[PATCH 4/7] reftable/record: reuse refnames when decoding log records

From: Patrick Steinhardt <ps@pks.im>
Date: Mar 5, 2024, 12:11 UTC
Message-ID: <f2b59e58a4e54e2a3d1df2c73c4f7acd92a57604.1709640322.git.ps@pks.im>
In-Reply-To: <cover.1709640322.git.ps@pks.im>

When decoding a log record we always reallocate their refname arrays. This results in quite a lot of needless allocation churn.

Refactor the code to grow the array as required only. Like this, we should usually only end up reallocating the array a small handful of times when iterating over many refs. Before:

    HEAP SUMMARY:
        in use at exit: 13,473 bytes in 122 blocks
      total heap usage: 4,068,487 allocs, 4,068,365 frees, 332,011,793 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 13,473 bytes in 122 blocks
      total heap usage: 3,068,488 allocs, 3,068,366 frees, 307,122,961 bytes allocated

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/record.c          | 2 +-
 reftable/reftable-record.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/reftable/record.c b/reftable/record.c
index d816de6d93..82780b2d06 100644
--- a/reftable/record.c
+++ b/reftable/record.c
@@ -861,7 +861,7 @@ static int reftable_log_record_decode(void *rec, struct strbuf key,
 	if (key.len <= 9 || key.buf[key.len - 9] != 0)
 		return REFTABLE_FORMAT_ERROR;
 
-	r->refname = reftable_realloc(r->refname, key.len - 8);
+	REFTABLE_ALLOC_GROW(r->refname, key.len - 8, r->refname_cap);
 	memcpy(r->refname, key.buf, key.len - 8);
 	ts = get_be64(key.buf + key.len - 8);
 
diff --git a/reftable/reftable-record.h b/reftable/reftable-record.h
index 2659ea008c..d28f38175c 100644
--- a/reftable/reftable-record.h
+++ b/reftable/reftable-record.h
@@ -74,6 +74,7 @@ int reftable_ref_record_equal(const struct reftable_ref_record *a,
 /* reftable_log_record holds a reflog entry */
 struct reftable_log_record {
 	char *refname;
+	size_t refname_cap;
 	uint64_t update_index; /* logical timestamp of a transactional update.
 				*/

-- 
2.44.0