Call fo comments - raising vfs.ufs.dirhash_reclaimage?

Davide Italiano davide at freebsd.org
Mon Oct 7 17:34:27 UTC 2013


> 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.

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare


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