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Home > Archive > PostgreSQL Performance > March 2006 > Re: Best OS & Configuration for Dual Xeon w/4GB & Adaptec
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Re: Best OS & Configuration for Dual Xeon w/4GB & Adaptec
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| Jim C. Nasby 2006-03-21, 7:28 am |
| On Tue, Mar 21, 2006 at 11:03:26PM +1200, Mark Kirkwood wrote:
> Jim C. Nasby wrote:
>
> Exactly.
>
> The so-called limit (controllable via various sysctl's) is on the amount
> of memory used for kvm mapped pages, not cached pages, i.e - its a
> subset of the cached pages that are set up for immediate access (the
> others require merely to be shifted from the 'Inactive' queue to this
> one before they can be operated on - a relatively cheap operation).
>
> So its really all about accounting, in a sense - whether pages end up in
> the 'Buf' or 'Inactive' queue, they are still cached!
So what's the difference between Buf and Active then? Just that active
means it's a code page, or that it's been directly mapped into a
processes memory (perhaps via mmap)?
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
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| Jim C. Nasby 2006-03-21, 7:28 am |
| On Tue, Mar 21, 2006 at 12:22:31PM +0000, Alex Hayward wrote:
> On Tue, 21 Mar 2006, Jim C. Nasby wrote:
>
>
> I don't think that Buf and Active are mutually exclusive. Try adding up
> Active, Inactive, Cache, Wired, Buf and Free - it'll come to more than
> your physical memory.
>
> Active gives an amount of physical memory. Buf gives an amount of
> kernel-space virtual memory which provide the kernel with a window on to
> pages in the other categories. In fact, I don't think that 'Buf' really
> belongs in the list as it doesn't represent a 'type' of page at all.
Ahhh, I get it... a KVM (what's that stand for anyway?) is required any
time the kernel wants to access a page that doesn't belong to it, right?
And actually, I just checked 4 machines and adding all the queues plus
buf together didn't add up to total memory except on one of them (there
adding just the queues came close; 1507.6MB on a 1.5GB machine).
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
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