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Subject: Re: Cygwin multithreading performance
To: cygwin AT cygwin DOT com
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From: Mark Geisert <mark AT maxrnd DOT com>
Message-ID: <566367C8.5020703@maxrnd.com>
Date: Sat, 5 Dec 2015 14:40:08 -0800
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Kacper Michajlow wrote:
> 2015-12-05 11:51 GMT+01:00 Mark Geisert <mark AT maxrnd DOT com>:
>> Mark Geisert wrote:
>> 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

We're not familiar at all with MinGW.  Could you locate the source for 
MinGW's pthread_mutex_lock() online and give us a link to it?  And BTW, 
which Windows are you running and on what kind of hardware (bitness and 
#CPUS/threads)?

It looks like we're going to have to compare actual pthread_mutex_lock() 
implementations.  Inspecting source is nice but I don't want to be 
chasing a mirage so I really hope there's a pthread_mutex_lock() 
function inside the MinGW git you are running.  gdb could easily answer 
that question.  Could you please do an 'info func pthread_mutex_lock' 
after starting MinGW git under MinGW gdb with a breakpoint at main() (so 
libraries are loaded).

..mark


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