Re: questions about VM_KMEM_SIZE_SCALE

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