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Date: Wed, 25 Jun 2003 23:20:47 -0700
From: David Schultz <das@FreeBSD.ORG>
To: Jay Kuri <jay@oneway.com>
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cc: freebsd-current@FreeBSD.ORG
Subject: Re: questions about VM_KMEM_SIZE_SCALE
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On Tue, Jun 24, 2003, Jay Kuri wrote:
> 
> Hi there,
> 
> Can anyone shed some light on the implications of adjusting
> VM_KMEM_SIZE_SCALE?  In particular I'm wondering if I increase this to,
> say, 2, what happens?  I must admit I don't know how KVA is different from
> KVM or total RAM... so the note in kern_malloc ("on an x86 with 256M KVA,
> try to keep VM_KMEM_SIZE_MAX at 80M or below") doesn't shed enough light
> on the matter.  What are the implications of VM_KMEM_SIZE getting large?
> 
> 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.