| Message ID | 20181018180522.17642-1-pclouds@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [PATCH/RFC] thread-utils: better wrapper to avoid #ifdef NO_PTHREADS | expand |
On Thu, Oct 18, 2018 at 08:05:22PM +0200, Nguyễn Thái Ngọc Duy wrote: > On Thu, Oct 18, 2018 at 7:09 PM Jeff King <peff@peff.net> wrote: > > > In this particular case though I think we should be able to avoid so > > > much #if if we make a wrapper for pthread api that would return an > > > error or something when pthread is not available. But similar > > > situation may happen elsewhere too. > > > > Yeah, I think that is generally the preferred method anyway, just > > because of readability and simplicity. > > I've wanted to do this for a while, so let's test the water and see if > it's well received. > > This patch is a proof of concept that adds just enough macros so that > I can build index-pack.c on a single thread mode with zero #ifdef > related to NO_PTHREADS. > > Besides readability and simplicity, it reduces the chances of breaking > conditional builds (e.g. you rename a variable name but forgot that > the variable is in #if block that is not used by your > compiler/platform). Yes, I love this. We're already halfway there with things like read_lock() in index-pack and elsewhere, which are conditionally no-ops. The resulting code is much easier to read, I think. > Performance-wise I don't think there is any loss for single thread > mode. I rely on compilers recognizing HAVE_THREADS being a constant > and remove dead code or at least optimize in favor of non-dead code. > > Memory-wise, yes we use some more memory in single thread mode. But we > don't have zillions of mutexes or thread id, so a bit extra memory > does not worry me so much. Yeah, I don't think carrying around a handful of ints is going to be a big deal. I also think we may want to make a fundamental shift in our view of thread support. In the early days, it was "well, this is a thing that modern systems can take advantage of for certain commands". But these days I suspect it is more like "there are a handful of legacy systems that do not even support threads". I don't think we should break the build on those legacy systems, but it's probably OK to stop thinking of it as "non-threaded platforms are the default and must pay zero cost" and more as "threaded platforms are the default, and non-threaded ones are OK to pay a small cost as long as they still work". > @@ -74,4 +79,29 @@ int init_recursive_mutex(pthread_mutex_t *m) > pthread_mutexattr_destroy(&a); > } > return ret; > +#else > + return ENOSYS; > +#endif > +} I suspect some of these ENOSYS could just become a silent success. ("yep, I initialized your dummy mutex"). But it probably doesn't matter much either way, as we would not generally even bother checking this return. > +#ifdef NO_PTHREADS > +int dummy_pthread_create(pthread_t *pthread, const void *attr, > + void *(*fn)(void *), void *data) > +{ > + return ENOSYS; > } Whereas for this one, ENOSYS makes a lot of sense (we should avoid the threaded code-path anyway when we see that online_cpus()==1, and this would let us know when we mess that up). > +int dummy_pthread_init(void *data) > +{ > + /* > + * Do nothing. > + * > + * The main purpose of this function is to break compiler's > + * flow analysis or it may realize that functions like > + * pthread_mutex_init() is no-op, which means the (static) > + * variable is not used/initialized at all and trigger > + * -Wunused-variable > + */ > + return ENOSYS; > +} It might be worth marking the dummy variables as MAYBE_UNUSED, exactly to avoid this kind of compiler complaint. -Peff
Jeff King <peff@peff.net> writes: > I also think we may want to make a fundamental shift in our view of > thread support. In the early days, it was "well, this is a thing that > modern systems can take advantage of for certain commands". But these > days I suspect it is more like "there are a handful of legacy systems > that do not even support threads". > > I don't think we should break the build on those legacy systems, but > it's probably OK to stop thinking of it as "non-threaded platforms are > the default and must pay zero cost" and more as "threaded platforms are > the default, and non-threaded ones are OK to pay a small cost as long as > they still work". Good suggestion.
