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@xxxxxxxx> 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
