use a hashmap to make remotes faster

Remotes are stored as an array, so looking one up or adding one without duplication is an O(n) operation. Reading an entire config file full of remotes is O(n^2) in the number of remotes. For a repository with tens of thousands of remotes, the running time can hit multiple minutes.

Hash tables are way faster. So we add a hashmap from remote name to struct remote and use it for all lookups. The time to add a new remote to a repo that already has 50,000 remotes drops from ~2 minutes to < 1 second.

We retain the old array of remotes so iterators proceed in config-file order.

Signed-off-by Patrick Reynolds <[email protected]>

Remotes are stored as an array, so looking one up or adding one without duplication is an O(n) operation. Reading an entire config file full of remotes is O(n^2) in the number of remotes. For a repository with tens of thousands of remotes, the running time can hit multiple minutes.

Hash tables are way faster. So we add a hashmap from remote name to struct remote and use it for all lookups. The time to add a new remote to a repo that already has 50,000 remotes drops from ~2 minutes to < 1 second.

We retain the old array of remotes so iterators proceed in config-file order.

Signed-off-by Patrick Reynolds <[email protected]>

---
mark function with no arguments as taking void

 remote.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++-----------------
 remote.h |  3 +++
 2 files changed, 49 insertions(+), 17 deletions(-)

remote.c +46 -17

index a0701f6..52533e4 100644
@@ -42,6 +42,7 @@ struct rewrites {
 static struct remote **remotes;
 static int remotes_alloc;
 static int remotes_nr;
+static struct hashmap remotes_hash;
 
 static struct branch **branches;
 static int branches_alloc;

@@ -136,26 +137,51 @@ static void add_url_alias(struct remote *remote, const char *url)
 	add_pushurl_alias(remote, url);
 }
 
+struct remotes_hash_key {
+	const char *str;
+	int len;
+};
+
+static int remotes_hash_cmp(const struct remote *a, const struct remote *b, const struct remotes_hash_key *key)
+{
+	if (key)
+		return strncmp(a->name, key->str, key->len) || a->name[key->len];
+	else
+		return strcmp(a->name, b->name);
+}
+
+static inline void init_remotes_hash(void)
+{
+	if (!remotes_hash.cmpfn)
+		hashmap_init(&remotes_hash, (hashmap_cmp_fn)remotes_hash_cmp, 0);
+}
+
 static struct remote *make_remote(const char *name, int len)
 {
-	struct remote *ret;
-	int i;
+	struct remote *ret, *replaced;
+	struct remotes_hash_key lookup;
+	struct hashmap_entry lookup_entry;
 
-	for (i = 0; i < remotes_nr; i++) {
-		if (len ? (!strncmp(name, remotes[i]->name, len) &&
-			   !remotes[i]->name[len]) :
-		    !strcmp(name, remotes[i]->name))
-			return remotes[i];
-	}
+	if (!len)
+		len = strlen(name);
+
+	init_remotes_hash();
+	lookup.str = name;
+	lookup.len = len;
+	hashmap_entry_init(&lookup_entry, memhash(name, len));
+
+	if ((ret = hashmap_get(&remotes_hash, &lookup_entry, &lookup)) != NULL)
+		return ret;
 
 	ret = xcalloc(1, sizeof(struct remote));
 	ret->prune = -1;  /* unspecified */
 	ALLOC_GROW(remotes, remotes_nr + 1, remotes_alloc);
 	remotes[remotes_nr++] = ret;
-	if (len)
-		ret->name = xstrndup(name, len);
-	else
-		ret->name = xstrdup(name);
+	ret->name = xstrndup(name, len);
+
+	hashmap_entry_init(ret, lookup_entry.hash);
+	replaced = hashmap_put(&remotes_hash, ret);
+	assert(replaced == NULL);  /* no previous entry overwritten */
 	return ret;
 }
 

@@ -722,13 +748,16 @@ struct remote *pushremote_get(const char *name)
 
 int remote_is_configured(const char *name)
 {
-	int i;
+	struct remotes_hash_key lookup;
+	struct hashmap_entry lookup_entry;
 	read_config();
 
-	for (i = 0; i < remotes_nr; i++)
-		if (!strcmp(name, remotes[i]->name))
-			return 1;
-	return 0;
+	init_remotes_hash();
+	lookup.str = name;
+	lookup.len = strlen(name);
+	hashmap_entry_init(&lookup_entry, memhash(name, lookup.len));
+
+	return hashmap_get(&remotes_hash, &lookup_entry, &lookup) != NULL;
 }
 
 int for_each_remote(each_remote_fn fn, void *priv)

remote.h +1 -0

index 917d383..81cb5ff 100644
@@ -1,6 +1,7 @@
 #ifndef REMOTE_H
 #define REMOTE_H
 
+#include "hashmap.h"
 #include "parse-options.h"
 
 enum {

remote.h +0 -0

 	const char *name;
 	int origin;
 

remote.h enum { · line 15

Quoted 3 lines

struct remote {

remote.h +2 -0

+	struct hashmap_entry ent;  /* must be first */
+

I stumbled about this comment "/* must be first */" when reading the changelog.

Why does it need to be first?
Is it a common reason I'm just not aware of,
or would it make sense to put the reason into the comment as well?