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Add a section describing how to tune ARM kernel options to use an MD_ROOT

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filesystem larger than about 50-55 MiB.

The description of VM_KMEM_SIZE_SCALE is roughly as hand-wavy as my
understanding of the option, but at least mentioning that it's a factor
and giving an empirical datapoint that works will give folks some idea
of what to tweak if they have problems.
This commit is contained in:
ian 2017-12-26 22:05:55 +00:00
parent 31c02595d7
commit d6156b173d
1 changed files with 42 additions and 1 deletions
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43
share/man/man4/md.4
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@ -7,7 +7,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd December 7, 2017
.Dd December 26, 2017
.Dt MD 4
.Os
.Sh NAME
@ -93,6 +93,47 @@ disk found in the
.Xr mdconfig 8
man page.
Other tools will also create these images, such as NanoBSD.
.Sh ARM KERNEL OPTIONS
On armv6 and armv7 architectures, an MD_ROOT image larger than
approximately 55 MiB may require building a custom kernel using
several tuning options related to kernel memory usage.
.Bl -tag -width indent
.It Cd options LOCORE_MAP_MB=<num>
This configures how much memory is mapped for the kernel during
the early initialization stages.
The value must be at least as large as the kernel plus all preloaded
modules, including the root image.
There is no downside to setting this value too large, as long
as it does not exceed the amount of physical memory.
The default is 64 MiB.
.It Cd options NKPT2PG=<num>
This configures the number of kernel L2 page table pages to
preallocate during kernel initialization.
Each L2 page can map 4 MiB of kernel space.
The value must be large enough to map the kernel plus all preloaded
modules, including the root image.
The default value is 32, which is sufficient to map 128 MiB.
.It Cd options VM_KMEM_SIZE_SCALE=<num>
This configures the amount of kernel virtual address (KVA) space to
dedicate to the kmem_arena map.
The value is the ratio of physical to virtual pages.
The default value of 3 allocates a page of KVA for each 3 pages
of physical ram in the system.
The kernel and modules, including the root image, also consume KVA.
The combination of a large root image and the default scaling
may preallocate so much KVA to kmem_arena that there is not enough
remaining address space to allocate kernel stacks, IO buffers,
and other resources that are not part of kmem_arena.
Overallocating kmem_arena space is likely to manifest as failure to
launch userland processes with "cannot allocate kernel stack" messages.
Setting the value too high may result in kernel failure to allocate
memory because kmem_arena is too small, and the failure may require
significant runtime to manifest.
Empirically, a value of 5 works well for a 200 MiB root image on
a system with 2 GiB of physical ram.
.El
.Sh SEE ALSO
.Xr gpart 8 ,
.Xr loader 8 ,