Re: [PATCH 2/2] core.fsyncobjectfiles: batch disk flushes

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Aug 25, 2021, 18:52 UTC
Message-ID: <nycvar.QRO.7.76.6.2108251551100.55@tvgsbejvaqbjf.bet>
In-Reply-To: <d1e68d4a2afc1d0ba74af64680bea09f412f21cc.1629856293.git.gitgitgadget@gmail.com>

Hi Neeraj,

continuing my review here, inlined.

On Wed, 25 Aug 2021, Neeraj Singh via GitGitGadget wrote:

Show 13 quoted lines

> From: Neeraj Singh <neerajsi@microsoft.com>
>
> When adding many objects to a repo with core.fsyncObjectFiles set to
> true, the cost of fsync'ing each object file can become prohibitive.
>
> One major source of the cost of fsync is the implied flush of the
> hardware writeback cache within the disk drive. Fortunately, Windows,
> MacOS, and Linux each offer mechanisms to write data from the filesystem
> page cache without initiating a hardware flush.
>
> This patch introduces a new 'core.fsyncObjectFiles = 2' option that
> takes advantage of the bulk-checkin infrastructure to batch up hardware
> flushes.

It makes sense, but I would recommend using a more easily explained value than `2`. Maybe `delayed`? Or `bulk` or `batched`?

The way this would be implemented would look somewhat like the implementation for `core.abbrev`, which also accepts a string ("auto") or a Boolean (or even an integral number), see https://github.com/git/git/blob/v2.33.0/config.c#L1367-L1381:

	if (!strcmp(var, "core.abbrev")) {
		if (!value)
			return config_error_nonbool(var);
		if (!strcasecmp(value, "auto"))
			default_abbrev = -1;
		else if (!git_parse_maybe_bool_text(value))
			default_abbrev = the_hash_algo->hexsz;
		else {
			int abbrev = git_config_int(var, value);
			if (abbrev < minimum_abbrev || abbrev > the_hash_algo->hexsz)
				return error(_("abbrev length out of range: %d"), abbrev);
			default_abbrev = abbrev;
		}
		return 0;
	}

Show 24 quoted lines

> When the new mode is enabled we do the following for new objects:
>
> 1. Create a tmp_obj_XXXX file and write the object data to it.
> 2. Issue a pagecache writeback request and wait for it to complete.
> 3. Record the tmp name and the final name in the bulk-checkin state for
>    later name.
>
> At the end of the entire transaction we:
> 1. Issue a fsync against the lock file to flush the hardware writeback
>    cache, which should by now have processed the tmp file writes.
> 2. Rename all of the temp files to their final names.
> 3. When updating the index and/or refs, we will issue another fsync
>    internal to that operation.
>
> On a filesystem with a singular journal that is updated during name
> operations (e.g. create, link, rename, etc), such as NTFS and HFS+, we
> would expect the fsync to trigger a journal writeout so that this
> sequence is enough to ensure that the user's data is durable by the time
> the git command returns.
>
> This change also updates the MacOS code to trigger a real hardware flush
> via fnctl(fd, F_FULLFSYNC) when fsync_or_die is called. Previously, on
> MacOS there was no guarantee of durability since a simple fsync(2) call
> does not flush any hardware caches.

You included a very nice table with performance numbers in the cover letter. Maybe include that here, in the commit message?

