Martin J. Bligh described at OLS the "blunderbuss effect", i.e. the
inefficiency of the dentry cache shrinker at freeing whole pages, since we
could leave (worst-case) one dentry per page because it's at the end of the
LRU list.
Pinned dentries (in first place libfs ones, but he also includes directories
one - I think they are just hard to free, not really pinned) are allocated
from the common dentry_cache, i.e. mixed with normal ones - why don't we fix
that?
It seems that adding an (optional) flag to a new __d_alloc (with d_alloc
becoming its wrapper) would be enough, since dentries are always allocated
directly by filesystems (either on lookup or on creation of the pinned
dentry). Or call it d_alloc_lively().
Also, it seems that the slab allocator willl allocate objects at fixed
locations inside a page (even with page colouring, colour_offset is fixed
per-slab and saved)*, once that slab has been allocated... so if we add a
"DCACHE_FREED" flag and zero slabs content on alloc (at least for this slab),
we could maybe enumerate all dentries in a page and try to free them, to
finally free the whole slab.
* Otherwise this problem could probably be fixed some way.
--
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade
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