Thin pack generation: optimization.

Jens Axboe noticed that recent "git push" has become very slow
since we made --thin transfer the default.

Thin pack generation to push a handful revisions that touch
relatively small number of paths out of huge tree was stupid; it
registered _everything_ from the excluded revisions.  As a
result, "Counting objects" phase was unnecessarily expensive.

This changes the logic to register the blobs and trees from
excluded revisions only for paths we are actually going to send
to the other end.

Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 236 insertions, 48 deletions

diff --git a/pack-objects.c b/pack-objects.c
index 9346392..09f4f2c 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -453,7 +453,7 @@ static void rehash_objects(void)
 	if (object_ix_hashsz < 1024)
 		object_ix_hashsz = 1024;
 	object_ix = xrealloc(object_ix, sizeof(int) * object_ix_hashsz);
-	object_ix = memset(object_ix, 0, sizeof(int) * object_ix_hashsz);
+	memset(object_ix, 0, sizeof(int) * object_ix_hashsz);
 	for (i = 0, oe = objects; i < nr_objects; i++, oe++) {
 		int ix = locate_object_entry_hash(oe->sha1);
 		if (0 <= ix)
@@ -505,21 +505,6 @@ static unsigned name_hash(struct name_path *path, const char *name)
 	 * but close enough.
 	 */
 	hash = (name_hash<<DIRBITS) | (hash & ((1U<<DIRBITS )-1));
-
-	if (0) { /* debug */
-		n = name + strlen(name);
-		if (n != name && n[-1] == '\n')
-			n--;
-		while (name <= --n)
-			fputc(*n, stderr);
-		for (p = path; p; p = p->up) {
-			fputc('/', stderr);
-			n = p->elem + p->len;
-			while (p->elem <= --n)
-				fputc(*n, stderr);
-		}
-		fprintf(stderr, "\t%08x\n", hash);
-	}
 	return hash;
 }
 
@@ -587,56 +572,254 @@ static int add_object_entry(const unsigned char *sha1, unsigned hash, int exclud
 	return status;
 }
 
-static void add_pbase_tree(struct tree_desc *tree, struct name_path *up)
+struct pbase_tree_cache {
+	unsigned char sha1[20];
+	int ref;
+	int temporary;
+	void *tree_data;
+	unsigned long tree_size;
+};
+
+static struct pbase_tree_cache *(pbase_tree_cache[256]);
+static int pbase_tree_cache_ix(const unsigned char *sha1)
+{
+	return sha1[0] % ARRAY_SIZE(pbase_tree_cache);
+}
+static int pbase_tree_cache_ix_incr(int ix)
+{
+	return (ix+1) % ARRAY_SIZE(pbase_tree_cache);
+}
+
+static struct pbase_tree {
+	struct pbase_tree *next;
+	/* This is a phony "cache" entry; we are not
+	 * going to evict it nor find it through _get()
+	 * mechanism -- this is for the toplevel node that
+	 * would almost always change with any commit.
+	 */
+	struct pbase_tree_cache pcache;
+} *pbase_tree;
+
+static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
+{
+	struct pbase_tree_cache *ent, *nent;
+	void *data;
+	unsigned long size;
+	char type[20];
+	int neigh;
+	int my_ix = pbase_tree_cache_ix(sha1);
+	int available_ix = -1;
+
+	/* pbase-tree-cache acts as a limited hashtable.
+	 * your object will be found at your index or within a few
+	 * slots after that slot if it is cached.
+	 */
+	for (neigh = 0; neigh < 8; neigh++) {
+		ent = pbase_tree_cache[my_ix];
+		if (ent && !memcmp(ent->sha1, sha1, 20)) {
+			ent->ref++;
+			return ent;
+		}
+		else if (((available_ix < 0) && (!ent || !ent->ref)) ||
+			 ((0 <= available_ix) &&
+			  (!ent && pbase_tree_cache[available_ix])))
+			available_ix = my_ix;
+		if (!ent)
+			break;
+		my_ix = pbase_tree_cache_ix_incr(my_ix);
+	}
+
+	/* Did not find one.  Either we got a bogus request or
+	 * we need to read and perhaps cache.
+	 */
+	data = read_sha1_file(sha1, type, &size);
+	if (!data)
+		return NULL;
+	if (strcmp(type, tree_type)) {
+		free(data);
+		return NULL;
+	}
+
+	/* We need to either cache or return a throwaway copy */
+
+	if (available_ix < 0)
+		ent = NULL;
+	else {
+		ent = pbase_tree_cache[available_ix];
+		my_ix = available_ix;
+	}
+
+	if (!ent) {
+		nent = xmalloc(sizeof(*nent));
+		nent->temporary = (available_ix < 0);
+	}
+	else {
+		/* evict and reuse */
+		free(ent->tree_data);
+		nent = ent;
+	}
+	memcpy(nent->sha1, sha1, 20);
+	nent->tree_data = data;
+	nent->tree_size = size;
+	nent->ref = 1;
+	if (!nent->temporary)
+		pbase_tree_cache[my_ix] = nent;
+	return nent;
+}
+
+static void pbase_tree_put(struct pbase_tree_cache *cache)
+{
+	if (!cache->temporary) {
+		cache->ref--;
+		return;
+	}
+	free(cache->tree_data);
+	free(cache);
+}
+
+static int name_cmp_len(const char *name)
+{
+	int i;
+	for (i = 0; name[i] && name[i] != '\n' && name[i] != '/'; i++)
+		;
+	return i;
+}
+
+static void add_pbase_object(struct tree_desc *tree,
+			     struct name_path *up,
+			     const char *name,
+			     int cmplen)
 {
 	while (tree->size) {
 		const unsigned char *sha1;
-		const char *name;
-		unsigned mode, hash;
+		const char *entry_name;
+		int entry_len;
+		unsigned mode;
 		unsigned long size;
 		char type[20];
 
