Re: Cygwin multithreading performance

[Kacper Michajlow <kasper93 at gmail dot com>]

2015-12-05 14:07 GMT+01:00 Kacper Michajlow <kasper93@gmail.com>:
> 2015-12-05 11:51 GMT+01:00 Mark Geisert <mark@maxrnd.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.
>
> Nah, there is no threading enabled when there is no pthreads. How
> would that work? :D See thread-utils.h
>
> #ifndef NO_PTHREADS
> #include <pthread.h>
>
> extern int online_cpus(void);
> extern int init_recursive_mutex(pthread_mutex_t*);
>
> #else
>
> #define online_cpus() 1
>
> #endif
>
>
> Looks like there is indeed a bug in git code when passing "--threads"
> explicitly to "git pack-objects", because they show warning about
> threads being unsupported, but doesn't overwrite delta_search_threads
> value. I will go to git's ML about it. This is completely not related
> to our issue.

Obviously I was wrong. There is
#define ll_find_deltas(l, s, w, d, p)    find_deltas(l, &s, w, d, p)
So 'delta_search_threads' value is never used. Still not related to
cygwin issue tho ;)

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