Merge branch 'jt/reftable-geometric-compaction'

The strategy to compact multiple tables of reftables after many
operations accumulate many entries has been improved to avoid
accumulating too many tables uncollected.

* jt/reftable-geometric-compaction:
  reftable/stack: use geometric table compaction
  reftable/stack: add env to disable autocompaction
  reftable/stack: expose option to disable auto-compaction

Author: gitster

refs/reftable-backend.c

@@ -18,6 +18,7 @@
 #include "../reftable/reftable-merged.h"
 #include "../setup.h"
 #include "../strmap.h"
+#include "parse.h"
 #include "refs-internal.h"
 
 /*
@@ -247,6 +248,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	refs->write_options.block_size = 4096;
 	refs->write_options.hash_id = repo->hash_algo->format_id;
 	refs->write_options.default_permissions = calc_shared_perm(0666 & ~mask);
+	refs->write_options.disable_auto_compact =
+		!git_env_bool("GIT_TEST_REFTABLE_AUTOCOMPACTION", 1);
 
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.

reftable/reftable-writer.h

@@ -46,6 +46,9 @@ struct reftable_write_options {
 	 *   is a single line, and add '\n' if missing.
 	 */
 	unsigned exact_log_message : 1;
+
+	/* boolean: Prevent auto-compaction of tables. */
+	unsigned disable_auto_compact : 1;
 };
 
 /* reftable_block_stats holds statistics for a single block type */

reftable/stack.c

@@ -680,7 +680,7 @@ int reftable_addition_commit(struct reftable_addition *add)
 	if (err)
 		goto done;
 
-	if (!add->stack->disable_auto_compact) {
+	if (!add->stack->config.disable_auto_compact) {
 		/*
 		 * Auto-compact the stack to keep the number of tables in
 		 * control. It is possible that a concurrent writer is already
@@ -1216,75 +1216,76 @@ static int segment_size(struct segment *s)
 	return s->end - s->start;
 }
 
-int fastlog2(uint64_t sz)
-{
-	int l = 0;
-	if (sz == 0)
-		return 0;
-	for (; sz; sz /= 2) {
-		l++;
-	}
-	return l - 1;
-}
-
-struct segment *sizes_to_segments(size_t *seglen, uint64_t *sizes, size_t n)
-{
-	struct segment *segs = reftable_calloc(n, sizeof(*segs));
-	struct segment cur = { 0 };
-	size_t next = 0, i;
-
-	if (n == 0) {
-		*seglen = 0;
-		return segs;
-	}
-	for (i = 0; i < n; i++) {
-		int log = fastlog2(sizes[i]);
-		if (cur.log != log && cur.bytes > 0) {
-			struct segment fresh = {
-				.start = i,
-			};
-
-			segs[next++] = cur;
-			cur = fresh;
-		}
-
-		cur.log = log;
-		cur.end = i + 1;
-		cur.bytes += sizes[i];
-	}
-	segs[next++] = cur;
-	*seglen = next;
-	return segs;
-}
-
 struct segment suggest_compaction_segment(uint64_t *sizes, size_t n)
 {
-	struct segment min_seg = {
-		.log = 64,
-	};
-	struct segment *segs;
-	size_t seglen = 0, i;
-
-	segs = sizes_to_segments(&seglen, sizes, n);
-	for (i = 0; i < seglen; i++) {
-		if (segment_size(&segs[i]) == 1)
-			continue;
+	struct segment seg = { 0 };
+	uint64_t bytes;
+	size_t i;
 
-		if (segs[i].log < min_seg.log)
-			min_seg = segs[i];
-	}
+	/*
+	 * If there are no tables or only a single one then we don't have to
+	 * compact anything. The sequence is geometric by definition already.
+	 */
+	if (n <= 1)
+		return seg;
 