-		sha1 = tree_entry_extract(tree, &name, &mode);
+		sha1 = tree_entry_extract(tree, &entry_name, &mode);
 		update_tree_entry(tree);
-		if (!has_sha1_file(sha1))
-			continue;
-		if (sha1_object_info(sha1, type, &size))
+		entry_len = strlen(entry_name);
+		if (entry_len != cmplen ||
+		    memcmp(entry_name, name, cmplen) ||
+		    !has_sha1_file(sha1) ||
+		    sha1_object_info(sha1, type, &size))
 			continue;
-
-		hash = name_hash(up, name);
-		if (!add_object_entry(sha1, hash, 1))
-			continue;
-
+		if (name[cmplen] != '/') {
+			unsigned hash = name_hash(up, name);
+			add_object_entry(sha1, hash, 1);
+			return;
+		}
 		if (!strcmp(type, tree_type)) {
 			struct tree_desc sub;
-			void *elem;
 			struct name_path me;
+			struct pbase_tree_cache *tree;
+			const char *down = name+cmplen+1;
+			int downlen = name_cmp_len(down);
+
+			tree = pbase_tree_get(sha1);
+			if (!tree)
+				return;
+			sub.buf = tree->tree_data;
+			sub.size = tree->tree_size;
+
+			me.up = up;
+			me.elem = entry_name;
+			me.len = entry_len;
+			add_pbase_object(&sub, &me, down, downlen);
+			pbase_tree_put(tree);
+		}
+	}
+}
 
