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SubjectRe: [RFC] Alignment of fields in struct dentry
On Sep 14, 2006  23:02 +0200, J�rn Engel wrote:
> On Thu, 14 September 2006 12:33:25 -0600, Andreas Dilger wrote:
> > I think it makes sense to keep d_inode in the first part of the dentry
> > always, because it is by far the most referenced field in the dentry,
> > along with the critical fields from prune_dcache(), shrink_dcache_anon(),
> > dget(), dput(), d_lookup().
>
> d_inode is definitely one of the hotter fields in there. It just
> happens to cause the misalignment. Bah, I don't see a good solution.

As is d_lock, which didn't exist when those comments were made.

> > While not totally accurate in terms of runtime frequency of use, the counts
> > in the code:
> >
> > fs/*.[ch] fs/*/*.[ch] size32 size64 prune_dc shrk_dc_anon d_lookup
> > d_inode 384 2131 4 8
> > d_lock 104 529 4 4 1 2
> > d_count 18 66 4 4 1 2
> > d_lru 18 18 4_ 8 1 1
> > d_hash 37 154 4 8_ 2 1
> > d_name 73 908 12_ 16 1
> > d_flags 26 104 4 4 2
> > d_mounted 7 7 4 4
> > d_parent 40 231 4 8_ 2
> > d_op 37 269 4 8
> > d_rcu/d_child 3+22 3+45 8 16
>
> [ d_hash is 8/16, actually ]

Doh!

> d_hash, d_name and d_parent belong way up to the top of the list, imo.
> d_lookup() should be the hottest function of all, as the comment in
> the structure definition already indicates. Maybe the solution is to
> rearrange the fields with those going to the top?
>
> Using your scheme (slightly reduced) we now have:
> size32 size64 funky?
> d_count 4 4
> d_flags 4 4
> d_lock 4 4_ y
> d_inode 4_ 8
> d_hash 8 16--
> d_parent 4 8_
> d_name 12-- 16___
> d_lru 8_ 16_
> d_rcu/d_child 8 16__
> d_subdirs 8___ 16_
> d_alias 8 16____
> d_time 4 8
> d_op 4_ 8_
> d_sb 4 8
> d_fsdata 4 8__
> d_cookie 0 0 y
> d_mounted 4 4
> d_iname 36____ 36
>
> With the two funky fields possibly growing, depending on kernel
> config. [_-] mark 16-, 32- 64- and 128-byte boundaries, depending on
> len. What really frightens me is that a 32-byte boundary goes right
> through d_name on 32bit machines.

Actually, splitting d_name like this is not so bad (as long as the
compiler doesn't add padding) because the important fields (hash
and len) are first and are compared for all non-matching dentries
in __d_lookup().

> Iirc, my PIII has 32-byte cachelines. Not good.
>
> How about moving [d_hash,d_parent,d_name] to the front? Something
> like:
> size32 size64 funky?
> d_hash 8 16_
> d_parent 4 8
> d_name 12- 16--
> d_inode 4 8_
> d_count 4__ 4
> d_flags 4 4
> d_lock 4 4 y
>
> d_mounted 4 4
>
> d_lru 8 16
> d_rcu/d_child 8 16
> d_subdirs 8 16
> d_alias 8 16
> d_time 4 8
> d_op 4 8
> d_sb 4 8
> d_fsdata 4 8
> d_cookie 0 0 y
> d_iname 36 36
>
> Now d_lookup() should use a single cacheline, even on my aged
> notebook, and the other hot fields remain at the top. d_mounted is
> also moved up to remove the misalignment on 64bit. Might be worth
> a benchmark or two to see whether it makes a difference...

Might not be too hard (even if it temporarily kills performance)
to add atomic counters for each of these fields where they are
referenced in dcache.c, namei.c and e.g. fs/ext3/*.c (which is
only using d_inode, d_name, and d_parent). Run a find and a
kernel compile and dump the counters at shutdown.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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