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pseudo-merge: implement support for finding existing merges

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This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.

Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).

Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.

This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:

  - non-matching pseudo-merge bitmaps are rejected quickly as soon as
    they differ in a single bit

  - already-matched pseudo-merge bitmaps are discarded from subsequent
    rounds of search

  - the number of pseudo-merges is generally small, even for large
    repositories

In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Taylor Blau
2024-05-23 17:27:21 -04:00
committed by Junio C Hamano
parent 94c1addf86
commit 7252d9a036
6 changed files with 174 additions and 2 deletions
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15
pack-bitmap-write.c
View File

@@ -19,6 +19,10 @@
#include "tree-walk.h"
#include "pseudo-merge.h"
#include "oid-array.h"
#include "config.h"
#include "alloc.h"
#include "refs.h"
#include "strmap.h"
struct bitmapped_commit {
struct commit *commit;
@@ -465,6 +469,7 @@ static int fill_bitmap_tree(struct bitmap_writer *writer,
}
static int reused_bitmaps_nr;
static int reused_pseudo_merge_bitmaps_nr;
static int fill_bitmap_commit(struct bitmap_writer *writer,
struct bb_commit *ent,
@@ -490,7 +495,7 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
struct bitmap *remapped = bitmap_new();
if (commit->object.flags & BITMAP_PSEUDO_MERGE)
old = NULL;
old = pseudo_merge_bitmap_for_commit(old_bitmap, c);
else
old = bitmap_for_commit(old_bitmap, c);
/*
@@ -501,7 +506,10 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
if (old && !rebuild_bitmap(mapping, old, remapped)) {
bitmap_or(ent->bitmap, remapped);
bitmap_free(remapped);
reused_bitmaps_nr++;
if (commit->object.flags & BITMAP_PSEUDO_MERGE)
reused_pseudo_merge_bitmaps_nr++;
else
reused_bitmaps_nr++;
continue;
}
bitmap_free(remapped);
@@ -631,6 +639,9 @@ int bitmap_writer_build(struct bitmap_writer *writer,
the_repository);
trace2_data_intmax("pack-bitmap-write", the_repository,
"building_bitmaps_reused", reused_bitmaps_nr);
trace2_data_intmax("pack-bitmap-write", the_repository,
"building_bitmaps_pseudo_merge_reused",
reused_pseudo_merge_bitmaps_nr);
stop_progress(&writer->progress);