Show 36 quoted lines

> Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
> ---
>  Documentation/config/core.txt |  17 ++++--
>  Makefile                      |   4 ++
>  builtin/add.c                 |   3 +-
>  builtin/update-index.c        |   3 +
>  bulk-checkin.c                | 105 +++++++++++++++++++++++++++++++---
>  bulk-checkin.h                |   4 +-
>  config.c                      |   4 +-
>  config.mak.uname              |   2 +
>  configure.ac                  |   8 +++
>  git-compat-util.h             |   7 +++
>  object-file.c                 |  12 +---
>  wrapper.c                     |  36 ++++++++++++
>  write-or-die.c                |   2 +-
>  13 files changed, 177 insertions(+), 30 deletions(-)
>
> diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
> index c04f62a54a1..3b672c2db67 100644
> --- a/Documentation/config/core.txt
> +++ b/Documentation/config/core.txt
> @@ -548,12 +548,17 @@ core.whitespace::
>    errors. The default tab width is 8. Allowed values are 1 to 63.
>
>  core.fsyncObjectFiles::
> -	This boolean will enable 'fsync()' when writing object files.
> -+
> -This is a total waste of time and effort on a filesystem that orders
> -data writes properly, but can be useful for filesystems that do not use
> -journalling (traditional UNIX filesystems) or that only journal metadata
> -and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
> +	A boolean value or the number '2', indicating the level of durability
> +	applied to object files.
> ++
> +This setting controls how much effort Git makes to ensure that data added to
> +the object store are durable in the case of an unclean system shutdown. If

In addition to the content, I also like a lot that this tempers down the language to be a lot more agreeable to read.

Show 66 quoted lines

> +'false', Git allows data to remain in file system caches according to operating
> +system policy, whence they may be lost if the system loses power or crashes. A
> +value of 'true' instructs Git to force objects to stable storage immediately
> +when they are added to the object store. The number '2' is an experimental
> +value that also preserves durability but tries to perform hardware flushes in a
> +batch.
>
>  core.preloadIndex::
>  	Enable parallel index preload for operations like 'git diff'
> diff --git a/Makefile b/Makefile
> index 9573190f1d7..cb950ee43d3 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1896,6 +1896,10 @@ ifdef HAVE_CLOCK_MONOTONIC
>  	BASIC_CFLAGS += -DHAVE_CLOCK_MONOTONIC
>  endif
>
> +ifdef HAVE_SYNC_FILE_RANGE
> +	BASIC_CFLAGS += -DHAVE_SYNC_FILE_RANGE
> +endif
> +
>  ifdef NEEDS_LIBRT
>  	EXTLIBS += -lrt
>  endif
> diff --git a/builtin/add.c b/builtin/add.c
> index 09e684585d9..c58dfcd4bc3 100644
> --- a/builtin/add.c
> +++ b/builtin/add.c
> @@ -670,7 +670,8 @@ int cmd_add(int argc, const char **argv, const char *prefix)
>
>  	if (chmod_arg && pathspec.nr)
>  		exit_status |= chmod_pathspec(&pathspec, chmod_arg[0], show_only);
> -	unplug_bulk_checkin();
> +
> +	unplug_bulk_checkin(&lock_file);
>
>  finish:
>  	if (write_locked_index(&the_index, &lock_file,
> diff --git a/builtin/update-index.c b/builtin/update-index.c
> index f1f16f2de52..64d025cf49e 100644
> --- a/builtin/update-index.c
> +++ b/builtin/update-index.c
> @@ -5,6 +5,7 @@
>   */
>  #define USE_THE_INDEX_COMPATIBILITY_MACROS
>  #include "cache.h"
> +#include "bulk-checkin.h"
>  #include "config.h"
>  #include "lockfile.h"
>  #include "quote.h"
> @@ -1152,6 +1153,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
>  		struct strbuf unquoted = STRBUF_INIT;
>
>  		setup_work_tree();
> +		plug_bulk_checkin();
>  		while (getline_fn(&buf, stdin) != EOF) {
>  			char *p;
>  			if (!nul_term_line && buf.buf[0] == '"') {
> @@ -1166,6 +1168,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
>  				chmod_path(set_executable_bit, p);
>  			free(p);
>  		}
> +		unplug_bulk_checkin(&lock_file);
>  		strbuf_release(&unquoted);
>  		strbuf_release(&buf);
>  	}

This change to `cmd_update_index()`, would it make sense to separate it out into its own commit? I think it would, as it is a slight change of behavior of the `--stdin` mode, no?

