On Wed, Dec 20, 2023 at 10:17:17AM +0100, Patrick Steinhardt wrote:

When reading ref records of type "val1" we store its object ID in an
allocated array. This results in an additional allocation for every
single ref record we read, which is rather inefficient especially when
iterating over refs.

Refactor the code to instead use a static array of `GIT_MAX_RAWSZ`
bytes. While this means that `struct ref_record` is bigger now, we
typically do not store all refs in an array anyway and instead only
handle a limited number of records at the same point in time.

Using `git show-ref --quiet` in a repository with ~350k refs this leads
to a significant drop in allocations. Before:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 2,116,683 allocs, 2,116,491 frees, 76,098,060 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 1,419,031 allocs, 1,418,839 frees, 62,145,036 bytes allocated

Signed-off-by: Patrick Steinhardt <redacted>

I screwed up the rebase. The following diff is required on top:

diff --git a/reftable/readwrite_test.c b/reftable/readwrite_test.c
index 56c0b4db5d..87b238105c 100644
--- a/reftable/readwrite_test.c
+++ b/reftable/readwrite_test.c

@@ -809,11 +809,10 @@ static void test_write_multiple_indices(void)
 	writer = reftable_new_writer(&strbuf_add_void, &writer_buf, &opts);
 	reftable_writer_set_limits(writer, 1, 1);
 	for (i = 0; i < 100; i++) {
-		unsigned char hash[GIT_SHA1_RAWSZ] = {i};
 		struct reftable_ref_record ref = {
 			.update_index = 1,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash,
+			.value.val1 = {i},
 		};
 
 		strbuf_reset(&buf);

Will fix in v2 of this series.

Patrick