On 10/23/2018 4:28 PM, Jeff King wrote: > On Thu, Oct 18, 2018 at 08:05:22PM +0200, Nguyễn Thái Ngọc Duy wrote: > >> On Thu, Oct 18, 2018 at 7:09 PM Jeff King <peff@peff.net> wrote: >>>> In this particular case though I think we should be able to avoid so >>>> much #if if we make a wrapper for pthread api that would return an >>>> error or something when pthread is not available. But similar >>>> situation may happen elsewhere too. >>> >>> Yeah, I think that is generally the preferred method anyway, just >>> because of readability and simplicity. >> >> I've wanted to do this for a while, so let's test the water and see if >> it's well received. >> >> This patch is a proof of concept that adds just enough macros so that >> I can build index-pack.c on a single thread mode with zero #ifdef >> related to NO_PTHREADS. >> >> Besides readability and simplicity, it reduces the chances of breaking >> conditional builds (e.g. you rename a variable name but forgot that >> the variable is in #if block that is not used by your >> compiler/platform). > > Yes, I love this. We're already halfway there with things like > read_lock() in index-pack and elsewhere, which are conditionally no-ops. > The resulting code is much easier to read, I think. > I am also very much in favor of this. I updated a couple of places threading is being used that I've been working in (preload-index and read-cache) and both are much simplified using your proof of concept patch. >> Performance-wise I don't think there is any loss for single thread >> mode. I rely on compilers recognizing HAVE_THREADS being a constant >> and remove dead code or at least optimize in favor of non-dead code. >> >> Memory-wise, yes we use some more memory in single thread mode. But we >> don't have zillions of mutexes or thread id, so a bit extra memory >> does not worry me so much. > > Yeah, I don't think carrying around a handful of ints is going to be a > big deal. > Just to be complete, there _is_ an additional cost. Today, code paths that are only executed when there are pthreads available are excluded from the binary (via #ifdef). With this change, those code paths would now be included causing some code bloat to NO_PTHREAD threaded images. One example of this is in read-cache.c where the ieot read/write functions aren't included for NO_PTHREAD but now would be. > I also think we may want to make a fundamental shift in our view of > thread support. In the early days, it was "well, this is a thing that > modern systems can take advantage of for certain commands". But these > days I suspect it is more like "there are a handful of legacy systems > that do not even support threads". > > I don't think we should break the build on those legacy systems, but > it's probably OK to stop thinking of it as "non-threaded platforms are > the default and must pay zero cost" and more as "threaded platforms are > the default, and non-threaded ones are OK to pay a small cost as long as > they still work". > I agree though I'm still curious if there are still no-threaded platforms taking new versions of git. Perhaps we should do the depreciation warning you suggested elsewhere and see how much push back we get. It's unlikely we'd get lucky and be able to stop supporting them completely but it's worth asking! >> @@ -74,4 +79,29 @@ int init_recursive_mutex(pthread_mutex_t *m) >> pthread_mutexattr_destroy(&a); >> } >> return ret; >> +#else >> + return ENOSYS; >> +#endif >> +} > > I suspect some of these ENOSYS could just become a silent success. > ("yep, I initialized your dummy mutex"). But it probably doesn't matter > much either way, as we would not generally even bother checking this > return. > >> +#ifdef NO_PTHREADS >> +int dummy_pthread_create(pthread_t *pthread, const void *attr, >> + void *(*fn)(void *), void *data) >> +{ >> + return ENOSYS; >> } > > Whereas for this one, ENOSYS makes a lot of sense (we should avoid the > threaded code-path anyway when we see that online_cpus()==1, and this > would let us know when we mess that up). > This highlights something anyone writing multi-threaded code will need to pay attention to that wasn't an issue before. If you attempt to create more threads than online_cpus(), the pthread_create() call will fail and needs to be handled gracefully. One example of this is in preload-index.c where (up to) 20 threads are created irrespective of what online_cpus() returns and if pthread_create() fails, it just dies. The logic would need to be updated for this to work correctly. I still think this is a much simpler issue to deal with than what we have today with having to write/debug multiple code paths but I did want to point it out for completeness. >> +int dummy_pthread_init(void *data) >> +{ >> + /* >> + * Do nothing. >> + * >> + * The main purpose of this function is to break compiler's >> + * flow analysis or it may realize that functions like >> + * pthread_mutex_init() is no-op, which means the (static) >> + * variable is not used/initialized at all and trigger >> + * -Wunused-variable >> + */ >> + return ENOSYS; >> +} > > It might be worth marking the dummy variables as MAYBE_UNUSED, exactly > to avoid this kind of compiler complaint. > > -Peff >
On Fri, Oct 26, 2018 at 4:09 PM Ben Peart <peartben@gmail.com> wrote: > I agree though I'm still curious if there are still no-threaded > platforms taking new versions of git. Perhaps we should do the > depreciation warning you suggested elsewhere and see how much push back > we get. It's unlikely we'd get lucky and be able to stop supporting > them completely but it's worth asking! NO_PTHREADS can also be used even though the platform supports multithread: to make keep git execution in a single core/thread. It might matter on hosted systems with limited cpu power and you don't want git to hog it all. Yes it can also be achieved by setting a zillion config keys to "1", this way is just simpler.
