questions about VM_KMEM_SIZE_SCALE

[Jay Kuri]

Thank you both very much.  This fills in the gaps nicely and I think I
have a pretty good grip on this now.  I feel like I can tune it a bit
without worry now.  (and avoid those nasty panics. :-) )  Thank you very
very much.

Jay

On Thu, 26 Jun 2003, Terry Lambert wrote:

> 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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Nothing fails like success because you do not learn 
anything from it. The only thing we ever learn from 
is failure. Success only confirms our superstitions.

        Jay Kuri	 jay at oneway.com