-			elem = read_sha1_file(sha1, type, &sub.size);
-			sub.buf = elem;
-			if (sub.buf) {
-				me.up = up;
-				me.elem = name;
-				me.len = strlen(name);
-				add_pbase_tree(&sub, &me);
-				free(elem);
-			}
+static unsigned *done_pbase_paths;
+static int done_pbase_paths_num;
+static int done_pbase_paths_alloc;
+static int done_pbase_path_pos(unsigned hash)
+{
+	int lo = 0;
+	int hi = done_pbase_paths_num;
+	while (lo < hi) {
+		int mi = (hi + lo) / 2;
+		if (done_pbase_paths[mi] == hash)
+			return mi;
+		if (done_pbase_paths[mi] < hash)
+			hi = mi;
+		else
+			lo = mi + 1;
+	}
+	return -lo-1;
+}
+
+static int check_pbase_path(unsigned hash)
+{
+	int pos = (!done_pbase_paths) ? -1 : done_pbase_path_pos(hash);
+	if (0 <= pos)
+		return 1;
+	pos = -pos - 1;
+	if (done_pbase_paths_alloc <= done_pbase_paths_num) {
+		done_pbase_paths_alloc = alloc_nr(done_pbase_paths_alloc);
+		done_pbase_paths = xrealloc(done_pbase_paths,
+					    done_pbase_paths_alloc *
+					    sizeof(unsigned));
+	}
+	done_pbase_paths_num++;
+	if (pos < done_pbase_paths_num)
+		memmove(done_pbase_paths + pos + 1,
+			done_pbase_paths + pos,
+			(done_pbase_paths_num - pos - 1) * sizeof(unsigned));
+	done_pbase_paths[pos] = hash;
+	return 0;
+}
+
+static void add_preferred_base_object(char *name, unsigned hash)
+{
+	struct pbase_tree *it;
+	int cmplen = name_cmp_len(name);
+
+	if (check_pbase_path(hash))
+		return;
+
+	for (it = pbase_tree; it; it = it->next) {
+		if (cmplen == 0) {
+			hash = name_hash(NULL, "");
+			add_object_entry(it->pcache.sha1, hash, 1);
+		}
+		else {
+			struct tree_desc tree;
+			tree.buf = it->pcache.tree_data;
+			tree.size = it->pcache.tree_size;
+			add_pbase_object(&tree, NULL, name, cmplen);
 		}
 	}
 }
 
 static void add_preferred_base(unsigned char *sha1)
 {
-	struct tree_desc tree;
-	void *elem;
+	struct pbase_tree *it;
+	void *data;
+	unsigned long size;
+	unsigned char tree_sha1[20];
 
-	elem = read_object_with_reference(sha1, tree_type, &tree.size, NULL);
-	tree.buf = elem;
-	if (!tree.buf)
+	data = read_object_with_reference(sha1, tree_type, &size, tree_sha1);
+	if (!data)
 		return;
-	if (add_object_entry(sha1, name_hash(NULL, ""), 1))
-		add_pbase_tree(&tree, NULL);
-	free(elem);
+
+	for (it = pbase_tree; it; it = it->next) {
+		if (!memcmp(it->pcache.sha1, tree_sha1, 20)) {
+			free(data);
+			return;
+		}
+	}
+
+	it = xcalloc(1, sizeof(*it));
+	it->next = pbase_tree;
+	pbase_tree = it;
+
+	memcpy(it->pcache.sha1, tree_sha1, 20);
+	it->pcache.tree_data = data;
+	it->pcache.tree_size = size;
 }
 
 static void check_object(struct object_entry *entry)
@@ -1051,6 +1234,7 @@ int main(int argc, char **argv)
 	char line[PATH_MAX + 20];
 	int window = 10, depth = 10, pack_to_stdout = 0;
 	struct object_entry **list;
+	int num_preferred_base = 0;
 	int i;
 
 	setup_git_directory();
@@ -1116,6 +1300,7 @@ int main(int argc, char **argv)
 
 	for (;;) {
 		unsigned char sha1[20];
+		unsigned hash;
 
 		if (!fgets(line, sizeof(line), stdin)) {
 			if (feof(stdin))
@@ -1132,12 +1317,15 @@ int main(int argc, char **argv)
 			if (get_sha1_hex(line+1, sha1))
 				die("expected edge sha1, got garbage:\n %s",
 				    line+1);
-			add_preferred_base(sha1);
+			if (num_preferred_base++ < window)
+				add_preferred_base(sha1);
 			continue;
 		}
 		if (get_sha1_hex(line, sha1))
 			die("expected sha1, got garbage:\n %s", line);
-		add_object_entry(sha1, name_hash(NULL, line+41), 0);
+		hash = name_hash(NULL, line+41);
+		add_preferred_base_object(line+41, hash);
+		add_object_entry(sha1, hash, 0);
 	}
 	if (progress)
 		fprintf(stderr, "Done counting %d objects.\n", nr_objects);