vm_lowmem event handler for dirhash

Julian Elischer julian at elischer.org
Wed May 27 16:57:33 UTC 2009


Nick Barkas wrote:
> On Wed, May 27, 2009 at 01:12:38PM +0200, Roman Divacky wrote:
>> 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.

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.

it would be good to see all the graphs rescaled to show in %,
rather than absolute numbers..


> 
> Nick
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