Re: [RFC] Alignment of fields in struct dentry

[Jörn Engel]

On Fri, 15 September 2006 22:44:07 +0200, Arnd Bergmann wrote:
> Am Friday 15 September 2006 12:27 schrieb Jörn Engel:
> > -};
> > +}__attribute__((aligned(64))); /* make sure the dentry is 128/192 bytes
> > +                                  on 32/64 bit independently of config
> > +                                  options.  d_iname will vary in length
> > +                                  a bit. */

Wow.  Mutt really didn't like quoting this.

> I'd guess that a 32 byte alignment is much better here, 64 byte sounds
> excessive. It should have the same effect with the current dentry layout
> and default config options, but would keep the d_iname length in the
> 16-44 byte range instead of 16-76 byte as your patch does.
> 
> Since all important fields are supposed to be kept in 32 bytes anyway,
> they are still either at the start or the end of a given cache line,
> but never cross two.

Another take would be to use a cacheline.  But I guess the difference
between 32/64/cacheline is mostly academic, given the rate of changes
to struct dentry.

Unless the minimum length for inline names is configurable, as in the
patch below.  (Note to self: I really should finish my test setup
before posting any further patches without performance numbers.)

Jörn

-- 
Don't worry about people stealing your ideas. If your ideas are any good,
you'll have to ram them down people's throats.
-- Howard Aiken quoted by Ken Iverson quoted by Jim Horning quoted by
   Raph Levien, 1979

--- slab/include/linux/dcache.h~dentry_size	2006-09-14 22:19:51.000000000 +0200
+++ slab/include/linux/dcache.h	2006-09-18 23:22:44.000000000 +0200
@@ -77,8 +77,6 @@ full_name_hash(const unsigned char *name
 
 struct dcookie_struct;
 
-#define DNAME_INLINE_LEN_MIN 36
-
 struct dentry {
 	atomic_t d_count;
 	unsigned int d_flags;		/* protected by d_lock */
@@ -111,8 +109,8 @@ struct dentry {
 	struct dcookie_struct *d_cookie; /* cookie, if any */
 #endif
 	int d_mounted;
-	unsigned char d_iname[DNAME_INLINE_LEN_MIN];	/* small names */
-};
+	unsigned char d_iname[CONFIG_DCACHE_INLINE_LEN]; /* small names */
+}__cacheline_aligned;
 
 /*
  * dentry->d_lock spinlock nesting subclasses:
--- slab/fs/Kconfig~dentry_size	2006-08-11 13:03:43.000000000 +0200
+++ slab/fs/Kconfig	2006-09-18 23:22:44.000000000 +0200
@@ -1931,5 +1931,18 @@ endmenu
 
 source "fs/nls/Kconfig"
 
+config DCACHE_INLINE_LEN
+	int "Length of inline filenames"
+	depends on EXPERIMENTAL
+	default 20
+	help
+	  Filenames up to this length are stored inline in a struct dentry.
+	  For most people something short like 20 is ok.  Developers with
+	  extensive use of git or ccache may want to set this to 38 or 44,
+	  respectively, as those programs create and often access thousands
+	  of files of that length.
+
+	  If unsure, choose 20.
+
 endmenu
 
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