pnetcdf-write-nb.c non-blocking

[Wei-keng Liao]

Hi, Rob,

You are right about the prohibition of reusing buffers in nonblocking APIs.

Just to make this limitation more clear. Reusing user buffers or using
overlapped user buffers are prohibited in general. This limitation is not
only for across nonlocking APIs but also for mixed nonblocking and blocking
APIs. The reason is because the user buffer may be used internally by
PnetCDF to do an in-place byte swap operation.

However, if PnetCDF is built with --disable-in-place-swap, then
the write buffer will be copied to a temporary buffer before committed
to the file. In this case, the write buffers might technically be reused
or overlapped.


Wei-keng

On Sep 14, 2015, at 2:45 PM, Rob Latham wrote:

> 
> 
> On 09/07/2015 04:25 PM, Wei-keng Liao wrote:
>> Hi,
>> 
>> The example program is buggy.
>> The write buffers for the two iput calls should be different.
>> FYI. Once a nonblocking iput/iget API is posted, the contents
>> of the buffers should not be touched before the call to
>> ncmpi_wait_all/ncmpi_wait. Otherwise, the results can be incorrect.
>> 
>> Please see the bug fix in http://trac.mcs.anl.gov/projects/parallel-netcdf/changeset/2095
>> Thanks for reporting this problem.
> 
> Hi Wei-keng:  I wrote that example and did indeed follow the rule that the contents of the buffers should not be touched before the call to ncmpi_wait_all. -- the 'data' buffer is the same for both calls and is unmodified.
> 
> My mistake was that I remembered the buffer reuse rule (for both MPI and pnetcdf) as "don't change the buffer" but it's more than that: I just wanted to make it clear that the rules for buffer reuse prohibit not only modification but any reads as well.
> 
> ==rob
> 
>> 
>> Wei-keng
>> 
>> On Sep 7, 2015, at 1:52 PM, W. Miah wrote:
>> 
>>> Hello,
>>> 
>>> I have downloaded pNetCDF and am browsing through the example code
>>> parallel-netcdf-1.6.0/examples/tutorial/pnetcdf-write-nb.c (attached
>>> with this email) which should write the MPI ranks into two arrays, but
>>> instead seems to be writing random values:
>>> 
>>> [miahw at bengal tutorial]$ ncdump output.nc
>>> netcdf output {
>>> dimensions:
>>>    d1 = 4 ;
>>> variables:
>>>    int v1(d1) ;
>>>    int v2(d1) ;
>>> 
>>> // global attributes:
>>>        :string = "Hello World\n",
>>>            "" ;
>>> data:
>>> 
>>> v1 = 0, 16777216, 33554432, 50331648 ;
>>> 
>>> v2 = 0, 16777216, 33554432, 50331648 ;
>>> }
>>> 
>>> I have looked at the code but can't see anything wrong with it. Could
>>> it be that the library has a bug?
>>> 
>>> Regards,
>>> 
>>> --
>>> web: http://miahw.wordpress.com
>>> gnupg: 2E29 B22F
>>> <pnetcdf-write-nb.c>
>> 
> 
> -- 
> Rob Latham
> Mathematics and Computer Science Division
> Argonne National Lab, IL USA