Re: [PATCH v2 11/19] pack-objects: use bitmaps when packing objects

On Wed, Oct 30, 2013 at 2:36 PM, Jeff King [off-list ref] wrote:

On Sat, Oct 26, 2013 at 05:25:14PM +0700, Nguyen Thai Ngoc Duy wrote:

quoted

quoted

For bitmaps to be used, the following must be true:

  1. We must be packing to stdout (as a normal `pack-objects` from
     `upload-pack` would do).

  2. There must be a .bitmap index containing at least one of the
     "have" objects that the client is asking for.

  3. Bitmaps must be enabled (they are enabled by default, but can be
     disabled by setting `pack.usebitmaps` to false, or by using
     `--no-use-bitmap-index` on the command-line).

If any of these is not true, we fall back to doing a normal walk of the
object graph.

I haven't read the bitmap creation code yet. But it probably does not
matter. If the client requests a shallow fetch, you probably want to
fall back to normal walk too.

One other criterion I should have mentioned: we must be using the
internal rev-list. That prevented us in v1.8.4.1 and earlier from using
bitmaps for shallow fetches. But as of v1.8.4.2, we always use
pack-objects' rev-walker. We may need to pass --no-use-bitmap-index for
shallow fetches.

I don't think a new option is needed. The code just needs to check if
there are any commit grafts. If there are, fall back to the old way.
That covers both shallow fetches and some rare grafted repos. I think
refs/replace/* does not impact rev walking, so we should be fine if
it's used.

As for repos that are themselves shallow, I do not know how doing a
"repack -b" would fare. Probably not well.

-- 
Duy