diffcore-rename: use a mem_pool for exact rename detection's hashmap

Exact rename detection, via insert_file_table(), uses a hashmap to store
files by oid.  Use a mem_pool for the hashmap entries so these can all be
allocated and deallocated together.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:      204.2  ms ±  3.0  ms   202.5  ms ±  3.2  ms
    mega-renames:      1.076 s ±  0.015 s     1.072 s ±  0.012 s
    just-one-mega:   364.1  ms ±  7.0  ms   357.3  ms ±  3.9  ms

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/diffcore-rename.c b/diffcore-rename.c
index 4ef0459cfb500049154ad72270e5ec5368702cf4..73d884099eb7a70f2204e1d225c0a609ad80a7c5 100644 (file)
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -317,10 +317,11 @@ static int find_identical_files(struct hashmap *srcs,
 }
 
 static void insert_file_table(struct repository *r,
+                             struct mem_pool *pool,
                              struct hashmap *table, int index,
                              struct diff_filespec *filespec)
 {
-       struct file_similarity *entry = xmalloc(sizeof(*entry));
+       struct file_similarity *entry = mem_pool_alloc(pool, sizeof(*entry));
 
        entry->index = index;
        entry->filespec = filespec;
@@ -336,7 +337,8 @@ static void insert_file_table(struct repository *r,
  * and then during the second round we try to match
  * cache-dirty entries as well.
  */
-static int find_exact_renames(struct diff_options *options)
+static int find_exact_renames(struct diff_options *options,
+                             struct mem_pool *pool)
 {
        int i, renames = 0;
        struct hashmap file_table;
@@ -346,7 +348,7 @@ static int find_exact_renames(struct diff_options *options)
         */
        hashmap_init(&file_table, NULL, NULL, rename_src_nr);
        for (i = rename_src_nr-1; i >= 0; i--)
-               insert_file_table(options->repo,
+               insert_file_table(options->repo, pool,
                                  &file_table, i,
                                  rename_src[i].p->one);
 
@@ -354,8 +356,8 @@ static int find_exact_renames(struct diff_options *options)
        for (i = 0; i < rename_dst_nr; i++)
                renames += find_identical_files(&file_table, i, options);
 
-       /* Free the hash data structure and entries */
-       hashmap_clear_and_free(&file_table, struct file_similarity, entry);
+       /* Free the hash data structure (entries will be freed with the pool) */
+       hashmap_clear(&file_table);
 
        return renames;
 }
@@ -1341,6 +1343,7 @@ void diffcore_rename_extended(struct diff_options *options,
        int num_destinations, dst_cnt;
        int num_sources, want_copies;
        struct progress *progress = NULL;
+       struct mem_pool local_pool;
        struct dir_rename_info info;
        struct diff_populate_filespec_options dpf_options = {
                .check_binary = 0,
@@ -1409,11 +1412,18 @@ void diffcore_rename_extended(struct diff_options *options,
                goto cleanup; /* nothing to do */
 
        trace2_region_enter("diff", "exact renames", options->repo);
+       mem_pool_init(&local_pool, 32*1024);
        /*
         * We really want to cull the candidates list early
         * with cheap tests in order to avoid doing deltas.
         */
-       rename_count = find_exact_renames(options);
+       rename_count = find_exact_renames(options, &local_pool);
+       /*
+        * Discard local_pool immediately instead of at "cleanup:" in order
+        * to reduce maximum memory usage; inexact rename detection uses up
+        * a fair amount of memory, and mem_pools can too.
+        */
+       mem_pool_discard(&local_pool, 0);
        trace2_region_leave("diff", "exact renames", options->repo);
 
        /* Did we only want exact renames? */