Cygwin multithreading performance

Mark Geisert
Sat Dec 5 10:51:00 GMT 2015

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...


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)


#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


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.


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