Re: [patch 00/52] vfs scalability patches updated

[Paul E. McKenney]

On Thu, Jul 01, 2010 at 10:35:35AM -0700, Linus Torvalds wrote:
> On Wed, Jun 30, 2010 at 5:40 AM, Nick Piggin <npiggin@xxxxxxx> wrote:
> >>
> >> That's a pretty big ouch. Why does RCU freeing of inodes cause that
> >> much regression? The RCU freeing is out of line, so where does the big
> >> impact come from?
> >
> > That comes mostly from inability to reuse the cache-hot inode structure,
> > and the cost to go over the deferred RCU list and free them after they
> > get cache cold.
> 
> 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?

In this case, the workload can be very update-heavy, so this type-safe
(vs. identity-safe) approach indeed makes a lot of sense.  But if this
was a read-heavy situation (think SELinux or many areas in networking),
the read-side simplifications and speedups that often come with
identity safety would probably more than make up for the occasional
grace-period-induced cache miss.

So, as a -very- rough rule of thumb, when less than a few percent
of the accesses are updates, you most likely want identity safety.
If more than half of the accesses can be updates, you probably want
SLAB_DESTROY_BY_RCU-style type safety instead -- or maybe just straight
locking.  If you are somewhere in between, pick one randomly, if
it works, go with it, otherwise try something else.  ;-)

In this situation, a create/rename/delete workload would be quite update
heavy, so, as you say, SLAB_DESTROY_BY_RCU is well worth looking into.

							Thanx, Paul

> 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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