Show 37 quoted lines

> diff --git a/bulk-checkin.c b/bulk-checkin.c
> index b023d9959aa..71004db863e 100644
> --- a/bulk-checkin.c
> +++ b/bulk-checkin.c
> @@ -3,6 +3,7 @@
>   */
>  #include "cache.h"
>  #include "bulk-checkin.h"
> +#include "lockfile.h"
>  #include "repository.h"
>  #include "csum-file.h"
>  #include "pack.h"
> @@ -10,6 +11,17 @@
>  #include "packfile.h"
>  #include "object-store.h"
>
> +struct object_rename {
> +	char *src;
> +	char *dst;
> +};
> +
> +static struct bulk_rename_state {
> +	struct object_rename *renames;
> +	uint32_t alloc_renames;
> +	uint32_t nr_renames;
> +} bulk_rename_state;
> +
>  static struct bulk_checkin_state {
>  	unsigned plugged:1;
>
> @@ -21,13 +33,15 @@ static struct bulk_checkin_state {
>  	struct pack_idx_entry **written;
>  	uint32_t alloc_written;
>  	uint32_t nr_written;
> -} state;
> +
> +} bulk_checkin_state;

While it definitely looks better after this patch, having the new code _and_ the rename in the same set of changes makes it a bit harder to review and to spot bugs.

Could I ask you to split this rename out into its own, preparatory patch ("preparatory" meaning that it should be ordered before the patch that adds support for the new fsync mode)?

Show 7 quoted lines

>
>  static void finish_bulk_checkin(struct bulk_checkin_state *state)
>  {
>  	struct object_id oid;
>  	struct strbuf packname = STRBUF_INIT;
>  	int i;
> +	unsigned old_plugged;

Since this variable is designed to hold the value of the `plugged` field of the `bulk_checkin_state`, which is declared as `unsigned plugged:1;`, we probably want a `:1` here, too.

Also: is it really "old", rather than "orig"? I would have expected the
name `orig_plugged` or `save_plugged`.

Show 10 quoted lines

>
>  	if (!state->f)
>  		return;
> @@ -55,13 +69,42 @@ static void finish_bulk_checkin(struct bulk_checkin_state *state)
>
>  clear_exit:
>  	free(state->written);
> +	old_plugged = state->plugged;
>  	memset(state, 0, sizeof(*state));
> +	state->plugged = old_plugged;

Unfortunately, I lack the context to understand the purpose of this. Is the idea that `plugged` gives an indication whether we're still within that batch that should be fsync'ed all at once?

I only see one caller where this would make a difference, and that caller is `deflate_to_pack()`. Maybe we should just start that function with `unsigned save_plugged:1 = state->plugged;` and restore it after the `while (1)` loop?

Show 30 quoted lines

>
>  	strbuf_release(&packname);
>  	/* Make objects we just wrote available to ourselves */
>  	reprepare_packed_git(the_repository);
>  }
>
> +static void do_sync_and_rename(struct bulk_rename_state *state, struct lock_file *lock_file)
> +{
> +	if (state->nr_renames) {
> +		int i;
> +
> +		/*
> +		 * Issue a full hardware flush against the lock file to ensure
> +		 * that all objects are durable before any renames occur.
> +		 * The code in fsync_and_close_loose_object_bulk_checkin has
> +		 * already ensured that writeout has occurred, but it has not
> +		 * flushed any writeback cache in the storage hardware.
> +		 */
> +		fsync_or_die(get_lock_file_fd(lock_file), get_lock_file_path(lock_file));
> +
> +		for (i = 0; i < state->nr_renames; i++) {
> +			if (finalize_object_file(state->renames[i].src, state->renames[i].dst))
> +				die_errno(_("could not rename '%s'"), state->renames[i].src);
> +
> +			free(state->renames[i].src);
> +			free(state->renames[i].dst);
> +		}
> +
> +		free(state->renames);
> +		memset(state, 0, sizeof(*state));

Hmm. There is a lot of `memset()`ing going on, and I am not quite sure that I like what I am seeing. It does not help that there are now two very easily-confused structs: `bulk_rename_state` and `bulk_checkin_state`. Which made me worried at first that we might be resetting the `renames` field inadvertently in `finish_bulk_checkin()`.

Maybe we can do this instead?