On Sat, Oct 27, 2018 at 09:26:28AM +0200, Duy Nguyen wrote: > On Fri, Oct 26, 2018 at 4:09 PM Ben Peart <peartben@gmail.com> wrote: > > I agree though I'm still curious if there are still no-threaded > > platforms taking new versions of git. Perhaps we should do the > > depreciation warning you suggested elsewhere and see how much push back > > we get. It's unlikely we'd get lucky and be able to stop supporting > > them completely but it's worth asking! > > NO_PTHREADS can also be used even though the platform supports > multithread: to make keep git execution in a single core/thread. It > might matter on hosted systems with limited cpu power and you don't > want git to hog it all. Yes it can also be achieved by setting a > zillion config keys to "1", this way is just simpler. Yeah, I wondered about that use case (also with your patches, and whether they might run into problems on systems that _do_ have pthreads, but just don't want to compile with them). But I think that is pretty easily solved by just having a single runtime option (e.g., to just pretend that oneline_cpus is always 1 by default). -Peff
diff --git a/Makefile b/Makefile index 5bf1af369e..ef852031bd 100644 --- a/Makefile +++ b/Makefile @@ -981,6 +981,7 @@ LIB_OBJS += sub-process.o LIB_OBJS += symlinks.o LIB_OBJS += tag.o LIB_OBJS += tempfile.o +LIB_OBJS += thread-utils.o LIB_OBJS += tmp-objdir.o LIB_OBJS += trace.o LIB_OBJS += trailer.o @@ -1664,7 +1665,6 @@ ifdef NO_PTHREADS else BASIC_CFLAGS += $(PTHREAD_CFLAGS) EXTLIBS += $(PTHREAD_LIBS) - LIB_OBJS += thread-utils.o endif ifdef HAVE_PATHS_H diff --git a/builtin/index-pack.c b/builtin/index-pack.c index 2004e25da2..bbd66ca025 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -42,9 +42,7 @@ struct base_data { }; struct thread_local { -#ifndef NO_PTHREADS pthread_t thread; -#endif struct base_data *base_cache; size_t base_cache_used; int pack_fd; @@ -98,8 +96,6 @@ static uint32_t input_crc32; static int input_fd, output_fd; static const char *curr_pack; -#ifndef NO_PTHREADS - static struct thread_local *thread_data; static int nr_dispatched; static int threads_active; @@ -179,26 +175,6 @@ static void cleanup_thread(void) free(thread_data); } -#else - -#define read_lock() -#define read_unlock() - -#define counter_lock() -#define counter_unlock() - -#define work_lock() -#define work_unlock() - -#define deepest_delta_lock() -#define deepest_delta_unlock() - -#define type_cas_lock() -#define type_cas_unlock() - -#endif - - static int mark_link(struct object *obj, int type, void *data, struct fsck_options *options) { if (!obj) @@ -364,22 +340,20 @@ static NORETURN void bad_object(off_t offset, const char *format, ...) static inline struct thread_local *get_thread_data(void) { -#ifndef NO_PTHREADS - if (threads_active) - return pthread_getspecific(key); - assert(!threads_active && - "This should only be reached when all threads are gone"); -#endif + if (HAVE_THREADS) { + if (threads_active) + return pthread_getspecific(key); + assert(!threads_active && + "This should only be reached when all threads are gone"); + } return ¬hread_data; } -#ifndef NO_PTHREADS static void set_thread_data(struct thread_local *data) { if (threads_active) pthread_setspecific(key, data); } -#endif static struct base_data *alloc_base_data(void) { @@ -1092,7 +1066,6 @@ static void resolve_base(struct object_entry *obj) find_unresolved_deltas(base_obj); } -#ifndef NO_PTHREADS static void *threaded_second_pass(void *data) { set_thread_data(data); @@ -1116,7 +1089,6 @@ static void *threaded_second_pass(void *data) } return NULL; } -#endif /* * First pass: @@ -1213,7 +1185,6 @@ static void resolve_deltas(void) progress = start_progress(_("Resolving deltas"), nr_ref_deltas + nr_ofs_deltas); -#ifndef NO_PTHREADS nr_dispatched = 0; if (nr_threads > 1 || getenv("GIT_FORCE_THREADS")) { init_thread(); @@ -1229,7 +1200,6 @@ static void