Re: [PATCH][RFC] %pd - for printing dentry name

[Matthew Wilcox <matthew@wil.cx>]

On Tue, Feb 02, 2010 at 07:56:36AM -0800, Linus Torvalds wrote:
> On Tue, 2 Feb 2010, Matthew Wilcox wrote:
> > 
> > How about doing this:
> > 
> >  struct qstr {
> > -	const unsigned char *name;
> > +	const unsigned char name[0];
> >  }
> > 
> >  struct dentry {
> > -	struct qstr d_name;
> > +	struct qstr *d_name;
> > -	unsigned char d_iname[DNAME_INLINE_LEN_MIN];    /* small names */
> > +	union {
> > +		struct qstr d_iname;
> > +		char pad[DNAME_INLINE_LEN_MIN];
> > +	};
> >  }
> > 
> > Doesn't increase the size of struct dentry, and puts the hash and len
> > with the name.  Increases long name allocations by 8 bytes each.
> 
> Conceptually nice, but in practice that's absolutely horrible.
> 
> Why? Because now the dentry lookup logic has to follow an additional 
> pointer just to verify the hash and the length of the name. That's some of 
> the hottest code we have, and the _last_ thing we want is another pointer 
> dereference and cache access in the path that looks up the dentry hash 
> chains.

I'd thought it was in the same cacheline ... but that's not generally
true since dentries are so large.  And the d_name is at the other end
of the dentry, so they're guaranteed to be in different cachelines.  Bum.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."