Re: [PATCH v4] libgfortran: Replace mutex with rwlock

["Zhu, Lipeng" <lipeng.zhu@intel.com>]

On 1/1/1970 8:00 AM, Bernhard Reutner-Fischer wrote:
> On Mon,  8 May 2023 17:44:43 +0800
> Lipeng Zhu <lipeng.zhu@intel.com> wrote:
> 
>> This patch try to introduce the rwlock and split the read/write to
>> unit_root tree and unit_cache with rwlock instead of the mutex to
>> increase CPU efficiency. In the get_gfc_unit function, the percentage
>> to step into the insert_unit function is around 30%, in most
>> instances, we can get the unit in the phase of reading the unit_cache
>> or unit_root tree. So split the read/write phase by rwlock would be an
>> approach to make it more parallel.
>>
>> BTW, the IPC metrics can gain around 9x in our test server with 220
>> cores. The benchmark we used is https://github.com/rwesson/NEAT
> 
> See commentary typos below.
> You did not state if you regression tested the patch?
I use valgrind --tool=helgrind or --tool=drd to test 'make 
check-fortran'. Is it necessary to add an additional unit test for this 
patch?

> Other than that it LGTM but i cannot approve it.
Thank you for your kind help for this patch, is there anything that I 
can do or can you help to push this patch forward?

> 
>> diff --git a/libgfortran/io/async.h b/libgfortran/io/async.h index
>> ad226c8e856..0033cc74252 100644
>> --- a/libgfortran/io/async.h
>> +++ b/libgfortran/io/async.h
>> @@ -210,6 +210,128 @@
>>       DEBUG_PRINTF ("%s" DEBUG_RED "ACQ:" DEBUG_NORM " %-30s %78p\n", aio_prefix, #mutex,
> mutex); \
Thanks, corrected in Patch v5.

>>     } while (0)
>>   
>> +#ifdef __GTHREAD_RWLOCK_INIT
>> +#define RWLOCK_DEBUG_ADD(rwlock) do {		\
>> +    aio_rwlock_debug *n;				\
>> +    n = xmalloc (sizeof(aio_rwlock_debug));	\
> 
> Missing space before the open brace: sizeof (
> 
Thanks, corrected in Patch v5.

>> diff --git a/libgfortran/io/unit.c b/libgfortran/io/unit.c index
>> 82664dc5f98..62f1db21d34 100644
>> --- a/libgfortran/io/unit.c
>> +++ b/libgfortran/io/unit.c
>> @@ -33,34 +33,36 @@ see the files COPYING3 and COPYING.RUNTIME
>> respectively.  If not, see
>>   
>>   
>>   /* IO locking rules:
>> -   UNIT_LOCK is a master lock, protecting UNIT_ROOT tree and UNIT_CACHE.
>> +   UNIT_RWLOCK is a master lock, protecting UNIT_ROOT tree and UNIT_CACHE.
>> +   And use the rwlock to spilt read and write phase to UNIT_ROOT tree
>> +   and UNIT_CACHE to increase CPU efficiency.
> 
> s/spilt/split. Maybe:
> 
> Using an rwlock improves efficiency by allowing us to separate readers and writers of both UNIT_ROOT
> and UNIT_CACHE.
> 
Thanks, corrected in Patch v5.

>> @@ -350,6 +356,17 @@ retry:
>>         if (c == 0)
>>   	break;
>>       }
>> +  /* We did not find a unit in the cache nor in the unit list, create a new
>> +    (locked) unit and insert into the unit list and cache.
>> +    Manipulating either or both the unit list and the unit cache requires to
>> +    hold a write-lock [for obvious reasons]:
>> +    1. By separating the read/write lock, it will greatly reduce the contention
>> +       at the read part, while write part is not always necessary or most
>> +       unlikely once the unit hit in cache.
> 
> +    By separating the read/write lock, we will greatly reduce the contention
> +    on the read part, while the write part is unlikely once the unit hits
> +    the cache.
> 
>> +    2. We try to balance the implementation complexity and the performance
>> +       gains that fit into current cases we observed by just using a
>> +       pthread_rwlock. */
> 
> Let's drop 2.

Got it, thanks!
> thanks,