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From: Taylor Blau <me@ttaylorr.com>
To: Abhradeep Chakraborty via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, Kaartic Sivaram <kaartic.sivaraam@gmail.com>,
	Derrick Stolee <derrickstolee@github.com>,
	Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Subject: Re: [PATCH v2 2/6] pack-bitmap-write.c: write lookup table extension
Date: Mon, 27 Jun 2022 12:05:24 -0400	[thread overview]
Message-ID: <YrnVRLvbG4TK8pXY@nand.local> (raw)
In-Reply-To: <d118f1d45e6202925d4efd5435acdd08545bf132.1656249017.git.gitgitgadget@gmail.com>

On Sun, Jun 26, 2022 at 01:10:13PM +0000, Abhradeep Chakraborty via GitGitGadget wrote:
> From: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
>
> The bitmap lookup table extension was documentated by an earlier
> change, but Git does not yet knowhow to write that extension.
>
> Teach git to write bitmap lookup table extension. The table contains
> the list of `N` <commit pos, offset, xor offset>` triplets. These
> triplets are sorted according to their commit pos (ascending order).
> The meaning of each data in the i'th triplet is given below:
>
>   - Commit pos is the position of the commit in the pack-index
>     (or midx) to which the i'th bitmap belongs. It is a 4 byte
>     network byte order integer.
>
>   - offset is the position of the i'th bitmap.
>
>   - xor offset denotes the position of the triplet with whose
>     bitmap the current triplet's bitmap need to xor with.
>
> Co-authored-by: Taylor Blau <me@ttaylorr.com>
> Mentored-by: Taylor Blau <me@ttaylorr.com>
> Co-mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
> Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
> ---
>  pack-bitmap-write.c | 72 +++++++++++++++++++++++++++++++++++++++++++--
>  pack-bitmap.h       |  5 ++--
>  2 files changed, 73 insertions(+), 4 deletions(-)
>
> diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
> index c43375bd344..899a4a941e1 100644
> --- a/pack-bitmap-write.c
> +++ b/pack-bitmap-write.c
> @@ -650,7 +650,9 @@ static const struct object_id *oid_access(size_t pos, const void *table)
>
>  static void write_selected_commits_v1(struct hashfile *f,
>  				      struct pack_idx_entry **index,
> -				      uint32_t index_nr)
> +				      uint32_t index_nr,
> +				      uint64_t *offsets,

We should probably leave this as a pointer to an off_t, since that is a
more appropriate type for keeping track of an offset within a file (and
indeed it is the return type of hashfile_total()).

But since it's platform-dependent, we should make sure to cast it to a
uint64_t before writing it as part of the lookup table.

> +				      uint32_t *commit_positions)
>  {
>  	int i;
>
> @@ -663,6 +665,11 @@ static void write_selected_commits_v1(struct hashfile *f,
>  		if (commit_pos < 0)
>  			BUG("trying to write commit not in index");
>
> +		if (offsets)
> +			offsets[i] = hashfile_total(f);

This makes sense to store here, since we can't easily recover this
information later on.

> +		if (commit_positions)
> +			commit_positions[i] = commit_pos;

This one I'm not as sure about. It would be nice to not have
write_selected_commits_v1() be responsible for writing this down, too.
And I think it's easy enough to recover later on, since we're just doing
a search over "index" (see above the "oid_pos" call).

I think that oid_pos() call could be hidden behind a function that takes
an object_id pointer, an index (double pointer) of pack_idx_entry
structs, and a length.

Its implementation would be something like:

    static int commit_bitmap_writer_pos(struct object_id *oid,
                                        struct pack_idx_entry **index,
                                        uint32_t index_nr)
    {
        return oid_pos(oid, index, index_nr, oid_access);
    }

and then we could replace any calls like commit_positions[i] with one
that first takes `i` to the appropriate object_id in selected commit
order.

That would be strictly less efficient, but not in a way that I think
matters, and it would definitely be cleaner to not rely on a side-effect
of write_selected_commits_v1().

Something in the middle there would be to have write_lookup_table()
assemble that list of commit_positions itself, something like:

    uint32_t *commit_positions;

    ALLOC_ARRAY(commit_positions, writer.selected_nr);

    for (i = 0; i < writer.selected_nr; i++) {
        int pos = oid_pos(&writer.selected[i].commit->object.oid,
                          index, index_nr);
        if (pos < 0)
            BUG("trying to write commit not in index");
        commit_positions[i] = pos;
    }

    ...

    free(commit_positions);

That at least removes a side-effect from the implementation of
write_selected_commits_v1() and brings the creation of the
commit_positions array closer to where it's being used, while still
maintaining the constant-time lookups. So that may be a good
alternative, but I'm curious of your thoughts.

