Re: [RFC] Alignment of fields in struct dentry

[Andreas Dilger <adilger@clusterfs.com>]

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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