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Date: Thu, 29 Aug 2013 11:49:35 -0700 (PDT)
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Subject: Re: Suggest changing dirhash defaults for FreeBSD 9.2.
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Here is a more recent dialog between the developers.

<quote
Nick Barkas
http://markmail.org/message/3sufphda2exjmhnq#query:+page:1+mid:3sufphda2exjmhnq+state:results

On Wed, May 27, 2009 at 12:36:49PM +0200, Nick Barkas wrote:

Some time during the next week or so, I plan on committing the attached
patch. It adds a vm_lowmem event handler to the dirhash code in UFS2 so that
dirhashes will be deleted when the system is low on memory. This allows one
to increase the maximum amount of memory available for dirhash on machines
that have memory to spare (via the vfs.ufs.dirhash_maxmem sysctl), and
hopefully just improving behaviour in low memory situations. I worked on
this last year for the summer of code with David Malone as my mentor.

cool! do you have any performance numbers? graphs? :) what value do you
recommend for the dirhash_maxmem sysctl?

Oh yes, I have many graphs: http://wiki.freebsd.org/DirhashDynamicMemory
When I ran those tests a few months ago, I used 64MB for dirhash_maxmem on a
system with 1GB of memory. I have not tried other amounts of memory besides
that, at least that I can recall, so please let me know what you find if you
experiment with other values. Performance improvements and sometimes
degradations changed depending on the type of work load, and the results on
7.x were also sometimes quite different from -current. I feel that the tests
I did were pretty artificial though, so it would be great to hear about any
results found with more realistic testing.

Nick 
&lt;/quote>

<quote
David Malone
http://markmail.org/message/3sufphda2exjmhnq#query:+page:1+mid:aerievrnrmkezehk+state:results

On Wed, May 27, 2009 at 09:57:31AM -0700, Julian Elischer wrote:

I was initially impressed by the numbers until I saw the scales.. a
difference between 475.5 and 474 is not that significant, but if your graph
scale is from 473 to 477, it looks at first glace very impressive.

I think we felt the real gains here would be the ability to set the dirhash
memory limits to a much larger value without having to worry about it
chewing all your memory. The results basically confirm that we haven't
introduced any serious regressions ;-)

David. 
&lt;/quote>



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