resolve_deltas(void) cleanup_thread(); return; } -#endif for (i = 0; i < nr_objects; i++) { struct object_entry *obj = &objects[i]; @@ -1531,11 +1501,11 @@ static int git_index_pack_config(const char *k, const char *v, void *cb) if (nr_threads < 0) die(_("invalid number of threads specified (%d)"), nr_threads); -#ifdef NO_PTHREADS - if (nr_threads != 1) - warning(_("no threads support, ignoring %s"), k); - nr_threads = 1; -#endif + if (!HAVE_THREADS) { + if (nr_threads != 1) + warning(_("no threads support, ignoring %s"), k); + nr_threads = 1; + } return 0; } return git_default_config(k, v, cb); @@ -1723,12 +1693,12 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix) nr_threads = strtoul(arg+10, &end, 0); if (!arg[10] || *end || nr_threads < 0) usage(index_pack_usage); -#ifdef NO_PTHREADS - if (nr_threads != 1) - warning(_("no threads support, " - "ignoring %s"), arg); - nr_threads = 1; -#endif + if (!HAVE_THREADS) { + if (nr_threads != 1) + warning(_("no threads support, " + "ignoring %s"), arg); + nr_threads = 1; + } } else if (starts_with(arg, "--pack_header=")) { struct pack_header *hdr; char *c; @@ -1791,14 +1761,12 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix) if (strict) opts.flags |= WRITE_IDX_STRICT; -#ifndef NO_PTHREADS - if (!nr_threads) { + if (HAVE_THREADS && !nr_threads) { nr_threads = online_cpus(); /* An experiment showed that more threads does not mean faster */ if (nr_threads > 3) nr_threads = 3; } -#endif curr_pack = open_pack_file(pack_name); parse_pack_header(); diff --git a/thread-utils.c b/thread-utils.c index a2135e0743..d205a474e0 100644 --- a/thread-utils.c +++ b/thread-utils.c @@ -20,6 +20,9 @@ int online_cpus(void) { +#ifdef NO_PTHREADS + return 1; +#else #ifdef _SC_NPROCESSORS_ONLN long ncpus; #endif @@ -59,10 +62,12 @@ int online_cpus(void) #endif return 1; +#endif } int init_recursive_mutex(pthread_mutex_t *m) { +#ifndef NO_PTHREADS pthread_mutexattr_t a; int ret; @@ -74,4 +79,29 @@ int init_recursive_mutex(pthread_mutex_t *m) pthread_mutexattr_destroy(&a); } return ret; +#else + return ENOSYS; +#endif +} + +#ifdef NO_PTHREADS +int dummy_pthread_create(pthread_t *pthread, const void *attr, + void *(*fn)(void *), void *data) +{ + return ENOSYS; } + +int dummy_pthread_init(void *data) +{ + /* + * Do nothing. + * + * The main purpose of this function is to break compiler's + * flow analysis or it may realize that functions like + * pthread_mutex_init() is no-op, which means the (static) + * variable is not used/initialized at all and trigger + * -Wunused-variable + */ + return ENOSYS; +} +#endif diff --git a/thread-utils.h b/thread-utils.h index d9a769d190..b8c6500c42 100644 --- a/thread-utils.h +++ b/thread-utils.h @@ -4,12 +4,44 @@ #ifndef NO_PTHREADS #include <pthread.h> -extern int online_cpus(void); -extern int init_recursive_mutex(pthread_mutex_t*); +#define HAVE_THREADS 1 #else -#define online_cpus() 1 +#define HAVE_THREADS 0 + +/* + * macros instead of typedefs because pthread definitions may have + * been pulled in by some system dependencies even though the user + * wants to disable pthread. + */ +#define pthread_t int +#define pthread_mutex_t int + +#define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex) +#define pthread_mutex_lock(mutex) +#define pthread_mutex_unlock(mutex) +#define pthread_mutex_destroy(mutex) + +#define pthread_key_create(key, attr) dummy_pthread_init(key) +#define pthread_key_delete(key) + +#define pthread_create(thread, attr, fn, data) \ + dummy_pthread_create(thread, attr, fn, data) +#define pthread_join(thread, reval) ENOSYS + +#define pthread_setspecific(key, data) +#define pthread_getspecific(key) NULL + +int dummy_pthread_create(pthread_t *pthread, const void *attr, + void *(*fn)(void *), void *data); + +int dummy_pthread_init(void *); #endif + +int online_cpus(void); +int init_recursive_mutex(pthread_mutex_t*); + + #endif /* THREAD_COMPAT_H */