Cygwin multithreading performance
Mark Geisert
mark@maxrnd.com
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...
#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.
..mark
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