		FREE_AND_NULL(state->renames);
		state->nr_renames = state->alloc_renames = 0;

Show 241 quoted lines

> +	}
> +}
> +
>  static int already_written(struct bulk_checkin_state *state, struct object_id *oid)
>  {
>  	int i;
> @@ -256,25 +299,69 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
>  	return 0;
>  }
>
> +static void add_rename_bulk_checkin(struct bulk_rename_state *state,
> +				    const char *src, const char *dst)
> +{
> +	struct object_rename *rename;
> +
> +	ALLOC_GROW(state->renames, state->nr_renames + 1, state->alloc_renames);
> +
> +	rename = &state->renames[state->nr_renames++];
> +	rename->src = xstrdup(src);
> +	rename->dst = xstrdup(dst);
> +}
> +
> +int fsync_and_close_loose_object_bulk_checkin(int fd, const char *tmpfile,
> +					      const char *filename)
> +{
> +	if (fsync_object_files) {
> +		/*
> +		 * 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 renaming files as part of do_sync_and_rename.
> +		 */
> +		if (bulk_checkin_state.plugged &&
> +		    fsync_object_files == 2 &&
> +		    git_fsync(fd, FSYNC_WRITEOUT_ONLY) >= 0) {
> +			add_rename_bulk_checkin(&bulk_rename_state, tmpfile, filename);
> +			if (close(fd))
> +				die_errno(_("error when closing loose object file"));
> +
> +			return 0;
> +
> +		} else {
> +			fsync_or_die(fd, "loose object file");
> +		}
> +	}
> +
> +	if (close(fd))
> +		die_errno(_("error when closing loose object file"));
> +
> +	return finalize_object_file(tmpfile, filename);
> +}
> +
>  int index_bulk_checkin(struct object_id *oid,
>  		       int fd, size_t size, enum object_type type,
>  		       const char *path, unsigned flags)
>  {
> -	int status = deflate_to_pack(&state, oid, fd, size, type,
> +	int status = deflate_to_pack(&bulk_checkin_state, oid, fd, size, type,
>  				     path, flags);
> -	if (!state.plugged)
> -		finish_bulk_checkin(&state);
> +	if (!bulk_checkin_state.plugged)
> +		finish_bulk_checkin(&bulk_checkin_state);
>  	return status;
>  }
>
>  void plug_bulk_checkin(void)
>  {
> -	state.plugged = 1;
> +	bulk_checkin_state.plugged = 1;
>  }
>
> -void unplug_bulk_checkin(void)
> +void unplug_bulk_checkin(struct lock_file *lock_file)
>  {
> -	state.plugged = 0;
> -	if (state.f)
> -		finish_bulk_checkin(&state);
> +	bulk_checkin_state.plugged = 0;
> +	if (bulk_checkin_state.f)
> +		finish_bulk_checkin(&bulk_checkin_state);
> +
> +	do_sync_and_rename(&bulk_rename_state, lock_file);
>  }
> diff --git a/bulk-checkin.h b/bulk-checkin.h
> index b26f3dc3b74..8efb01ed669 100644
> --- a/bulk-checkin.h
> +++ b/bulk-checkin.h
> @@ -6,11 +6,13 @@
>
>  #include "cache.h"
>
> +int fsync_and_close_loose_object_bulk_checkin(int fd, const char *tmpfile, 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);
>
>  void plug_bulk_checkin(void);
> -void unplug_bulk_checkin(void);
> +void unplug_bulk_checkin(struct lock_file *);
>
>  #endif
> diff --git a/config.c b/config.c
> index f33abeab851..375bdb24b0a 100644
> --- a/config.c
> +++ b/config.c
> @@ -1509,7 +1509,9 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
>  	}
>
>  	if (!strcmp(var, "core.fsyncobjectfiles")) {
> -		fsync_object_files = git_config_bool(var, value);
> +		int is_bool;
> +
> +		fsync_object_files = git_config_bool_or_int(var, value, &is_bool);
>  		return 0;
>  	}
>
> diff --git a/config.mak.uname b/config.mak.uname
> index 69413fb3dc0..8c07f2265a8 100644
> --- a/config.mak.uname
> +++ b/config.mak.uname
> @@ -53,6 +53,7 @@ ifeq ($(uname_S),Linux)
>  	HAVE_CLOCK_MONOTONIC = YesPlease
>  	# -lrt is needed for clock_gettime on glibc <= 2.16
>  	NEEDS_LIBRT = YesPlease
> +	HAVE_SYNC_FILE_RANGE = YesPlease
>  	HAVE_GETDELIM = YesPlease
>  	SANE_TEXT_GREP=-a
>  	FREAD_READS_DIRECTORIES = UnfortunatelyYes
