Re: [PATCH 2/3] dio: scale unaligned IO tracking via multiple lists

[Dave Chinner <david@fromorbit.com>]

On Mon, Nov 08, 2010 at 10:36:06AM -0500, Jeff Moyer wrote:
> Dave Chinner <david@fromorbit.com> writes:
> 
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > To avoid concerns that a single list and lock tracking the unaligned
> > IOs will not scale appropriately, create multiple lists and locks
> > and chose them by hashing the unaligned block being zeroed.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> >  fs/direct-io.c |   49 ++++++++++++++++++++++++++++++++++++-------------
> >  1 files changed, 36 insertions(+), 13 deletions(-)
> >
> > diff --git a/fs/direct-io.c b/fs/direct-io.c
> > index 1a69efd..353ac52 100644
> > --- a/fs/direct-io.c
> > +++ b/fs/direct-io.c
> > @@ -152,8 +152,28 @@ struct dio_zero_block {
> >  	atomic_t	ref;		/* reference count */
> >  };
> >  
> > -static DEFINE_SPINLOCK(dio_zero_block_lock);
> > -static LIST_HEAD(dio_zero_block_list);
> > +#define DIO_ZERO_BLOCK_NR	37LL
> 
> I'm always curious to know how these numbers are derived.  Why 37?

It's a prime number large enough to give enough lists to minimise
contention whilst providing decent distribution for 8 byte aligned
addresses with low overhead. XFS uses the same sort of waitqueue
hashing for global IO completion wait queues used by truncation
and inode eviction (see xfs_ioend_wait()).

Seemed reasonable (and simple!) just to copy that design pattern
for another global IO completion wait queue....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com