[RFC PATCH v2 2/7] object-file: pass down unpack-objects.c flags for "bulk" checkin

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Mar 23, 2022, 14:18 UTC
Message-ID: <RFC-patch-v2-2.7-c6f776fc2bc-20220323T140753Z-avarab@gmail.com>
In-Reply-To: <RFC-cover-v2-0.7-00000000000-20220323T140753Z-avarab@gmail.com>

Remove much of this as a POC for exploring some of what I mentioned in https://lore.kernel.org/git/220322.86mthinxnn.gmgdl@evledraar.gmail.com/

This commit is obviously not what we *should* do as end-state, but demonstrates what's needed (I think) for a bare-minimum implementation of just the "bulk" syncing method for loose objects without the part where we do the tmp-objdir.c dance.

Performance with this is already quite promising. Benchmarking with:

	git hyperfine -L rev ns/batched-fsync,HEAD -s 'make CFLAGS=-O3' \
	    	-p 'rm -rf r.git && git init --bare r.git' \
		'./git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack'

I.e. unpacking a small packfile (my dotfiles) yields, on a Linux ramdisk:

	Benchmark 1: ./git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'ns/batched-fsync
	  Time (mean ± σ):     815.9 ms ±   8.2 ms    [User: 522.9 ms, System: 287.9 ms]
	  Range (min … max):   805.6 ms … 835.9 ms    10 runs

	Benchmark 2: ./git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'HEAD
	  Time (mean ± σ):     779.4 ms ±  15.4 ms    [User: 505.7 ms, System: 270.2 ms]
	  Range (min … max):   763.1 ms … 813.9 ms    10 runs

	Summary
	  './git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'HEAD' ran
	    1.05 ± 0.02 times faster than './git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'ns/batched-fsync'

Doing the same with "strace --summary-only", which probably helps to emulate cases with slower syscalls is ~15% faster than using the tmp-objdir indirection:

	Summary
	  'strace --summary-only ./git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'HEAD' ran
	    1.16 ± 0.01 times faster than 'strace --summary-only ./git -C r.git -c core.fsync=loose-object -c core.fsyncMethod=batch unpack-objects </tmp/pack-dotfiles.pack' in 'ns/batched-fsync'

Which makes sense in terms of syscalls. In my case HEAD has ~101k calls, and the parent topic is making ~129k calls, with around 2x the number of unlink(), link() as expected.

Of course some users will want to use the tmp-objdir.c method. So a version of this commit could be rewritten to come earlier in the series, with the "bulk" on top being optional.

It seems to me that it's a much better strategy to do this whole thing in close_loose_object() after passing down the new HASH_N_OBJECTS / HASH_N_OBJECTS_FIRST / HASH_N_OBJECTS_LAST flags.

Doing that for the "builtin/add.c" and "builtin/unpack-objects.c" code having its {un,}plug_bulk_checkin() removed here is then just a matter of passing down a similar set of flags indicating whether we're dealing with N objects, and if so if we're dealing with the last one or not.

As we'll see in subsequent commits doing it this way also effortlessly integrates with other HASH_* flags. E.g. for "update-index" the code being rm'd here doesn't handle the interaction with "HASH_WRITE_OBJECT" properly, but once we've moved all this sync bootstrapping logic to close_loose_object() we'll never get to it if we're not actually writing something.

This code currently doesn't use the HASH_N_OBJECTS_FIRST flag, but that's what we'd use later to optionally call tmp_objdir_create().

Aside: This also changes logic that was a bit confusing and repetitive
in close_loose_object(). Previously we'd first call
batch_fsync_enabled(FSYNC_COMPONENT_LOOSE_OBJECT) which is just as
shorthand for:

	fsync_components & FSYNC_COMPONENT_LOOSE_OBJECT &&
	fsync_method == FSYNC_METHOD_BATCH

We'd then proceed to call fsync_component_or_die(FSYNC_COMPONENT_LOOSE_OBJECT) later in the same function, which is just a way of calling fsync_or_die() if:

	fsync_components & FSYNC_COMPONENT_LOOSE_OBJECT

Now we instead just define a local "fsync_loose" variable by checking "fsync_components & FSYNC_COMPONENT_LOOSE_OBJECT", which shows us that the previous case of fsync_component_or_die(...)" could just be added to the existing "fsync_object_files > 0" branch.

