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Re: Cygwin multithreading performance
- From: Kacper Michajlow <kasper93 at gmail dot com>
- To: cygwin at cygwin dot com
- Date: Sat, 5 Dec 2015 14:07:54 +0100
- Subject: Re: Cygwin multithreading performance
- Authentication-results: sourceware.org; auth=none
- References: <CABPLASTtRK4mNxh0M_AnZgjJQ15kWPx+L=U=VCU3Wwi7jV_57A at mail dot gmail dot com> <564E3017 dot 90205 at maxrnd dot com> <CABPLASTLrH_udLuu2F-m5P6dkENW1Z4YHEudp4NG0-FGLJgPMg at mail dot gmail dot com> <5650379B dot 4030405 at maxrnd dot com> <20151121105301 dot GE2755 at calimero dot vinschen dot de> <5652C402 dot 7040006 at maxrnd dot com> <24780-1448274431-7444 at sneakemail dot com> <5653B52B dot 5000804 at maxrnd dot com> <20151126093427 dot GJ2755 at calimero dot vinschen dot de> <5656DDEF dot 9070603 at maxrnd dot com> <5662C199 dot 7040906 at maxrnd dot 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.
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