> +static int table_cmp(const void *_va, const void *_vb, void *commit_positions)

OK, so this is sorting the table in order of the commit positions. I
would rename the commit_positions parameter to something like "void
*_data", and then have commit_positions be the result of the cast, like
"uint32_t *commit_positions = _data";

> +{
> +	int8_t result = 0;

int8_t isn't an often used type in Git's codebase, but we can get rid of
this variable altogether and just return immediately from each case,
e.g.:

    if (a < b)
        return -1;
    else if (a > b)
        return 1;
    return 0;

or similar.

> +	uint32_t *positions = (uint32_t *) commit_positions;

Explicit cast isn't need here since you're going up from void*.

> +static void write_lookup_table(struct hashfile *f,
> +			       uint64_t *offsets,
> +			       uint32_t *commit_positions)
> +{
> +	uint32_t i;
> +	uint32_t *table, *table_inv;
> +
> +	ALLOC_ARRAY(table, writer.selected_nr);
> +	ALLOC_ARRAY(table_inv, writer.selected_nr);
> +
> +	for (i = 0; i < writer.selected_nr; i++)
> +		table[i] = i;
> +
> +	QSORT_S(table, writer.selected_nr, table_cmp, commit_positions);

I think the construction of table and table_inv could definitely benefit
from a comment here indicating what they're used for and what they
contain (e.g., "table maps abc to xyz").

> +	for (i = 0; i < writer.selected_nr; i++)
> +		table_inv[table[i]] = i;
> +
> +	for (i = 0; i < writer.selected_nr; i++) {
> +		struct bitmapped_commit *selected = &writer.selected[table[i]];
> +		uint32_t xor_offset = selected->xor_offset;
> +
> +		hashwrite_be32(f, commit_positions[table[i]]);
> +		hashwrite_be64(f, offsets[table[i]]);
> +		hashwrite_be32(f, xor_offset ?
> +				table_inv[table[i] - xor_offset]: 0xffffffff);

Nit: missing space before ':'.

Thanks,
Taylor

  parent reply	other threads:[~2022-06-27 16:07 UTC|newest]

