Re: Cygwin multithreading performance

[Kacper Michajlow <kasper93 at gmail dot com>]

2015-12-05 23:40 GMT+01:00 Mark Geisert <mark@maxrnd.com>:
> Kacper Michajlow wrote:
>>
>> 2015-12-05 11:51 GMT+01:00 Mark Geisert <mark@maxrnd.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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Hmm, thinking about it mingw doesn't have pthread implementation or
any wrapper for it. If someone needs pthread they would probably go
for pthreads-w32 implementation.

I started to wonder because I don't recall git would need pthreads to
compile on Windows. And indeed they have a wrapper for Windows API...
https://github.com/git/git/blob/master/compat/win32/pthread.h
https://github.com/git/git/blob/master/compat/win32/pthread.c

Though it is not really a matter that "native" git build is fast and
all, but that Cygwin's one really struggles if it comes to MT workload
.

And this not only issue with git unfortunately. Download speeds are
also limited on Cygwin. I know POSIX compatibility layers comes with a
price but I would love to see improvements in those areas.
Cygwin:
Receiving objects: 100% (230458/230458), 78.41 MiB | 1.53 MiB/s, done.
"native" git:
Receiving objects: 100% (230458/230458), 78.41 MiB | 18.54 MiB/s, done.

I'm on Windows 10 x64 and i7 5820K (6C/12T).

-Kacper

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