> @@ -133,6 +134,7 @@ ifeq ($(uname_S),Darwin)
>  	COMPAT_OBJS += compat/precompose_utf8.o
>  	BASIC_CFLAGS += -DPRECOMPOSE_UNICODE
>  	BASIC_CFLAGS += -DPROTECT_HFS_DEFAULT=1
> +	BASIC_CFLAGS += -DFSYNC_DOESNT_FLUSH=1
>  	HAVE_BSD_SYSCTL = YesPlease
>  	FREAD_READS_DIRECTORIES = UnfortunatelyYes
>  	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
> diff --git a/configure.ac b/configure.ac
> index 031e8d3fee8..c711037d625 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1090,6 +1090,14 @@ AC_COMPILE_IFELSE([CLOCK_MONOTONIC_SRC],
>  	[AC_MSG_RESULT([no])
>  	HAVE_CLOCK_MONOTONIC=])
>  GIT_CONF_SUBST([HAVE_CLOCK_MONOTONIC])
> +
> +#
> +# Define HAVE_SYNC_FILE_RANGE=YesPlease if sync_file_range is available.
> +GIT_CHECK_FUNC(sync_file_range,
> +	[HAVE_SYNC_FILE_RANGE=YesPlease],
> +	[HAVE_SYNC_FILE_RANGE])
> +GIT_CONF_SUBST([HAVE_SYNC_FILE_RANGE])
> +
>  #
>  # Define NO_SETITIMER if you don't have setitimer.
>  GIT_CHECK_FUNC(setitimer,
> diff --git a/git-compat-util.h b/git-compat-util.h
> index b46605300ab..d14e2436276 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -1210,6 +1210,13 @@ __attribute__((format (printf, 1, 2))) NORETURN
>  void BUG(const char *fmt, ...);
>  #endif
>
> +enum fsync_action {
> +    FSYNC_WRITEOUT_ONLY,
> +    FSYNC_HARDWARE_FLUSH
> +};
> +
> +int git_fsync(int fd, enum fsync_action action);
> +
>  /*
>   * Preserves errno, prints a message, but gives no warning for ENOENT.
>   * Returns 0 on success, which includes trying to unlink an object that does
> diff --git a/object-file.c b/object-file.c
> index 607e9e2f80b..5f04143dde0 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -1859,16 +1859,6 @@ int hash_object_file(const struct git_hash_algo *algo, const void *buf,
>  	return 0;
>  }
>
> -/* Finalize a file on disk, and close it. */
> -static int close_loose_object(int fd, const char *tmpfile, const char *filename)
> -{
> -	if (fsync_object_files)
> -		fsync_or_die(fd, "loose object file");
> -	if (close(fd) != 0)
> -		die_errno(_("error when closing loose object file"));
> -	return finalize_object_file(tmpfile, filename);
> -}
> -
>  /* Size of directory component, including the ending '/' */
>  static inline int directory_size(const char *filename)
>  {
> @@ -1982,7 +1972,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
>  			warning_errno(_("failed futimes() on %s"), tmp_file.buf);
>  	}
>
> -	return close_loose_object(fd, tmp_file.buf, filename.buf);
> +	return fsync_and_close_loose_object_bulk_checkin(fd, tmp_file.buf, filename.buf);
>  }
>
>  static int freshen_loose_object(const struct object_id *oid)
> diff --git a/wrapper.c b/wrapper.c
> index 563ad590df1..37a8b61a7df 100644
> --- a/wrapper.c
> +++ b/wrapper.c
> @@ -538,6 +538,42 @@ int xmkstemp_mode(char *filename_template, int mode)
>  	return fd;
>  }
>
> +int git_fsync(int fd, enum fsync_action action)
> +{
> +	if (action == FSYNC_WRITEOUT_ONLY) {
> +#ifdef __APPLE__
> +		/*
> +		 * on Mac OS X, fsync just causes filesystem cache writeback but does not
> +		 * flush hardware caches.
> +		 */
> +		return fsync(fd);
> +#endif
> +
> +#ifdef HAVE_SYNC_FILE_RANGE
> +		/*
> +		 * On linux 2.6.17 and above, sync_file_range is the way to issue
> +		 * a writeback without a hardware flush. An offset of 0 and size of 0
> +		 * indicates writeout of the entire file and the wait flags ensure that all
> +		 * dirty data is written to the disk (potentially in a disk-side cache)
> +		 * before we continue.
> +		 */
> +
> +		return sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WAIT_BEFORE |
> +						 SYNC_FILE_RANGE_WRITE |
> +						 SYNC_FILE_RANGE_WAIT_AFTER);
> +#endif
> +
> +		errno = ENOSYS;
> +		return -1;
> +	}

