Merge branch 'ps/reftable-iteration-perf-part2'

The code to iterate over refs with the reftable backend has seen
some optimization.

* ps/reftable-iteration-perf-part2:
  refs/reftable: precompute prefix length
  reftable: allow inlining of a few functions
  reftable/record: decode keys in place
  reftable/record: reuse refname when copying
  reftable/record: reuse refname when decoding
  reftable/merged: avoid duplicate pqueue emptiness check
  reftable/merged: circumvent pqueue with single subiter
  reftable/merged: handle subiter cleanup on close only
  reftable/merged: remove unnecessary null check for subiters
  reftable/merged: make subiters own their records
  reftable/merged: advance subiter on subsequent iteration
  reftable/merged: make `merged_iter` structure private
  reftable/pq: use `size_t` to track iterator index

Author: gitster

refs/reftable-backend.c

@@ -346,6 +346,7 @@ struct reftable_ref_iterator {
 	struct object_id oid;
 
 	const char *prefix;
+	size_t prefix_len;
 	unsigned int flags;
 	int err;
 };
@@ -374,8 +375,8 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 			continue;
 		}
 
-		if (iter->prefix &&
-		    strncmp(iter->prefix, iter->ref.refname, strlen(iter->prefix))) {
+		if (iter->prefix_len &&
+		    strncmp(iter->prefix, iter->ref.refname, iter->prefix_len)) {
 			iter->err = 1;
 			break;
 		}
@@ -484,6 +485,7 @@ static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_
 	iter = xcalloc(1, sizeof(*iter));
 	base_ref_iterator_init(&iter->base, &reftable_ref_iterator_vtable);
 	iter->prefix = prefix;
+	iter->prefix_len = prefix ? strlen(prefix) : 0;
 	iter->base.oid = &iter->oid;
 	iter->flags = flags;
 	iter->refs = refs;

reftable/block.c

@@ -291,9 +291,8 @@ static int restart_key_less(size_t idx, void *args)
 	/* the restart key is verbatim in the block, so this could avoid the
 	   alloc for decoding the key */
 	struct strbuf rkey = STRBUF_INIT;
-	struct strbuf last_key = STRBUF_INIT;
 	uint8_t unused_extra;
-	int n = reftable_decode_key(&rkey, &unused_extra, last_key, in);
+	int n = reftable_decode_key(&rkey, &unused_extra, in);
 	int result;
 	if (n < 0) {
 		a->error = 1;
@@ -326,35 +325,34 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 	if (it->next_off >= it->br->block_len)
 		return 1;
 
-	n = reftable_decode_key(&it->key, &extra, it->last_key, in);
+	n = reftable_decode_key(&it->last_key, &extra, in);
 	if (n < 0)
 		return -1;
-
-	if (!it->key.len)
+	if (!it->last_key.len)
 		return REFTABLE_FORMAT_ERROR;
 
 	string_view_consume(&in, n);
-	n = reftable_record_decode(rec, it->key, extra, in, it->br->hash_size);
+	n = reftable_record_decode(rec, it->last_key, extra, in, it->br->hash_size);
 	if (n < 0)
 		return -1;
 	string_view_consume(&in, n);
 
-	strbuf_swap(&it->last_key, &it->key);
 	it->next_off += start.len - in.len;
 	return 0;
 }
 
 int block_reader_first_key(struct block_reader *br, struct strbuf *key)
 {
-	struct strbuf empty = STRBUF_INIT;
-	int off = br->header_off + 4;
+	int off = br->header_off + 4, n;
 	struct string_view in = {
 		.buf = br->block.data + off,
 		.len = br->block_len - off,
 	};
-
 	uint8_t extra = 0;
-	int n = reftable_decode_key(key, &extra, empty, in);
+
+	strbuf_reset(key);
+
+	n = reftable_decode_key(key, &extra, in);
 	if (n < 0)
 		return n;
 	if (!key->len)
@@ -371,7 +369,6 @@ int block_iter_seek(struct block_iter *it, struct strbuf *want)
 void block_iter_close(struct block_iter *it)
 {
 	strbuf_release(&it->last_key);
-	strbuf_release(&it->key);
 }
 
 int block_reader_seek(struct block_reader *br, struct block_iter *it,
@@ -408,8 +405,8 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
 		if (err < 0)
 			goto done;
 
-		reftable_record_key(&rec, &it->key);
-		if (err > 0 || strbuf_cmp(&it->key, want) >= 0) {
+		reftable_record_key(&rec, &it->last_key);
+		if (err > 0 || strbuf_cmp(&it->last_key, want) >= 0) {
 			err = 0;
 			goto done;
 		}

reftable/block.h

@@ -84,12 +84,10 @@ struct block_iter {
 
 	/* key for last entry we read. */
 	struct strbuf last_key;
-	struct strbuf key;
 };
 
