On Wed, Jun 30, 2010 at 5:40 AM, Nick Piggin <npiggin@suse.de> wrote:
I do wonder if this isn't a big design bug.
Most of the time with RCU, we don't need to wait to actually do the
_freeing_ of the individual data structure, we only need to make sure
that the data structure remains of the same _type_. IOW, we can free
it (and re-use it), but the backing storage cannot be released to the
page cache. That's what SLAB_DESTROY_BY_RCU should give us.
Is that not possible in this situation? Do we really need to keep the
inode _identity_ around for RCU?
If you use just SLAB_DESTROY_BY_RCU, then inode re-use remains, and
cache behavior would be much improved. The usual requirement for
SLAB_DESTROY_BY_RCU is that you only touch a lock (and perhaps
re-validate the identity) in the RCU-reader paths. Could that be made
to work?
Because that 27% drop really is pretty distressing.
That said, open (of the non-creating kind), close, and stat are
certainly more important than creating and freeing files. So as a
trade-off, it's probably the right thing to do. But if we can get all
the improvement _without_ that big downside, that would obviously be
better yet.
Linus
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