linux-kernel - Re: [patch 1/2] fs: mnt_want

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Date:	Tue, 10 Mar 2009 16:03:38 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [patch 1/2] fs: mnt_want_write speedup

On Tue, Mar 10, 2009 at 08:48:57AM -0600, Matthew Wilcox wrote:
> On Tue, Mar 10, 2009 at 03:37:18PM +0100, Nick Piggin wrote:
> > costly, unfortunately). It results in about 900 bytes smaller code too. It
> > does increase the size of a vfsmount, however.
> 
> Only on 64-bit SMP systems, and then only by four bytes.  And, best of
> all, you can fix that if you care.  Look at this:
> 
>         /* --- cacheline 1 boundary (64 bytes) --- */
>         struct list_head           mnt_child;            /*    64    16 */
>         int                        mnt_flags;            /*    80     4 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         const char  *              mnt_devname;          /*    88     8 */
>         struct list_head           mnt_list;             /*    96    16 */
>         struct list_head           mnt_expire;           /*   112    16 */
> 
> So move mnt_flags to later in the struct (after the pointers), and move
> 
> > +#ifdef CONFIG_SMP
> > +     int *mnt_writers;
> > +#else
> > +     int mnt_writers;
> > +#endif
> 
> to be with the other pointers.  Bonus points for putting it between
>         struct mnt_namespace *     mnt_ns;               /*   184     8 */
> and
>         int                        mnt_id;               /*   192     4 */
> 
> so that it doesn't become a new 4-byte hole for those incredibly common
> 64-bit uniprocessor builds.  *cough*.

Oh good point, although yes I was more worried about mnt_writers in
the SMP case (yes I didn't state it very well). Basically I would be
worried if huge machinges have huge numbers of mounts.... but I think
a) if they did they would probably like the scalability improvements,
b) the improvement on smaller systems is so significant that 100s of
   CPU systems will have to find a way to cut down memory if it really
   was a problem for them.

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