that a memory zone must belong to a single NUMA node. The following configuration is incorrect, therefore not allowed: --numa guest_numa_id=0,memory_zones=mem0 --numa guest_numa_id=1,memory_zones=mem0

Example

--memory size=0
--memory-zone id=mem0,size=1G id=mem1,size=1G --memory-zone id=mem2,size=1G
--numa guest_numa_id=0,memory_zones=[mem0,mem2] --numa guest_numa_id=1,memory_zones=mem1

sgx_epc_sections

List of SGX EPC sections attached to the guest NUMA node identified by the guest_numa_id option. This allows for describing a list of SGX EPC sections which must be seen by the guest as belonging to the NUMA node guest_numa_id.

Multiple values can be provided to define the list. Each value is a string referring to an existing SGX EPC section identifier. Values are separated from each other with the , separator.

As soon as one tries to describe a list of values, [ and ] must be used to demarcate the list.

Example

--sgx-epc id=epc0,size=32M id=epc1,size=64M id=epc2,size=32M
--numa guest_numa_id=0,sgx_epc_sections=epc1 --numa guest_numa_id=1,sgx_epc_sections=[epc0,epc2]

PCI bus

Cloud Hypervisor supports only one PCI bus, which is why it has been tied to the NUMA node 0 by default. It is the user responsibility to organize the NUMA nodes correctly so that vCPUs and guest RAM which should be located on the same NUMA node as the PCI bus end up on the NUMA node 0.