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docs: update new prefault behavior in memory.md

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On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
This commit is contained in:
Philipp Schuster
2026-06-30 13:03:37 +02:00
committed by Bo Chen
parent 22cf10b6ab
commit f35c9842be
1 changed files with 14 additions and 10 deletions
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docs/memory.md
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@@ -157,11 +157,13 @@ _Example_
### `prefault` ### `prefault`
Specifies if the memory must be `mmap(2)` with `MAP_POPULATE` flag. Specifies whether guest memory should be prefaulted with `madvise(2)` using
`MADV_POPULATE_WRITE` after it is `mmap(2)`-ed.
By triggering prefault, one can allocate all required physical memory and create With prefault enabled, Cloud Hypervisor allocates the required physical memory
its page tables while calling `mmap`. With physical memory allocated, the number and creates its page tables before the VM starts. This reduces page faults at
of page faults will decrease during running, and performance will also improve. runtime and can surface insufficient backing memory during VM creation. If
prefaulting fails, VM creation fails with an error.
Note that boot of VM will be slower with `prefault` enabled because of allocating 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 physical memory and creating page tables in advance, and physical memory of the
@@ -170,7 +172,7 @@ specified size will be consumed quickly.
This option only takes effect at boot of VM. There is also a `prefault` option in 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. restore and its choice will overwrite `prefault` in memory.
By default this option is turned off. By default, this option is turned off.
_Example_ _Example_
@@ -429,11 +431,13 @@ _Example_
### `prefault` ### `prefault`
Specifies if the memory must be `mmap(2)` with `MAP_POPULATE` flag. Specifies whether this memory zone should be prefaulted with `madvise(2)` using
`MADV_POPULATE_WRITE` after it is `mmap(2)`-ed.
By triggering prefault, one can allocate all required physical memory and create With prefault enabled, Cloud Hypervisor allocates the required physical memory
its page tables while calling `mmap`. With physical memory allocated, the number and creates its page tables before the VM starts. This reduces page faults at
of page faults will decrease during running, and performance will also improve. runtime and can surface insufficient backing memory during VM creation. If
prefaulting fails, VM creation fails with an error.
Note that boot of VM will be slower with `prefault` enabled because of allocating 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 physical memory and creating page tables in advance, and physical memory of the
@@ -442,7 +446,7 @@ specified size will be consumed quickly.
This option only takes effect at boot of VM. There is also a `prefault` option in 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. restore and its choice will overwrite `prefault` in memory.
By default this option is turned off. By default, this option is turned off.
_Example_ _Example_