 #define BLOCK_ITER_INIT { \
 	.last_key = STRBUF_INIT, \
-	.key = STRBUF_INIT, \
 }
 
 /* initializes a block reader. */

reftable/iter.c

@@ -16,11 +16,6 @@ license that can be found in the LICENSE file or at
 #include "reader.h"
 #include "reftable-error.h"
 
-int iterator_is_null(struct reftable_iterator *it)
-{
-	return !it->ops;
-}
-
 static void filtering_ref_iterator_close(void *iter_arg)
 {
 	struct filtering_ref_iterator *fri = iter_arg;

reftable/iter.h

@@ -16,10 +16,6 @@ license that can be found in the LICENSE file or at
 #include "reftable-iterator.h"
 #include "reftable-generic.h"
 
-/* Returns true for a zeroed out iterator, such as the one returned from
- * iterator_destroy. */
-int iterator_is_null(struct reftable_iterator *it);
-
 /* iterator that produces only ref records that point to `oid` */
 struct filtering_ref_iterator {
 	int double_check;

reftable/merged.c

@@ -17,23 +17,37 @@ license that can be found in the LICENSE file or at
 #include "reftable-error.h"
 #include "system.h"
 
+struct merged_subiter {
+	struct reftable_iterator iter;
+	struct reftable_record rec;
+};
+
+struct merged_iter {
+	struct merged_subiter *subiters;
+	struct merged_iter_pqueue pq;
+	uint32_t hash_id;
+	size_t stack_len;
+	uint8_t typ;
+	int suppress_deletions;
+	ssize_t advance_index;
+};
+
 static int merged_iter_init(struct merged_iter *mi)
 {
 	for (size_t i = 0; i < mi->stack_len; i++) {
 		struct pq_entry e = {
 			.index = i,
+			.rec = &mi->subiters[i].rec,
 		};
 		int err;
 
-		reftable_record_init(&e.rec, mi->typ);
-		err = iterator_next(&mi->stack[i], &e.rec);
+		reftable_record_init(&mi->subiters[i].rec, mi->typ);
+		err = iterator_next(&mi->subiters[i].iter,
+				    &mi->subiters[i].rec);
 		if (err < 0)
 			return err;
-		if (err > 0) {
-			reftable_iterator_destroy(&mi->stack[i]);
-			reftable_record_release(&e.rec);
+		if (err > 0)
 			continue;
-		}
 
 		merged_iter_pqueue_add(&mi->pq, &e);
 	}
@@ -46,54 +60,66 @@ static void merged_iter_close(void *p)
 	struct merged_iter *mi = p;
 
 	merged_iter_pqueue_release(&mi->pq);
-	for (size_t i = 0; i < mi->stack_len; i++)
-		reftable_iterator_destroy(&mi->stack[i]);
-	reftable_free(mi->stack);
+	for (size_t i = 0; i < mi->stack_len; i++) {
+		reftable_iterator_destroy(&mi->subiters[i].iter);
+		reftable_record_release(&mi->subiters[i].rec);
+	}
+	reftable_free(mi->subiters);
 }
 
-static int merged_iter_advance_nonnull_subiter(struct merged_iter *mi,
-					       size_t idx)
+static int merged_iter_advance_subiter(struct merged_iter *mi, size_t idx)
 {
 	struct pq_entry e = {
 		.index = idx,
+		.rec = &mi->subiters[idx].rec,
 	};
 	int err;
 
-	reftable_record_init(&e.rec, mi->typ);
-	err = iterator_next(&mi->stack[idx], &e.rec);
-	if (err < 0)
+	err = iterator_next(&mi->subiters[idx].iter, &mi->subiters[idx].rec);
+	if (err)
 		return err;
 
-	if (err > 0) {
-		reftable_iterator_destroy(&mi->stack[idx]);
-		reftable_record_release(&e.rec);
-		return 0;
-	}
-
 	merged_iter_pqueue_add(&mi->pq, &e);
 	return 0;
 }
 
