[PATCH 15/21] maintenance: auto-size pack-files batch

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Jul 7, 2020, 14:21 UTC
Message-ID: <8be89707d2d5a8b527e104a2aeadb5ea7f65d476.1594131695.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.671.git.1594131695.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

When repacking during the 'pack-files' job, we use the --batch-size option in 'git multi-pack-index repack'. The initial setting used --batch-size=0 to repack everything into a single pack-file. This is not sustaintable for a large repository. The amount of work required is also likely to use too many system resources for a background job.

Update the 'pack-files' maintenance task by dynamically computing a --batch-size option based on the current pack-file structure.

The dynamic default size is computed with this idea in mind for a client repository that was cloned from a very large remote: there is likely one "big" pack-file that was created at clone time. Thus, do not try repacking it as it is likely packed efficiently by the server.

Instead, we select the second-largest pack-file, and create a batch size that is one larger than that pack-file. If there are three or more pack-files, then this guarantees that at least two will be combined into a new pack-file.

Of course, this means that the second-largest pack-file size is likely to grow over time and may eventually surpass the initially-cloned pack-file. Recall that the pack-file batch is selected in a greedy manner: the packs are considered from oldest to newest and are selected if they have size smaller than the batch size until the total selected size is larger than the batch size. Thus, that oldest "clone" pack will be first to repack after the new data creates a pack larger than that.

We also want to place some limits on how large these pack-files become, in order to bound the amount of time spent repacking. A maximum batch-size of two gigabytes means that large repositories will never be packed into a single pack-file using this job, but also that repack is rather expensive. This is a trade-off that is valuable to have if the maintenance is being run automatically or in the background. Users who truly want to optimize for space and performance (and are willing to pay the upfront cost of a full repack) can use the 'gc' task to do so.

Reported-by: Son Luong Ngoc <sluongng@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 builtin/gc.c           | 47 +++++++++++++++++++++++++++++++++++++++++-
 t/t7900-maintenance.sh |  5 +++--
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 259b0475c0..582219156a 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -1032,20 +1032,65 @@ static int multi_pack_index_expire(struct repository *r)
 	return result;
 }
 
+#define TWO_GIGABYTES (2147483647)
+#define UNSET_BATCH_SIZE ((unsigned long)-1)
+
+static off_t get_auto_pack_size(struct repository *r)
+{
+	/*
+	 * The "auto" value is special: we optimize for
+	 * one large pack-file (i.e. from a clone) and
+	 * expect the rest to be small and they can be
+	 * repacked quickly.
+	 *
+	 * The strategy we select here is to select a
+	 * size that is one more than the second largest
+	 * pack-file. This ensures that we will repack
+	 * at least two packs if there are three or more
+	 * packs.
+	 */
+	off_t max_size = 0;
+	off_t second_largest_size = 0;
+	off_t result_size;
+	struct packed_git *p;
+
+	reprepare_packed_git(r);
+	for (p = get_all_packs(r); p; p = p->next) {
+		if (p->pack_size > max_size) {
+			second_largest_size = max_size;
+			max_size = p->pack_size;
+		} else if (p->pack_size > second_largest_size)
+			second_largest_size = p->pack_size;
+	}
+
+	result_size = second_largest_size + 1;
+
+	/* But limit ourselves to a batch size of 2g */
+	if (result_size > TWO_GIGABYTES)
+		result_size = TWO_GIGABYTES;
+
+	return result_size;
+}
+
 static int multi_pack_index_repack(struct repository *r)
 {
 	int result;
 	struct argv_array cmd = ARGV_ARRAY_INIT;
+	struct strbuf batch_arg = STRBUF_INIT;
+
 	argv_array_pushl(&cmd, "-C", r->worktree,
 			 "multi-pack-index", "repack", NULL);
 
 	if (opts.quiet)
 		argv_array_push(&cmd, "--no-progress");
 
-	argv_array_push(&cmd, "--batch-size=0");
+	strbuf_addf(&batch_arg, "--batch-size=%"PRIuMAX,
+			    (uintmax_t)get_auto_pack_size(r));
+	argv_array_push(&cmd, batch_arg.buf);
 
 	close_object_store(r->objects);
 	result = run_command_v_opt(cmd.argv, RUN_GIT_CMD);
+	strbuf_release(&batch_arg);
 
 	if (result && multi_pack_index_verify(r)) {
 		warning(_("multi-pack-index verify failed after repack"));
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index a6be8456aa..43d32c131b 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -134,10 +134,11 @@ test_expect_success 'pack-files task' '
 	test_line_count = 4 packs-between &&
 
 	# the job deletes the two old packs, and does not write
-	# a new one because only one pack remains.
+	# a new one because the batch size is not high enough to
+	# pack the largest pack-file.
 	git maintenance run --task=pack-files &&
 	ls .git/objects/pack/*.pack >packs-after &&
-	test_line_count = 1 packs-after
+	test_line_count = 2 packs-after
 '
 
 test_done

-- 
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