Note: This commit reverts much of "core.fsyncmethod: batched disk
flushes for loose-objects". We'll set up new structures to bring what
it was doing back in a different way. I.e. to do the tmp-objdir
plug-in in object-file.c

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/unpack-objects.c |  2 --
 builtin/update-index.c   |  4 ---
 bulk-checkin.c           | 74 ----------------------------------------
 bulk-checkin.h           |  3 --
 cache.h                  |  5 ---
 object-file.c            | 37 ++++++++++++++------
 6 files changed, 26 insertions(+), 99 deletions(-)

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index ec40c6fd966..93da436581b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -507,7 +507,6 @@ static void unpack_all(void)
 	if (!quiet)
 		progress = start_progress(_("Unpacking objects"), nr_objects);
 	CALLOC_ARRAY(obj_list, nr_objects);
-	plug_bulk_checkin();
 	oflags = nr_objects > 1 ? HASH_N_OBJECTS : 0;
 	for (i = 0; i < nr_objects; i++) {
 		int nth = i + 1;
@@ -517,7 +516,6 @@ static void unpack_all(void)
 		unpack_one(i, oflags | f);
 		display_progress(progress, nth);
 	}
-	unplug_bulk_checkin();
 	stop_progress(&progress);
 
 	if (delta_list)
diff --git a/builtin/update-index.c b/builtin/update-index.c
index cbd2b0d633b..95ed3c47b2e 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -1118,8 +1118,6 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	parse_options_start(&ctx, argc, argv, prefix,
 			    options, PARSE_OPT_STOP_AT_NON_OPTION);
 
-	/* optimize adding many objects to the object database */
-	plug_bulk_checkin();
 	while (ctx.argc) {
 		if (parseopt_state != PARSE_OPT_DONE)
 			parseopt_state = parse_options_step(&ctx, options,
@@ -1194,8 +1192,6 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		strbuf_release(&buf);
 	}
 
-	/* by now we must have added all of the new objects */
-	unplug_bulk_checkin();
 	if (split_index > 0) {
 		if (git_config_get_split_index() == 0)
 			warning(_("core.splitIndex is set to false; "
diff --git a/bulk-checkin.c b/bulk-checkin.c
index a0dca79ba6a..577b135e39c 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -3,20 +3,15 @@
  */
 #include "cache.h"
 #include "bulk-checkin.h"
-#include "lockfile.h"
 #include "repository.h"
 #include "csum-file.h"
 #include "pack.h"
 #include "strbuf.h"
-#include "string-list.h"
-#include "tmp-objdir.h"
 #include "packfile.h"
 #include "object-store.h"
 
 static int bulk_checkin_plugged;
 
-static struct tmp_objdir *bulk_fsync_objdir;
-
 static struct bulk_checkin_state {
 	char *pack_tmp_name;
 	struct hashfile *f;
@@ -85,40 +80,6 @@ static void finish_bulk_checkin(struct bulk_checkin_state *state)
 	reprepare_packed_git(the_repository);
 }
 
-/*
- * Cleanup after batch-mode fsync_object_files.
- */
-static void do_batch_fsync(void)
-{
-	struct strbuf temp_path = STRBUF_INIT;
-	struct tempfile *temp;
-
-	if (!bulk_fsync_objdir)
-		return;
-
-	/*
-	 * Issue a full hardware flush against a temporary file to ensure
-	 * that all objects are durable before any renames occur. The code in
-	 * fsync_loose_object_bulk_checkin has already issued a writeout
-	 * request, but it has not flushed any writeback cache in the storage
-	 * hardware or any filesystem logs. This fsync call acts as a barrier
-	 * to ensure that the data in each new object file is durable before
-	 * the final name is visible.
-	 */
-	strbuf_addf(&temp_path, "%s/bulk_fsync_XXXXXX", get_object_directory());
-	temp = xmks_tempfile(temp_path.buf);
-	fsync_or_die(get_tempfile_fd(temp), get_tempfile_path(temp));
-	delete_tempfile(&temp);
-	strbuf_release(&temp_path);
-
-	/*
-	 * Make the object files visible in the primary ODB after their data is
-	 * fully durable.
-	 */
-	tmp_objdir_migrate(bulk_fsync_objdir);
-	bulk_fsync_objdir = NULL;
-}
-
 static int already_written(struct bulk_checkin_state *state, struct object_id *oid)
 {
 	int i;
@@ -313,26 +274,6 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
 	return 0;
 }
 
-void prepare_loose_object_bulk_checkin(void)
-{
-	if (bulk_checkin_plugged && !bulk_fsync_objdir)
-		bulk_fsync_objdir = tmp_objdir_create("bulk-fsync");
-}
-
-void fsync_loose_object_bulk_checkin(int fd, const char *filename)
-{
-	/*
-	 * If we have a plugged bulk checkin, we issue a call that
-	 * cleans the filesystem page cache but avoids a hardware flush
-	 * command. Later on we will issue a single hardware flush
-	 * before as part of do_batch_fsync.
-	 */
-	if (!bulk_fsync_objdir ||
-	    git_fsync(fd, FSYNC_WRITEOUT_ONLY) < 0) {
-		fsync_or_die(fd, filename);
-	}
-}
-
 int index_bulk_checkin(struct object_id *oid,
 		       int fd, size_t size, enum object_type type,
 		       const char *path, unsigned flags)
@@ -347,19 +288,6 @@ int index_bulk_checkin(struct object_id *oid,
 void plug_bulk_checkin(void)
 {
 	assert(!bulk_checkin_plugged);
-
-	/*
-	 * A temporary object directory is used to hold the files
-	 * while they are not fsynced.
-	 */
-	if (batch_fsync_enabled(FSYNC_COMPONENT_LOOSE_OBJECT)) {
-		bulk_fsync_objdir = tmp_objdir_create("bulk-fsync");
-		if (!bulk_fsync_objdir)
-			die(_("Could not create temporary object directory for core.fsyncMethod=batch"));
-
-		tmp_objdir_replace_primary_odb(bulk_fsync_objdir, 0);
-	}
-
 	bulk_checkin_plugged = 1;
 }
 
