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

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

On 5/9/2023 10:32 AM, Zhu, Lipeng wrote:
> 
> 
> 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?
> 
Hi Bernhard,

Is there any other refinement that need I to do for this patch?

Thanks.


>>
>>> 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,