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Date: Thu, 31 Jan 2013 10:32:09 +0200
From: Andriy Gapon <avg@FreeBSD.org>
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To: alc@FreeBSD.org
Subject: Re: kva size on amd64
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Cc: Alan Cox <alan.l.cox@gmail.com>, freebsd-arch@FreeBSD.org
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on 31/01/2013 10:10 Alan Cox said the following:
> In short, it will waste a non-trivial amount of physical memory.  Unlike user
> virtual address spaces, page table pages are preallocated for the kernel virtual
> address space.  More precisely, they are preallocated for the reserved (or
> defined) regions of the kernel map, i.e., every range of addresses that has a
> corresponding vm_map_entry.  The kmem map is one such reserved region.  So, if
> you always set your kmem map to its maximum possible size of ~300GB, then you
> are preallocating about 600MB of physical memory for page table pages that will
> never be used (except on machines with 300+ GB of DRAM).


Alan,

thank you very much for this information!

Would it make sense then to do either of the following:
- add some (non-trivial) code to auto-grow kmem map upon kva shortage
- set default vm_kmem_size to min(2 * mem_size, vm_kmem_size_max)
?

Perhaps something else?..

BTW, it seems that in OpenSolaris they do not limit kva size, but probably they
allocate kernel page tables in some different way (on demand perhaps).

-- 
Andriy Gapon