David Schultz wrote:
> On Tue, Jun 24, 2003, Jay Kuri wrote:
> > Does changing this affect memory available to user programs if it's unused
> > by the kernel?
>
> No, KVA_PAGES affects the memory available to user programs. (You
> have a 4 GB address space on i386 to split between user programs
> and the kernel.) Within the kernel's share of this address space,
> memory is split into submaps, such as the mb_map (for the
> network), buffer_map for the filesystem buffer cache, and the
> kmem_map for just about everything else. These submaps are
> size-limited to prevent any one of them from getting out of hand.
>
> The vm_kmem_map is sized automatically based on the amount of
> memory you have. Specifically,
>
> kmem_map_size = min(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE),
> VM_KMEM_SIZE_MAX)
>
> The default value for VM_KMEM_SIZE_SCALE is 3, and the default
> VM_KMEM_SIZE_MAX is 200MB.
David is exactly right in what he has said. Some minor ommisions
about the implications of what he has said, though, are:
1) The intent of doing this is to ensure that, for a given
amount of physical memory, you don't grab more than
1/VM_KMEM_SIZE_SCALE * 100 % of it (default: 50%) for
the kmem_map.
2) If you *need* a much larger kmem_map, you are limited to
200K, no matter how much physical memory you have, unless
you raise VM_KMEM_SIZE_MAX, as well.
3) If you want to explicitly control the memory size, you can
set VM_KMEM_SIZE_SCALE very, very large, which will cause
the second term in the "max()" expression to approach zero,
and then set VM_KMEM_SIZE_MAX = VM_KMEM_SIZE = <some value>
to disable the "min()" expression.
4) The kmem_map is used to allocate kernel structure having to
do with memory management; if you have a very large amount
of memory in your system, you should consider increasing
VM_KMEM_SIZE_MAX, at a minimum, but realize that you are
competing for KVA with all ather uses of kernel memory, so
if your KVA space is 2G, and you have 4G of RAM, and your
VM_KMEM_SIZE_SCALE is 2 and you set VM_KMEM_SIZE_MAX so it
doesn't clamp the top end, you can run yourself out of KVA
space. IMO, it would be good to use something like
min(Physical Memory, KVA space) in place of Physical Memory
as the first term of the min() expression.
5) It's generally useful to set VM_KMEM_SIZE_MAX huge, and
then use only VM_KMEM_SIZE_SCALE to adjust how much of
the physical RAM you allow the kmem_map to use (subject to
the limits in #4, above).
6) Physical memory allocated to backing KVA resident maps
like kmem_map don't reduce the amount of virtual memory
available to user space processes; however, actually using
the full KVA space worth of physical memory might mean that
user space processes compete for physical RAM where they
wouldn't before, and so it may mean swapping. Memory in
KVA maps is generally type-stable and can't be reclaimed
for user process use (i.e.: the kernel does not page, except
for speciall allocated memory sections, and they are not
generally used for anything important enough to get a map
entry).
-- Terry
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