On Monday, October 07, 2013 1:34:24 pm 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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> > http://lists.freebsd.org/mailman/listinfo/freebsd-fs
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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 think this looks good. One cosmetic nit is that I think this:
if (!try_lock())
continue;
else
memfreed += ufsdirhash_destroy();
Looks a bit odd. I would either drop the else (which the old code did in its
failsafe case) or just do this:
if (try_lock())
memfreed += ufsdirhash_destroy();
--
John Baldwin
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