[PATCH] index-pack: work around thread-unsafe pread()
- From
Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
- Date
- Mar 25, 2014, 13:41 UTC
- Message-ID
- <1395754901-19730-1-git-send-email-pclouds@gmail.com>
- In-Reply-To
- <CACsJy8AYs0-rmGZz2_KEkT2ibW-sTpm=Q9FxFhNGRYd2b6R+sA@mail.gmail.com>
pread() implementation for Cygwin and MSYS is not thread safe, which led to multithreading being disabled in c0f8654 (index-pack: Disable threading on cygwin - 2012-06-26). Work around it by opening one file handle per thread, so parallel pread() (on different file handle) can't step on each other. Also remove NO_THREAD_SAFE_PREAD that was introduced in c0f8654 because it's no longer used anywhere.
This workaround is unconditional, even for platforms with thread-safe pread() because the overhead is small (a couple file handles more) and not worth fragmenting the code.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> --- On Fri, Mar 21, 2014 at 8:51 AM, Duy Nguyen <pclouds@gmail.com> wrote: > On Thu, Mar 20, 2014 at 11:08 PM, Stefan Zager <szager@chromium.org> wrote: >> Duy, would you like to re-post your patch without the new pread implementation? > > I will but let me try out the sliding window idea first. My quick > tests on git.git show me we may only need 21k mmap instead of 177k > pread. That hints some potential performance improvement.
Here it is. But I still think it's worth measuring the simpler patch that protects pread() from the caller. I suspect we won't see any difference.
Makefile | 7 ------- builtin/index-pack.c | 27 +++++++++++++++++---------- config.mak.uname | 1 - 3 files changed, 17 insertions(+), 18 deletions(-)
diff --git a/Makefile b/Makefile index 3646391..0089fad 100644 --- a/Makefile +++ b/Makefile @@ -183,9 +183,6 @@ all:: # Define NO_STRUCT_ITIMERVAL if you don't have struct itimerval # This also implies NO_SETITIMER # -# Define NO_THREAD_SAFE_PREAD if your pread() implementation is not -# thread-safe. (e.g. compat/pread.c or cygwin) -# # Define NO_FAST_WORKING_DIRECTORY if accessing objects in pack files is # generally faster on your platform than accessing the working directory. # @@ -1336,10 +1333,6 @@ endif ifdef NO_PREAD COMPAT_CFLAGS += -DNO_PREAD COMPAT_OBJS += compat/pread.o - NO_THREAD_SAFE_PREAD = YesPlease -endif -ifdef NO_THREAD_SAFE_PREAD - BASIC_CFLAGS += -DNO_THREAD_SAFE_PREAD endif ifdef NO_FAST_WORKING_DIRECTORY BASIC_CFLAGS += -DNO_FAST_WORKING_DIRECTORY diff --git a/builtin/index-pack.c b/builtin/index-pack.c index a6b1c17..676d39d 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -40,17 +40,13 @@ struct base_data { int ofs_first, ofs_last; }; -#if !defined(NO_PTHREADS) && defined(NO_THREAD_SAFE_PREAD) -/* pread() emulation is not thread-safe. Disable threading. */ -#define NO_PTHREADS -#endif - struct thread_local { #ifndef NO_PTHREADS pthread_t thread; #endif struct base_data *base_cache; size_t base_cache_used; + int pack_fd; }; /* @@ -91,7 +87,8 @@ static off_t consumed_bytes; static unsigned deepest_delta; static git_SHA_CTX input_ctx; static uint32_t input_crc32; -static int input_fd, output_fd, pack_fd; +static int input_fd, output_fd; +static const char *curr_pack; #ifndef NO_PTHREADS @@ -134,6 +131,7 @@ static inline void unlock_mutex(pthread_mutex_t *mutex) */ static void init_thread(void) { + int i; init_recursive_mutex(&read_mutex); pthread_mutex_init(&counter_mutex, NULL); pthread_mutex_init(&work_mutex, NULL); @@ -141,11 +139,18 @@ static void init_thread(void) pthread_mutex_init(&deepest_delta_mutex, NULL); pthread_key_create(&key, NULL); thread_data = xcalloc(nr_threads, sizeof(*thread_data)); + for (i = 0; i < nr_threads; i++) { + thread_data[i].pack_fd = open(curr_pack, O_RDONLY); + if (thread_data[i].pack_fd == -1) + die_errno(_("unable to open %s"), curr_pack); + } + threads_active = 1; } static void cleanup_thread(void) { + int i; if (!threads_active) return; threads_active = 0; @@ -154,6 +159,8 @@ static void cleanup_thread(void) pthread_mutex_destroy(&work_mutex); if (show_stat) pthread_mutex_destroy(&deepest_delta_mutex); + for (i = 0; i < nr_threads; i++) + close(thread_data[i].pack_fd); pthread_key_delete(key); free(thread_data); } @@ -288,13 +295,13 @@ static const char *open_pack_file(const char *pack_name) output_fd = open(pack_name, O_CREAT|O_EXCL|O_RDWR, 0600); if (output_fd < 0) die_errno(_("unable to create '%s'"), pack_name); - pack_fd = output_fd; + nothread_data.pack_fd = output_fd; } else { input_fd = open(pack_name, O_RDONLY); if (input_fd < 0) die_errno(_("cannot open packfile '%s'"), pack_name); output_fd = -1; - pack_fd = input_fd; + nothread_data.pack_fd = input_fd; } git_SHA1_Init(&input_ctx); return pack_name; @@ -542,7 +549,7 @@ static void *unpack_data(struct object_entry *obj, do { ssize_t n = (len < 64*1024) ? len : 64*1024; - n = pread(pack_fd, inbuf, n, from); + n = pread(get_thread_data()->pack_fd, inbuf, n, from); if (n < 0) die_errno(_("cannot pread pack file")); if (!n) @@ -1490,7 +1497,7 @@ static void show_pack_info(int stat_only) int cmd_index_pack(int argc, const char **argv, const char *prefix) { int i, fix_thin_pack = 0, verify = 0, stat_only = 0; - const char *curr_pack, *curr_index; + const char *curr_index; const char *index_name = NULL, *pack_name = NULL; const char *keep_name = NULL, *keep_msg = NULL; char *index_name_buf = NULL, *keep_name_buf = NULL; diff --git a/config.mak.uname b/config.mak.uname index 6069a44..c9febe1 100644 --- a/config.mak.uname +++ b/config.mak.uname @@ -157,7 +157,6 @@ ifeq ($(uname_O),Cygwin) NO_SYMLINK_HEAD = YesPlease NO_IPV6 = YesPlease OLD_ICONV = UnfortunatelyYes - NO_THREAD_SAFE_PREAD = YesPlease # There are conflicting reports about this. # On some boxes NO_MMAP is needed, and not so elsewhere. # Try commenting this out if you suspect MMAP is more efficient
-- 1.9.1.345.ga1a145c