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docs: update prefault in memory
Update introduction of option `prefault`. In addition, this commit also did the following: - Rearrange options, synchronize order with `config.rs`. - Break long lines in `hugepages`. - Update old example of `hugepages` in memory zone. Signed-off-by: Li Yu <liyu.yukiteru@bytedance.com>
This commit is contained in:
Li Yu
Bo Chen
parent
7b7a3abfaa
commit
0468598fa3
159
docs/memory.md
159
docs/memory.md
@ -13,18 +13,19 @@ easiest way to get started with Cloud-Hypervisor.
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struct MemoryConfig {
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size: u64,
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mergeable: bool,
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shared: bool,
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hugepages: bool,
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hugepage_size: Option<u64>,
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hotplug_method: HotplugMethod,
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hotplug_size: Option<u64>,
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hotplugged_size: Option<u64>,
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shared: bool,
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hugepages: bool,
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hugepage_size: Option<u64>,
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prefault: bool,
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zones: Option<Vec<MemoryZoneConfig>>,
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}
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```
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```
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--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>"
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--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" [default: size=512M]
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```
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### `size`
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@ -59,46 +60,6 @@ _Example_
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--memory size=1G,mergeable=on
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```
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### `shared`
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Specifies if the memory must be `mmap(2)` with `MAP_SHARED` flag.
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By sharing a memory mapping, one can share the guest RAM with other processes
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running on the host. One can use this option when running vhost-user devices
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as part of the VM device model, as they will be driven by standalone daemons
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needing access to the guest RAM content.
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By default this option is turned off, which results in performing `mmap(2)`
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with `MAP_PRIVATE` flag.
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_Example_
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```
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--memory size=1G,shared=on
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```
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### `hugepages` and `hugepage_size`
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Specifies if the memory must be created and `mmap(2)` with `MAP_HUGETLB` and size
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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.
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By using hugepages, one can improve the overall performance of the VM, assuming
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the guest will allocate hugepages as well. Another interesting use case is VFIO
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as it speeds up the VM's boot time since the amount of IOMMU mappings are
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reduced.
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The user is responsible for ensuring there are sufficient huge pages of the specified size for the VMM to use.
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Failure to do so may result in strange VMM behaviour, e.g. error with `ReadKernelImage` is common.
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If there is a strange error with `hugepages` enabled, just disable it or check whether there are enough huge pages.
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By default this option is turned off.
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_Example_
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```
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--memory size=1G,hugepages=on,hugepage_size=2M
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```
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### `hotplug_method`
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Selects the way of adding and/or removing memory to/from a booted VM.
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@ -144,6 +105,72 @@ _Example_
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--memory size=1G,hotplug_method=virtio-mem,hotplug_size=1G,hotplugged_size=512M
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```
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### `shared`
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Specifies if the memory must be `mmap(2)` with `MAP_SHARED` flag.
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By sharing a memory mapping, one can share the guest RAM with other processes
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running on the host. One can use this option when running vhost-user devices
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as part of the VM device model, as they will be driven by standalone daemons
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needing access to the guest RAM content.
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By default this option is turned off, which results in performing `mmap(2)`
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with `MAP_PRIVATE` flag.
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_Example_
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```
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--memory size=1G,shared=on
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```
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### `hugepages` and `hugepage_size`
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Specifies if the memory must be created and `mmap(2)` with `MAP_HUGETLB` and size
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flags. This performs a memory mapping relying on the specified huge page size.
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If no huge page size is supplied the system's default huge page size is used.
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By using hugepages, one can improve the overall performance of the VM, assuming
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the guest will allocate hugepages as well. Another interesting use case is VFIO
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as it speeds up the VM's boot time since the amount of IOMMU mappings are
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reduced.
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The user is responsible for ensuring there are sufficient huge pages of the
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specified size for the VMM to use. Failure to do so may result in strange VMM
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behaviour, e.g. error with `ReadKernelImage` is common. If there is a strange
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error with `hugepages` enabled, just disable it or check whether there are enough
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huge pages.
