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    SubjectRe: [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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