-	while (min_seg.start > 0) {
-		size_t prev = min_seg.start - 1;
-		if (fastlog2(min_seg.bytes) < fastlog2(sizes[prev]))
+	/*
+	 * Find the ending table of the compaction segment needed to restore the
+	 * geometric sequence. Note that the segment end is exclusive.
+	 *
+	 * To do so, we iterate backwards starting from the most recent table
+	 * until a valid segment end is found. If the preceding table is smaller
+	 * than the current table multiplied by the geometric factor (2), the
+	 * compaction segment end has been identified.
+	 *
+	 * Tables after the ending point are not added to the byte count because
+	 * they are already valid members of the geometric sequence. Due to the
+	 * properties of a geometric sequence, it is not possible for the sum of
+	 * these tables to exceed the value of the ending point table.
+	 *
+	 * Example table size sequence requiring no compaction:
+	 * 	64, 32, 16, 8, 4, 2, 1
+	 *
+	 * Example table size sequence where compaction segment end is set to
+	 * the last table. Since the segment end is exclusive, the last table is
+	 * excluded during subsequent compaction and the table with size 3 is
+	 * the final table included:
+	 * 	64, 32, 16, 8, 4, 3, 1
+	 */
+	for (i = n - 1; i > 0; i--) {
+		if (sizes[i - 1] < sizes[i] * 2) {
+			seg.end = i + 1;
+			bytes = sizes[i];
 			break;
+		}
+	}
 
-		min_seg.start = prev;
-		min_seg.bytes += sizes[prev];
+	/*
+	 * Find the starting table of the compaction segment by iterating
+	 * through the remaining tables and keeping track of the accumulated
+	 * size of all tables seen from the segment end table. The previous
+	 * table is compared to the accumulated size because the tables from the
+	 * segment end are merged backwards recursively.
+	 *
+	 * Note that we keep iterating even after we have found the first
+	 * starting point. This is because there may be tables in the stack
+	 * preceding that first starting point which violate the geometric
+	 * sequence.
+	 *
+	 * Example compaction segment start set to table with size 32:
+	 * 	128, 32, 16, 8, 4, 3, 1
+	 */
+	for (; i > 0; i--) {
+		uint64_t curr = bytes;
+		bytes += sizes[i - 1];
+
+		if (sizes[i - 1] < curr * 2) {
+			seg.start = i - 1;
+			seg.bytes = bytes;
+		}
 	}
 
-	reftable_free(segs);
-	return min_seg;
+	return seg;
 }
 
 static uint64_t *stack_table_sizes_for_compaction(struct reftable_stack *st)

reftable/stack.h

@@ -19,7 +19,6 @@ struct reftable_stack {
 	int list_fd;
 
 	char *reftable_dir;
-	int disable_auto_compact;
 
 	struct reftable_write_options config;
 
@@ -33,12 +32,9 @@ int read_lines(const char *filename, char ***lines);
 
 struct segment {
 	size_t start, end;
-	int log;
 	uint64_t bytes;
 };
 
-int fastlog2(uint64_t sz);
-struct segment *sizes_to_segments(size_t *seglen, uint64_t *sizes, size_t n);
 struct segment suggest_compaction_segment(uint64_t *sizes, size_t n);
 
 #endif

reftable/stack_test.c

@@ -325,7 +325,7 @@ static void test_reftable_stack_transaction_api_performs_auto_compaction(void)
 		 * we can ensure that we indeed honor this setting and have
 		 * better control over when exactly auto compaction runs.
 		 */
-		st->disable_auto_compact = i != n;
+		st->config.disable_auto_compact = i != n;
 
 		err = reftable_stack_new_addition(&add, st);
 		EXPECT_ERR(err);
@@ -497,6 +497,7 @@ static void test_reftable_stack_add(void)
 	struct reftable_write_options cfg = {
 		.exact_log_message = 1,
 		.default_permissions = 0660,
+		.disable_auto_compact = 1,
 	};
 	struct reftable_stack *st = NULL;
 	char *dir = get_tmp_dir(__LINE__);
@@ -508,7 +509,6 @@ static void test_reftable_stack_add(void)
 
 	err = reftable_new_stack(&st, dir, cfg);
 	EXPECT_ERR(err);
-	st->disable_auto_compact = 1;
 
 	for (i = 0; i < N; i++) {
 		char buf[256];
@@ -770,59 +770,13 @@ static void test_reftable_stack_hash_id(void)
 	clear_dir(dir);
 }
 
