[Tux3] Thinking about losing the dleaf free space vars

[Shapor Naghibzadeh]

I think at this point the tracking used/free explicitly will make
detecting bugs easier.  Perhaps it should be left for now, and removed
later as an optimization.

Shapor

On Wed, Oct 1, 2008 at 2:07 PM, Daniel Phillips <phillips at phunq.net> wrote:
> Currently we have:
>
>        unsigned leaf_free(BTREE, vleaf *leaf)
>        {
>                return to_dleaf(leaf)->used - to_dleaf(leaf)->free;
>        }
>
> and the same result is produced by:
>
>        int leaf_free2(BTREE, void *vleaf)
>        {
>                struct dleaf *leaf = vleaf;
>                struct group *gdict = (void *)leaf + btree->sb->blocksize, *gstop = gdict - leaf->groups;
>                struct entry *edict = (void *)gstop, *entry = edict;
>                struct extent *extents = leaf->table;
>                for (struct group *group = gdict - 1; group >= gstop; group--)
>                        extents += (entry -= group->count)->limit;
>                return (void *)entry - (void *)extents;
>        }
>
> Which is pretty efficient.  I wonder whether it is worth maintaining
> the free/used variables in the dleaf at all?  There are only two places
> where we actually need to know: in filemap.c, to decide whether we need
> to split the leaf before inserting new extents, and in btree.c to
> decide whether we can merge two leaves.  In both cases we have just
> traversed the leaf contents anyway, and done most of the work of
> computing the leaf free space.
>
> Against this, we have a considerable amount of code for maintaining the
> free and used variables in the leaf during various operations.  I have
> nearly convinced myself that we should err on the side of less code and
> get rid of the in-leaf variables.
>
> Regards,
>
> Daniel
>
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