Re: Cygwin multithreading performance

[Mark Geisert <mark at maxrnd dot com>]

Mark Geisert wrote:
> Corinna Vinschen wrote:
> > On Nov 23 16:54, Mark Geisert wrote:
> > > John Hein wrote:
> > > > Mark Geisert wrote at 23:45 -0800 on Nov 22, 2015:
> > > >   > Corinna Vinschen wrote:
> > > >   > > On Nov 21 01:21, Mark Geisert wrote:
> > > >   > [...] so I wonder if there's
> > > >   > >> some unintentional serialization going on somewhere, but I
> > > > don't know yet
> > > >   > >> how I could verify that theory.
> > > >   > >
> > > >   > > If I'm allowed to make an educated guess, the big serializer
> > > > in Cygwin
> > > >   > > are probably the calls to malloc, calloc, realloc, free.  We
> > > > desperately
> > > >   > > need a new malloc implementation better suited to
> > > > multi-threading.
> [...]
> > > > Someone recently mentioned on this list they were working on porting
> > > > jemalloc.  That would be a good choice.
> > >
> > > Indeed; thanks for the reminder.  Somehow I hadn't followed that thread.
> >
> > Indeed^2.  Did you look into the locking any further to see if there's
> > more than one culprit?  I guess we've a rather long way to a "lock-less
> > kernel"...
[...]
> But that is just groundwork to identifying which locks are suffering the
> most contention.  To identify them at source level I think I'll also
> need to record the caller's RIP when they are being locked.

In the OP's very good testcase the most heavily contended locks, by far, 
are those internal to git's builtin/pack-objects.c.  I plan to show 
actual stats after some more cleanup, but I did notice something in that 
git source file that might explain the difference between Cygwin and 
MinGW when running this testcase...

#ifndef NO_PTHREADS

static pthread_mutex_t read_mutex;
#define read_lock()             pthread_mutex_lock(&read_mutex)
#define read_unlock()           pthread_mutex_unlock(&read_mutex)

static pthread_mutex_t cache_mutex;
#define cache_lock()            pthread_mutex_lock(&cache_mutex)
#define cache_unlock()          pthread_mutex_unlock(&cache_mutex)

static pthread_mutex_t progress_mutex;
#define progress_lock()         pthread_mutex_lock(&progress_mutex)
#define progress_unlock()       pthread_mutex_unlock(&progress_mutex)

#else

#define read_lock()             (void)0
#define read_unlock()           (void)0
#define cache_lock()            (void)0
#define cache_unlock()          (void)0
#define progress_lock()         (void)0
#define progress_unlock()       (void)0

#endif

Is it possible the MinGW version of git is compiled with NO_PTHREADS 
#defined?  If so, it would mean there's no locking being done at all and 
would explain the faster execution and near 100% CPU utilization when 
running under MinGW.

..mark


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