Call fo comments - raising vfs.ufs.dirhash_reclaimage?

[Peter Holm]

On Mon, Oct 07, 2013 at 07:34:24PM +0200, Davide Italiano wrote:
> > What would perhaps be better than a hardcoded reclaim age would be to use
> > an LRU-type approach and perhaps set a target percent to reclaim.  That is,
> > suppose you were to reclaim the oldest 10% of hashes on each lowmem call
> > (and make the '10%' the tunable value).  Then you will always make some amount
> > of progress in a low memory situation (and if the situation remains dire you
> > will eventually empty the entire cache), but the effective maximum age will
> > be more dynamic.  Right now if you haven't touched UFS in 5 seconds it
> > throws the entire thing out on the first lowmem event.  The LRU-approach would
> > only throw the oldest 10% out on the first call, but eventually throw it all out
> > if the situation remains dire.
> >
> > --
> > John Baldwin
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> 
> I liked your idea more than what's available in HEAD right now and I
> implemented it.
> http://people.freebsd.org/~davide/review/ufs_direclaimage.diff
> I was unsure what kind of heuristic I should choose to select which
> (10% of) entries should be evicted so I just removed the first 10%
> ones from the head of the ufs_dirhash list (which should be the
> oldest).
> The code keeps rescanning the cache until 10% (or, the percentage set
> via SYSCTL) of the entry are freed, but probably we can discuss if
> this limit could be relaxed and just do a single scan over the list.
> Unfortunately I haven't a testcase to prove the effectiveness (or
> non-effectiveness) of the approach but I think either Ivan or Peter
> could be able to give it a spin, maybe.
> 

I gave this patch a spin for 12 hours without finding any problems.
I can do more testing at a later time, if you want to.

- Peter