Author: ian Date: Tue Dec 26 22:05:55 2017 New Revision: 327226 URL: https://svnweb.freebsd.org/changeset/base/327226
Log: Add a section describing how to tune ARM kernel options to use an MD_ROOT 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. Modified: head/share/man/man4/md.4 Modified: head/share/man/man4/md.4 ============================================================================== --- head/share/man/man4/md.4 Tue Dec 26 20:56:55 2017 (r327225) +++ head/share/man/man4/md.4 Tue Dec 26 22:05:55 2017 (r327226) @@ -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 , _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
