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Date: Sun, 06 Jan 2008 22:42:43 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: Vadim Goncharov <vadimnuclight@tpu.ru>
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Cc: freebsd-current@freebsd.org
Subject: Re: When will ZFS become stable?
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Vadim Goncharov wrote:
> 06.01.08 @ 23:34 Kris Kennaway wrote:
> 
>>> What is the other 512 MB of the 1 GB used for?
>>
>> Everything else that the kernel needs address space for.  Buffer 
>> cache, mbuf allocation, etc.
> 
> Aren't they allocated from the same memory zones? I have a router with 
> 256 Mb RAM, it had a panic with ng_nat once due to exhausted kmem. So, 
> what these number from it's sysctl do really mean?
> 
> vm.kmem_size: 83415040
> vm.kmem_size_max: 335544320
> vm.kmem_size_scale: 3
> vm.kvm_size: 1073737728
> vm.kvm_free: 704638976
> 

I believe that mbufs are allocated from a separate map.  In your case 
you only have ~80MB available in your kmem_map, which is used for 
malloc() in the kernel.  It is possible that ng_nat in combination with 
the other kernel malloc usage exhausted this relatively small amount of 
space without mbuf use being a factor.

Kris