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By default this option is turned off.
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_Example_
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```
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--memory size=1G,hugepages=on,hugepage_size=2M
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```
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### `prefault`
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Specifies if the memory must be `mmap(2)` with `MAP_POPULATE` flag.
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By triggering prefault, one can allocate all required physical memory and create
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its page tables while calling `mmap`. With physical memory allocated, the number
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of page faults will decrease during running, and performance will also improve.
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Note that boot of VM will be slower with `prefault` enabled because of allocating
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physical memory and creating page tables in advance, and physical memory of the
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specified size will be consumed quickly.
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This option only takes effect at boot of VM. There is also a `prefault` option in
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restore and its choice will overwrite `prefault` in memory.
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By default this option is turned off.
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_Example_
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```
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--memory size=1G,prefault=on
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```
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## Advanced Parameters
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`MemoryZoneConfig` or what is known as `--memory-zone` from the CLI perspective
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@ -157,14 +184,16 @@ struct MemoryZoneConfig {
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file: Option<PathBuf>,
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shared: bool,
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hugepages: bool,
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hugepage_size: Option<u64>,
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host_numa_node: Option<u32>,
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hotplug_size: Option<u64>,
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hotplugged_size: Option<u64>,
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prefault: bool,
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}
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```
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```
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--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>,id=<zone_identifier>,hotplug_size=<hotpluggable_memory_size>,hotplugged_size=<hotplugged_memory_size>"
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--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"
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```
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This parameter expects one or more occurences, allowing for a list of memory
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@ -252,24 +281,30 @@ _Example_
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--memory-zone id=mem0,size=1G,shared=on
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```
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### `hugepages`
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### `hugepages` and `hugepage_size`
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Specifies if the memory zone must be `mmap(2)` with `MAP_HUGETLB` and
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`MAP_HUGE_2MB` flags. This performs a memory zone mapping relying on 2MiB
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pages instead of the default 4kiB pages.
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Specifies if the memory must be created and `mmap(2)` with `MAP_HUGETLB` and size
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flags. This performs a memory mapping relying on the specified huge page size.
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If no huge page size is supplied the system's default huge page size is used.
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By using hugepages, one can improve the overall performance of the VM, assuming
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the guest will allocate hugepages as well. Another interesting use case is VFIO
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as it speeds up the VM's boot time since the amount of IOMMU mappings are
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reduced.
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The user is responsible for ensuring there are sufficient huge pages of the
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specified size for the VMM to use. Failure to do so may result in strange VMM
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behaviour, e.g. error with `ReadKernelImage` is common. If there is a strange
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error with `hugepages` enabled, just disable it or check whether there are enough
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huge pages.
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By default this option is turned off.
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_Example_
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```
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--memory size=0
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--memory-zone id=mem0,size=1G,hugepages=on
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--memory-zone id=mem0,size=1G,hugepages=on,hugepage_size=2M
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```
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### `host_numa_node`
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@ -336,6 +371,30 @@ _Example_
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--memory-zone id=mem0,size=1G,hotplug_size=1G,hotplugged_size=512M
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```
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### `prefault`
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Specifies if the memory must be `mmap(2)` with `MAP_POPULATE` flag.
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By triggering prefault, one can allocate all required physical memory and create
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its page tables while calling `mmap`. With physical memory allocated, the number
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of page faults will decrease during running, and performance will also improve.
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Note that boot of VM will be slower with `prefault` enabled because of allocating
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physical memory and creating page tables in advance, and physical memory of the
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specified size will be consumed quickly.
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This option only takes effect at boot of VM. There is also a `prefault` option in
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restore and its choice will overwrite `prefault` in memory.
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By default this option is turned off.
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_Example_
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```
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--memory size=0
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--memory-zone id=mem0,size=1G,prefault=on
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```
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## NUMA settings
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`NumaConfig` or what is known as `--numa` from the CLI perspective has been
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