PostgreSQL Performance - Re: Best OS & Configuration for Dual Xeon w/4GB &

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Re: Best OS & Configuration for Dual Xeon w/4GB &

Mark Kirkwood

2006-03-21, 3:34 am

Scott Marlowe wrote:
> On Mon, 2006-03-20 at 08:45, Jim C. Nasby wrote:
>
>
>
> So, then, the inact is pretty much the same as kernel buffers in linux?
>

I think Freebsd 'Inactive' corresponds pretty closely to Linux's
'Inactive Dirty'|'Inactive Laundered'|'Inactive
Free'.

From what I can see, 'Buf' is a bit misleading e.g. read a 1G file
randomly and you increase 'Inactive' by about 1G - 'Buf' might get to
200M. However read the file again and you'll see zero i/o in vmstat or
gstat. From reading the Freebsd architecture docs, I think 'Buf'
consists of those pages from 'Inactive' or 'Active' that were last kvm
mapped for read/write operations. However 'Buf' is restricted to a
fairly small size (various sysctls), so really only provides a lower
bound on the file buffer cache activity.

Sorry to not really answer your question Scott - how are Linux kernel
buffers actually defined?

Cheers

Mark

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Jim C. Nasby

2006-03-21, 7:28 am

On Tue, Mar 21, 2006 at 03:51:35PM +1200, Mark Kirkwood wrote:
> Mark Kirkwood wrote:
>
> Hmmm - on second thoughts I think I've got that wrong :-(, since in
> Linux all the file buffer pages appear in 'Cached' don't they...
>
> (I also notice that 'Inactive Laundered' does not seem to be mentioned
> in vanilla - read non-Redhat - 2.6 kernels)
>
> So I think its more correct to say Freebsd 'Inactive' is similar to
> Linux 'Inactive' + some|most of Linux 'Cached'.
>
> A good discussion of how the Freebsd vm works is here:
>
> http://www.freebsd.org/doc/en_US.IS...andbook/vm.html
>
> In particular:
>
> "FreeBSD reserves a limited amount of KVM to hold mappings from struct
> bufs, but it should be made clear that this KVM is used solely to hold
> mappings and does not limit the ability to cache data."

It's worth noting that starting in either 2.4 or 2.6, linux pretty much
adopted the FreeBSD VM system (or so I've been told).
--
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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