Thread overview: 162+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-20 12:33 [PATCH 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty via GitGitGadget
2022-06-20 12:33 ` [PATCH 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-06-20 16:56   ` Derrick Stolee
2022-06-20 17:09     ` Taylor Blau
2022-06-21  8:31       ` Abhradeep Chakraborty
2022-06-22 16:26         ` Taylor Blau
2022-06-21  8:23     ` Abhradeep Chakraborty
2022-06-20 17:21   ` Taylor Blau
2022-06-21  9:22     ` Abhradeep Chakraborty
2022-06-22 16:29       ` Taylor Blau
2022-06-22 16:45         ` Abhradeep Chakraborty
2022-06-20 20:21   ` Derrick Stolee
2022-06-21 10:08     ` Abhradeep Chakraborty
2022-06-22 16:30       ` Taylor Blau
2022-06-20 12:33 ` [PATCH 2/6] pack-bitmap: prepare to read " Abhradeep Chakraborty via GitGitGadget
2022-06-20 20:49   ` Derrick Stolee
2022-06-21 10:28     ` Abhradeep Chakraborty
2022-06-20 22:06   ` Taylor Blau
2022-06-21 11:52     ` Abhradeep Chakraborty
2022-06-22 16:49       ` Taylor Blau
2022-06-22 17:18         ` Abhradeep Chakraborty
2022-06-22 21:34           ` Taylor Blau
2022-06-20 12:33 ` [PATCH 3/6] pack-bitmap-write.c: write " Abhradeep Chakraborty via GitGitGadget
2022-06-20 22:16   ` Taylor Blau
2022-06-21 12:50     ` Abhradeep Chakraborty
2022-06-22 16:51       ` Taylor Blau
2022-06-20 12:33 ` [PATCH 4/6] builtin/pack-objects.c: learn pack.writeBitmapLookupTable Taylor Blau via GitGitGadget
2022-06-20 22:18   ` Taylor Blau
2022-06-20 12:33 ` [PATCH 5/6] bitmap-commit-table: add tests for the bitmap lookup table Abhradeep Chakraborty via GitGitGadget
2022-06-22 16:54   ` Taylor Blau
2022-06-20 12:33 ` [PATCH 6/6] bitmap-lookup-table: add performance tests Abhradeep Chakraborty via GitGitGadget
2022-06-22 17:14   ` Taylor Blau
2022-06-26 13:10 ` [PATCH v2 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty via GitGitGadget
2022-06-26 13:10   ` [PATCH v2 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-06-27 14:18     ` Derrick Stolee
2022-06-27 15:48       ` Taylor Blau
2022-06-27 16:51       ` Abhradeep Chakraborty
2022-06-26 13:10   ` [PATCH v2 2/6] pack-bitmap-write.c: write " Abhradeep Chakraborty via GitGitGadget
2022-06-27 14:35     ` Derrick Stolee
2022-06-27 16:12       ` Taylor Blau
2022-06-27 17:10       ` Abhradeep Chakraborty
2022-06-27 16:05     ` Taylor Blau [this message]
2022-06-27 18:29       ` Abhradeep Chakraborty
2022-06-26 13:10   ` [PATCH v2 3/6] pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests Abhradeep Chakraborty via GitGitGadget
2022-06-27 14:43     ` Derrick Stolee
2022-06-27 17:42       ` Abhradeep Chakraborty
2022-06-27 17:49         ` Taylor Blau
2022-06-27 17:47     ` Taylor Blau
2022-06-27 18:39       ` Abhradeep Chakraborty
2022-06-29 20:11         ` Taylor Blau
2022-06-26 13:10   ` [PATCH v2 4/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-06-27 15:12     ` Derrick Stolee
2022-06-27 18:06       ` [PATCH v2 4/6] pack-bitmap: prepare to read lookup table Abhradeep Chakraborty
2022-06-27 18:32         ` Derrick Stolee
2022-06-27 21:49       ` [PATCH v2 4/6] pack-bitmap: prepare to read lookup table extension Taylor Blau
2022-06-28  8:59         ` [PATCH v2 4/6] pack-bitmap: prepare to read lookup table Abhradeep Chakraborty
2022-06-29 20:22           ` Taylor Blau
2022-06-30  6:58             ` [PATCH v2 4/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty
2022-06-27 21:38     ` Taylor Blau
2022-06-28 19:25       ` Abhradeep Chakraborty
2022-06-29 20:37         ` Taylor Blau
2022-06-29 20:41           ` Taylor Blau
2022-06-30  8:35           ` Abhradeep Chakraborty
2022-06-26 13:10   ` [PATCH v2 5/6] bitmap-lookup-table: add performance tests for lookup table Abhradeep Chakraborty via GitGitGadget
2022-06-27 21:53     ` Taylor Blau
2022-06-28  7:58       ` Abhradeep Chakraborty
2022-06-29 20:40         ` Taylor Blau
2022-06-26 13:10   ` [PATCH v2 6/6] p5310-pack-bitmaps.sh: enable pack.writeReverseIndex for testing Abhradeep Chakraborty via GitGitGadget
2022-06-27 21:50     ` Taylor Blau
2022-06-28  8:01       ` Abhradeep Chakraborty
2022-07-04  8:46   ` [PATCH v3 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty via GitGitGadget
2022-07-04  8:46     ` [PATCH v3 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-08 16:38       ` Philip Oakley
2022-07-09  7:53         ` Abhradeep Chakraborty
2022-07-10 15:01           ` Philip Oakley
2022-07-14 23:15             ` Taylor Blau
2022-07-15 10:36               ` Philip Oakley
2022-07-15 18:48             ` Abhradeep Chakraborty
2022-07-04  8:46     ` [PATCH v3 2/6] pack-bitmap-write.c: write " Abhradeep Chakraborty via GitGitGadget
2022-07-14 23:26       ` Taylor Blau
2022-07-15  2:22       ` Taylor Blau
2022-07-15 15:58         ` Abhradeep Chakraborty
2022-07-15 22:15           ` Taylor Blau
2022-07-16 11:50             ` Abhradeep Chakraborty
2022-07-26  0:34               ` Taylor Blau
2022-07-18  8:59       ` Martin Ågren
2022-07-04  8:46     ` [PATCH v3 3/6] pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests Abhradeep Chakraborty via GitGitGadget
2022-07-04  8:46     ` [PATCH v3 4/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-15  2:46       ` Taylor Blau