@@ -369,6 +297,4 @@ void unplug_bulk_checkin(void)
 	bulk_checkin_plugged = 0;
 	if (bulk_checkin_state.f)
 		finish_bulk_checkin(&bulk_checkin_state);
-
-	do_batch_fsync();
 }
diff --git a/bulk-checkin.h b/bulk-checkin.h
index 181d3447ff9..b26f3dc3b74 100644
--- a/bulk-checkin.h
+++ b/bulk-checkin.h
@@ -6,9 +6,6 @@
 
 #include "cache.h"
 
-void prepare_loose_object_bulk_checkin(void);
-void fsync_loose_object_bulk_checkin(int fd, const char *filename);
-
 int index_bulk_checkin(struct object_id *oid,
 		       int fd, size_t size, enum object_type type,
 		       const char *path, unsigned flags);
diff --git a/cache.h b/cache.h
index 72c91c91286..2f3831fa853 100644
--- a/cache.h
+++ b/cache.h
@@ -1772,11 +1772,6 @@ void fsync_or_die(int fd, const char *);
 int fsync_component(enum fsync_component component, int fd);
 void fsync_component_or_die(enum fsync_component component, int fd, const char *msg);
 
-static inline int batch_fsync_enabled(enum fsync_component component)
-{
-	return (fsync_components & component) && (fsync_method == FSYNC_METHOD_BATCH);
-}
-
 ssize_t read_in_full(int fd, void *buf, size_t count);
 ssize_t write_in_full(int fd, const void *buf, size_t count);
 ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset);
diff --git a/object-file.c b/object-file.c
index cd0ddb49e4b..dbeb3df502d 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1886,19 +1886,37 @@ void hash_object_file(const struct git_hash_algo *algo, const void *buf,
 	hash_object_file_literally(algo, buf, len, type_name(type), oid);
 }
 
+static void sync_loose_object_batch(int fd, const char *filename,
+				    const unsigned oflags)
+{
+	const int last = oflags & HASH_N_OBJECTS_LAST;
+
+	/*
+	 * We're doing a sync_file_range() (or equivalent) for 1..N-1
+	 * objects, and then a "real" fsync() for N. On some OS's
+	 * enabling core.fsync=loose-object && core.fsyncMethod=batch
+	 * improves the performance by a lot.
+	 */
+	if (last || (!last && git_fsync(fd, FSYNC_WRITEOUT_ONLY) < 0))
+		fsync_or_die(fd, filename);
+}
+
 /* Finalize a file on disk, and close it. */
-static void close_loose_object(int fd, const char *filename)
+static void close_loose_object(int fd, const char *filename,
+			       const unsigned oflags)
 {
+	int fsync_loose;
+
 	if (the_repository->objects->odb->will_destroy)
 		goto out;
 
-	if (batch_fsync_enabled(FSYNC_COMPONENT_LOOSE_OBJECT))
-		fsync_loose_object_bulk_checkin(fd, filename);
-	else if (fsync_object_files > 0)
+	fsync_loose = fsync_components & FSYNC_COMPONENT_LOOSE_OBJECT;
+
+	if (oflags & HASH_N_OBJECTS && fsync_loose &&
+	    fsync_method == FSYNC_METHOD_BATCH)
+		sync_loose_object_batch(fd, filename, oflags);
+	else if (fsync_object_files > 0 || fsync_loose)
 		fsync_or_die(fd, filename);
-	else
-		fsync_component_or_die(FSYNC_COMPONENT_LOOSE_OBJECT, fd,
-				       filename);
 
 out:
 	if (close(fd) != 0)
@@ -1962,9 +1980,6 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 	static struct strbuf tmp_file = STRBUF_INIT;
 	static struct strbuf filename = STRBUF_INIT;
 
-	if (batch_fsync_enabled(FSYNC_COMPONENT_LOOSE_OBJECT))
-		prepare_loose_object_bulk_checkin();
-
 	loose_object_path(the_repository, &filename, oid);
 
 	fd = create_tmpfile(&tmp_file, filename.buf);
@@ -2015,7 +2030,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 		die(_("confused by unstable object source data for %s"),
 		    oid_to_hex(oid));
 
-	close_loose_object(fd, tmp_file.buf);
+	close_loose_object(fd, tmp_file.buf, flags);
 
 	if (mtime) {
 		struct utimbuf utb;

-- 
2.35.1.1428.g1c1a0152d61