Hmm. I wonder whether we can do this more consistently with how Git usually does platform-specific things.

In the 3rd patch, the one where you implemented Windows-specific support, in the Git for Windows PR at https://github.com/git-for-windows/git/pull/3391, you introduce a `mingw_fsync_no_flush()` function and define `fsync_no_flush` to expand to that function name.

This is very similar to how Git does things. Take for example the `offset_1st_component` macro: https://github.com/git/git/blob/v2.33.0/git-compat-util.h#L386-L392

Unless defined in a platform-specific manner, it is defined in `git-compat-util.h`:

	#ifndef offset_1st_component
	static inline int git_offset_1st_component(const char *path)
	{
		return is_dir_sep(path[0]);
	}
	#define offset_1st_component git_offset_1st_component
	#endif

And on Windows, it is defined as following (https://github.com/git/git/blob/v2.33.0/compat/win32/path-utils.h#L34-L35), before the lines quoted above:

	int win32_offset_1st_component(const char *path);
	#define offset_1st_component win32_offset_1st_component

We could do the exact same thing here. Define a platform-specific `mingw_fsync_no_flush()` in `compat/mingw.h` and define the macro `fsync_no_flush` to point to it. In `git-compat-util.h`, in the `__APPLE__`-specific part, implement it via `fsync()`. And later, in the platform-independent part, _iff_ the macro has not yet been defined, implement an inline function that does that `HAVE_SYNC_FILE_RANGE` dance and falls back to `ENOSYS`.

That would contain the platform-specific `#ifdef` blocks to `git-compat-util.h`, which is exactly where we want them.

Show 6 quoted lines

> +
> +#ifdef __APPLE__
> +	return fcntl(fd, F_FULLFSYNC);
> +#else
> +	return fsync(fd);
> +#endif

Same thing here. We would probably want something like `fsync_with_flush` here.

It is my hope that you find my comments and suggestions helpful.

Thank you, Johannes

Show 21 quoted lines

> +}
> +
>  static int warn_if_unremovable(const char *op, const char *file, int rc)
>  {
>  	int err;
> diff --git a/write-or-die.c b/write-or-die.c
> index d33e68f6abb..8f53953d4ab 100644
> --- a/write-or-die.c
> +++ b/write-or-die.c
> @@ -57,7 +57,7 @@ void fprintf_or_die(FILE *f, const char *fmt, ...)
>
>  void fsync_or_die(int fd, const char *msg)
>  {
> -	while (fsync(fd) < 0) {
> +	while (git_fsync(fd, FSYNC_HARDWARE_FLUSH) < 0) {
>  		if (errno != EINTR)
>  			die_errno("fsync error on '%s'", msg);
>  	}
> --
> gitgitgadget
>