-static void test_log2(void)
-{
-	EXPECT(1 == fastlog2(3));
-	EXPECT(2 == fastlog2(4));
-	EXPECT(2 == fastlog2(5));
-}
-
-static void test_sizes_to_segments(void)
-{
-	uint64_t sizes[] = { 2, 3, 4, 5, 7, 9 };
-	/* .................0  1  2  3  4  5 */
-
-	size_t seglen = 0;
-	struct segment *segs =
-		sizes_to_segments(&seglen, sizes, ARRAY_SIZE(sizes));
-	EXPECT(segs[2].log == 3);
-	EXPECT(segs[2].start == 5);
-	EXPECT(segs[2].end == 6);
-
-	EXPECT(segs[1].log == 2);
-	EXPECT(segs[1].start == 2);
-	EXPECT(segs[1].end == 5);
-	reftable_free(segs);
-}
-
-static void test_sizes_to_segments_empty(void)
-{
-	size_t seglen = 0;
-	struct segment *segs = sizes_to_segments(&seglen, NULL, 0);
-	EXPECT(seglen == 0);
-	reftable_free(segs);
-}
-
-static void test_sizes_to_segments_all_equal(void)
-{
-	uint64_t sizes[] = { 5, 5 };
-	size_t seglen = 0;
-	struct segment *segs =
-		sizes_to_segments(&seglen, sizes, ARRAY_SIZE(sizes));
-	EXPECT(seglen == 1);
-	EXPECT(segs[0].start == 0);
-	EXPECT(segs[0].end == 2);
-	reftable_free(segs);
-}
-
 static void test_suggest_compaction_segment(void)
 {
-	uint64_t sizes[] = { 128, 64, 17, 16, 9, 9, 9, 16, 16 };
-	/* .................0    1    2  3   4  5  6 */
+	uint64_t sizes[] = { 512, 64, 17, 16, 9, 9, 9, 16, 2, 16 };
 	struct segment min =
 		suggest_compaction_segment(sizes, ARRAY_SIZE(sizes));
-	EXPECT(min.start == 2);
-	EXPECT(min.end == 7);
+	EXPECT(min.start == 1);
+	EXPECT(min.end == 10);
 }
 
 static void test_suggest_compaction_segment_nothing(void)
@@ -933,9 +887,21 @@ static void test_empty_add(void)
 	reftable_stack_destroy(st2);
 }
 
+static int fastlog2(uint64_t sz)
+{
+	int l = 0;
+	if (sz == 0)
+		return 0;
+	for (; sz; sz /= 2)
+		l++;
+	return l - 1;
+}
+
 static void test_reftable_stack_auto_compaction(void)
 {
-	struct reftable_write_options cfg = { 0 };
+	struct reftable_write_options cfg = {
+		.disable_auto_compact = 1,
+	};
 	struct reftable_stack *st = NULL;
 	char *dir = get_tmp_dir(__LINE__);
 
@@ -945,7 +911,6 @@ static void test_reftable_stack_auto_compaction(void)
 	err = reftable_new_stack(&st, dir, cfg);
 	EXPECT_ERR(err);
 
-	st->disable_auto_compact = 1; /* call manually below for coverage. */
 	for (i = 0; i < N; i++) {
 		char name[100];
 		struct reftable_ref_record ref = {
@@ -994,7 +959,7 @@ static void test_reftable_stack_add_performs_auto_compaction(void)
 		 * we can ensure that we indeed honor this setting and have
 		 * better control over when exactly auto compaction runs.
 		 */
-		st->disable_auto_compact = i != n;
+		st->config.disable_auto_compact = i != n;
 
 		strbuf_reset(&refname);
 		strbuf_addf(&refname, "branch-%04d", i);
@@ -1121,7 +1086,6 @@ static void test_reftable_stack_compaction_concurrent_clean(void)
 int stack_test_main(int argc, const char *argv[])
 {
 	RUN_TEST(test_empty_add);
-	RUN_TEST(test_log2);
 	RUN_TEST(test_names_equal);
 	RUN_TEST(test_parse_names);
 	RUN_TEST(test_read_file);
@@ -1142,9 +1106,6 @@ int stack_test_main(int argc, const char *argv[])
 	RUN_TEST(test_reftable_stack_update_index_check);
 	RUN_TEST(test_reftable_stack_uptodate);
 	RUN_TEST(test_reftable_stack_validate_refname);
-	RUN_TEST(test_sizes_to_segments);
-	RUN_TEST(test_sizes_to_segments_all_equal);
-	RUN_TEST(test_sizes_to_segments_empty);
 	RUN_TEST(test_suggest_compaction_segment);
 	RUN_TEST(test_suggest_compaction_segment_nothing);
 	return 0;