Subject: Re: [PATCH v2 12/24] pack-bitmap-write: fill bitmap with commit history

From: Taylor Blau <[email-masked]>
Date: Sat, 28 Nov 2020 12:21:20 -0500

On Tue, Nov 24, 2020 at 05:14:09PM -0800, Jonathan Tan wrote:
> > From: Derrick Stolee <[email-masked]>
> >
> > The fill_bitmap_commit() method assumes that every parent of the given
> > commit is already part of the current bitmap. Instead of making that
> > assumption, let's walk parents until we reach commits already part of
> > the bitmap. Set the value for that parent immediately after querying to
> > save time doing double calls to find_object_pos() and to avoid inserting
> > the parent into the queue multiple times.
>
> I see from the later patches that this has no effect until the part
> where we can skip commits, but as Junio says [1], it's worth mentioning
> it here. Maybe something like:
>
>   The fill_bitmap_commit() method assumes that every parent of the given
>   commit is already part of the current bitmap. This is currently
>   correct, but a subsequent patch will change the nature of the edges of
>   the graph from parent-child to ancestor-descendant. In preparation for
>   that, let's walk parents...

Thanks. Stolee and I worked a little on revising this last week, and I
think that the current log message is more along these lines. Here's
what we wrote:

    pack-bitmap-write: fill bitmap with commit history

    The current implementation of bitmap_writer_build() creates a
    reachability bitmap for every walked commit. After computing a bitmap
    for a commit, those bits are pushed to an in-progress bitmap for its
    children.

    fill_bitmap_commit() assumes the bits corresponding to objects
    reachable from the parents of a commit are already set. This means that
    when visiting a new commit, we only have to walk the objects reachable
    between it and any of its parents.

    A future change to bitmap_writer_build() will relax this condition so
    not all parents have their reachable objects set in the in-progress
    bitmap. Prepare for that by having 'fill_bitmap_commit()' walk
    parents until reaching commits whose bits are already set. Then, walk
    the trees for these commits as well.

    This has no functional change with the current implementation of
    bitmap_writer_build().

> >  static void fill_bitmap_commit(struct bb_commit *ent,
> > -			       struct commit *commit)
> > +			       struct commit *commit,
> > +			       struct prio_queue *queue)
>
> As far as I can see, this function expects an empty queue and always
> ends with the queue empty, and the only reason why we don't instantiate
> a new queue every time is so that we can save on the internal array
> allocation/deallocation. Maybe add a comment to that effect.

Sure. Would you find a comment like that more helpful above
'fill_bitmap_commit()', or above the declaration of 'queue' (in
'bitmap_writer_build()') itself?

Thanks,
Taylor