[PATCH v2] Improve the performance of --num-threads -d 31

["Zhou, Wenjian/周文?"]

Hi,

On 03/02/2016 11:05 AM, Minoru Usui wrote:
> Hi, Zhou
> 
>>>>>>> ===
>>>>>>>       producer                   Consumer
>>>>>>> ---------------------------------------------------------
>>>>>>>       pthread_mutex_lock()
>>>>>>>       pfn = info->current_pfn
>>>>>>>                                  info->current_pfn = end_pfn
>>>>>>>       info->current_pfn++
>>>>>>>         -> end_pfn + 1
>>>>>>>       pthread_mutex_unlock()
>>>>>>> ===
>>
>> How about just changing "info->current_pfn = end_pfn" to "info->current_pfn--" ?
>> Just like the first version of the patch.
> 
> If you don't get mutex lock in consumer side, this change is meaningless.
> Of course, info->current_pfn may equal to end_pfn at the end of the cycle,
> but there is a timing that info->current_pfn is bigger than end_pfn in processing producer thread.
> 
> The root cause is producer increments info->current_pfn everytime, even if info->current_pfn == end_pfn
> in following code.
> 

Actually, I didn't get what you mean exactly until this letter...

I think there is no problem if the info->current_pfn is larger than the end_pfn
in the function write_kdump_pages_parallel_cyclic(), for no other one will use
current_pfn here.
Since we can't and needn't prevent using info->current_pfn outside the function,
we should keep info->current_pfn correct before returning from the function.

> ===
>>>>> +			/* get next pfn */
>>>>> +			pthread_mutex_lock(&info->current_pfn_mutex);
>>>>> +			pfn = info->current_pfn;
>>>>> +			info->current_pfn++;                        # increment everytime
>>>>> +			page_flag_buf->ready = FLAG_FILLING;
>>>>> +			pthread_mutex_unlock(&info->current_pfn_mutex);
>>>>>
>>>>> -			buf_ready = TRUE;
>>>>> +			page_flag_buf->pfn = pfn;
>>>>>
>>>>> -			page_data_buf[index].pfn = pfn;
>>>>> -			page_data_buf[index].ready = 1;
>>>>> +			if (pfn >= kdump_thread_args->end_pfn) {
>>>>> +				page_data_buf[index].used = FALSE;
>>>>> +				page_flag_buf->ready = FLAG_READY;
>>>>> +				break;                             # not decrement
>>>>> +			}
> ===
> 
> If you don't allow info->current_pfn is bigger than end_pfn,
> you don't need to increment info->current_pfn when pfn >= kdump_thread_args->end_pfn like following.
> 
> ===
>                          /* get next pfn */
>                          pthread_mutex_lock(&info->current_pfn_mutex);
>                          pfn = info->current_pfn;
>                          page_flag_buf->pfn = pfn;
>                          if (pfn >= kdump_thread_args->end_pfn) {
>                                  page_data_buf[index].used = FALSE;
>                                  page_flag_buf->ready = FLAG_READY;
>                                  pthread_mutex_unlock(&info->current_pfn_mutex);
>                                  break;
>                          }
>                          page_flag_buf->ready = FLAG_FILLING;
>                          info->current_pfn++;
>                          pthread_mutex_unlock(&info->current_pfn_mutex);
> ===
> 
> If you allow info->current_pfn is bigger than end_pfn, producer doesn't need to change info->current_pfn.
> 

I also have thought about it.
It can keep current_pfn never larger than the end.
But it also makes the code a bit more complex.
If there aren't any special reason, I don't think it's worth to do it.

-- 
Thanks
Zhou