-static int merged_iter_advance_subiter(struct merged_iter *mi, size_t idx)
-{
-	if (iterator_is_null(&mi->stack[idx]))
-		return 0;
-	return merged_iter_advance_nonnull_subiter(mi, idx);
-}
-
 static int merged_iter_next_entry(struct merged_iter *mi,
 				  struct reftable_record *rec)
 {
 	struct pq_entry entry = { 0 };
-	int err = 0;
+	int err = 0, empty;
+
+	empty = merged_iter_pqueue_is_empty(mi->pq);
+
+	if (mi->advance_index >= 0) {
+		/*
+		 * When there are no pqueue entries then we only have a single
+		 * subiter left. There is no need to use the pqueue in that
+		 * case anymore as we know that the subiter will return entries
+		 * in the correct order already.
+		 *
+		 * While this may sound like a very specific edge case, it may
+		 * happen more frequently than you think. Most repositories
+		 * will end up having a single large base table that contains
+		 * most of the refs. It's thus likely that we exhaust all
+		 * subiters but the one from that base ref.
+		 */
+		if (empty)
+			return iterator_next(&mi->subiters[mi->advance_index].iter,
+					     rec);
+
+		err = merged_iter_advance_subiter(mi, mi->advance_index);
+		if (err < 0)
+			return err;
+		if (!err)
+			empty = 0;
+		mi->advance_index = -1;
+	}
 
-	if (merged_iter_pqueue_is_empty(mi->pq))
+	if (empty)
 		return 1;
 
 	entry = merged_iter_pqueue_remove(&mi->pq);
-	err = merged_iter_advance_subiter(mi, entry.index);
-	if (err < 0)
-		return err;
 
 	/*
 	  One can also use reftable as datacenter-local storage, where the ref
@@ -107,56 +133,34 @@ static int merged_iter_next_entry(struct merged_iter *mi,
 		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
 		int cmp;
 
-		/*
-		 * When the next entry comes from the same queue as the current
-		 * entry then it must by definition be larger. This avoids a
-		 * comparison in the most common case.
-		 */
-		if (top.index == entry.index)
-			break;
-
-		cmp = reftable_record_cmp(&top.rec, &entry.rec);
+		cmp = reftable_record_cmp(top.rec, entry.rec);
 		if (cmp > 0)
 			break;
 
 		merged_iter_pqueue_remove(&mi->pq);
 		err = merged_iter_advance_subiter(mi, top.index);
 		if (err < 0)
-			goto done;
-		reftable_record_release(&top.rec);
+			return err;
 	}
 
-	reftable_record_release(rec);
-	*rec = entry.rec;
-
-done:
-	if (err)
-		reftable_record_release(&entry.rec);
-	return err;
+	mi->advance_index = entry.index;
+	SWAP(*rec, *entry.rec);
+	return 0;
 }
 
-static int merged_iter_next(struct merged_iter *mi, struct reftable_record *rec)
+static int merged_iter_next_void(void *p, struct reftable_record *rec)
 {
+	struct merged_iter *mi = p;
 	while (1) {
 		int err = merged_iter_next_entry(mi, rec);
-		if (err == 0 && mi->suppress_deletions &&
-		    reftable_record_is_deletion(rec)) {
+		if (err)
+			return err;
+		if (mi->suppress_deletions && reftable_record_is_deletion(rec))
 			continue;
-		}
-
-		return err;
+		return 0;
 	}
 }
 
-static int merged_iter_next_void(void *p, struct reftable_record *rec)
-{
-	struct merged_iter *mi = p;
-	if (merged_iter_pqueue_is_empty(mi->pq))
-		return 1;
-
-	return merged_iter_next(mi, rec);
-}
-
 static struct reftable_iterator_vtable merged_iter_vtable = {
 	.next = &merged_iter_next_void,
 	.close = &merged_iter_close,
@@ -246,14 +250,15 @@ static int merged_table_seek_record(struct reftable_merged_table *mt,
 		.typ = reftable_record_type(rec),
 		.hash_id = mt->hash_id,
 		.suppress_deletions = mt->suppress_deletions,
+		.advance_index = -1,
 	};
 	struct merged_iter *p;
 	int err;
 
-	REFTABLE_CALLOC_ARRAY(merged.stack, mt->stack_len);
+	REFTABLE_CALLOC_ARRAY(merged.subiters, mt->stack_len);
 	for (size_t i = 0; i < mt->stack_len; i++) {
 		err = reftable_table_seek_record(&mt->stack[i],
-						 &merged.stack[merged.stack_len], rec);
+						 &merged.subiters[merged.stack_len].iter, rec);
 		if (err < 0)
 			goto out;
 		if (!err)

reftable/merged.h

@@ -9,7 +9,7 @@ license that can be found in the LICENSE file or at
 #ifndef MERGED_H
 #define MERGED_H
 
-#include "pq.h"
+#include "system.h"
 
 struct reftable_merged_table {
 	struct reftable_table *stack;
@@ -24,15 +24,6 @@ struct reftable_merged_table {
 	uint64_t max;
 };
 
-struct merged_iter {
-	struct reftable_iterator *stack;
-	uint32_t hash_id;
-	size_t stack_len;
-	uint8_t typ;
-	int suppress_deletions;
-	struct merged_iter_pqueue pq;
-};
-
 void merged_table_release(struct reftable_merged_table *mt);
 
 #endif