2022-07-15 16:38         ` Abhradeep Chakraborty
2022-07-15 22:20           ` Taylor Blau
2022-07-18  9:06             ` Martin Ågren
2022-07-18 19:25               ` Abhradeep Chakraborty
2022-07-18 23:26                 ` Martin Ågren
2022-07-26  0:45               ` Taylor Blau
2022-07-04  8:46     ` [PATCH v3 5/6] bitmap-lookup-table: add performance tests for lookup table Abhradeep Chakraborty via GitGitGadget
2022-07-15  2:53       ` Taylor Blau
2022-07-15 18:23         ` Abhradeep Chakraborty
2022-07-04  8:46     ` [PATCH v3 6/6] p5310-pack-bitmaps.sh: remove pack.writeReverseIndex Abhradeep Chakraborty via GitGitGadget
2022-07-04 16:35     ` [PATCH v3 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty
2022-07-06 19:21     ` Junio C Hamano
2022-07-07  8:48       ` Abhradeep Chakraborty
2022-07-07 18:09         ` Kaartic Sivaraam
2022-07-07 18:42           ` Abhradeep Chakraborty
2022-07-20 14:05     ` [PATCH v4 " Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 2/6] pack-bitmap-write.c: write " Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 3/6] pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 4/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 5/6] p5310-pack-bitmaps.sh: enable `pack.writeReverseIndex` Abhradeep Chakraborty via GitGitGadget
2022-07-20 14:05       ` [PATCH v4 6/6] bitmap-lookup-table: add performance tests for lookup table Abhradeep Chakraborty via GitGitGadget
2022-07-20 18:38       ` [PATCH v5 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty via GitGitGadget
2022-07-20 18:38         ` [PATCH v5 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-20 18:38         ` [PATCH v5 2/6] pack-bitmap-write.c: write " Abhradeep Chakraborty via GitGitGadget
2022-07-26  0:52           ` Taylor Blau
2022-07-26 18:22             ` Abhradeep Chakraborty
2022-07-20 18:38         ` [PATCH v5 3/6] pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests Abhradeep Chakraborty via GitGitGadget
2022-07-28 19:22           ` Johannes Schindelin
2022-08-02 12:40             ` Abhradeep Chakraborty
2022-08-02 15:35               ` Johannes Schindelin
2022-08-02 17:44                 ` Abhradeep Chakraborty
2022-08-08 13:06                   ` Johannes Schindelin
2022-08-08 13:58                     ` Abhradeep Chakraborty
2022-08-09  9:03                       ` Johannes Schindelin
2022-08-09 12:03                         ` Abhradeep Chakraborty
2022-08-09 12:07                           ` Abhradeep Chakraborty
2022-08-10  9:09                           ` Johannes Schindelin
2022-08-10  9:20                             ` Johannes Schindelin
2022-08-10 10:04                               ` Abhradeep Chakraborty
2022-08-10 17:51                                 ` Derrick Stolee
2022-08-12 18:51                                   ` Abhradeep Chakraborty
2022-08-12 19:22                                     ` Derrick Stolee
2022-08-13 10:59                                       ` Abhradeep Chakraborty
2022-08-16 21:57                                         ` Taylor Blau
2022-08-17 10:02                                           ` Abhradeep Chakraborty
2022-08-17 20:38                                             ` Taylor Blau
2022-08-19 21:49                                               ` Taylor Blau
2022-08-13 11:05                               ` Abhradeep Chakraborty
2022-08-16 18:47                             ` Taylor Blau
2022-07-20 18:38         ` [PATCH v5 4/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-07-26  1:13           ` Taylor Blau
2022-07-26 18:56             ` Abhradeep Chakraborty
2022-07-26 19:36             ` Eric Sunshine
2022-07-20 18:38         ` [PATCH v5 5/6] p5310-pack-bitmaps.sh: enable `pack.writeReverseIndex` Abhradeep Chakraborty via GitGitGadget
2022-07-26  1:18           ` Taylor Blau
2022-07-26  7:15             ` Ævar Arnfjörð Bjarmason
2022-07-26 13:32               ` Derrick Stolee
2022-07-26 13:54                 ` Ævar Arnfjörð Bjarmason
2022-07-26 18:17                   ` Abhradeep Chakraborty
2022-07-20 18:38         ` [PATCH v5 6/6] bitmap-lookup-table: add performance tests for lookup table Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55         ` [PATCH v6 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 1/6] Documentation/technical: describe bitmap lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 2/6] bitmap: move `get commit positions` code to `bitmap_writer_finish` Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 3/6] pack-bitmap-write.c: write lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 4/6] pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 5/6] pack-bitmap: prepare to read lookup table extension Abhradeep Chakraborty via GitGitGadget
2022-08-14 16:55           ` [PATCH v6 6/6] bitmap-lookup-table: add performance tests for lookup table Abhradeep Chakraborty via GitGitGadget
2022-08-19 21:21           ` [PATCH v6 0/6] [GSoC] bitmap: integrate a lookup table extension to the bitmap format Junio C Hamano
2022-08-22 14:42             ` Johannes Schindelin
2022-08-22 14:48               ` Taylor Blau
2022-08-25 22:16           ` Taylor Blau
2022-08-26 16:02             ` Junio C Hamano

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