pnetcdf strange behavior

[Richard Hedges]

It all depends on how the data is laid out.  If the 8th task will be 
writing bytes 7000000001 through 8000000000, the file size will be 
700000001 after that task writes only 1 byte, for example.

I would guess that this is the phenomena that you have observed.


At 7:55 PM -0400 4/12/07, Christopher Subich wrote:
>Hongzhang Shan wrote:
>>slower from 7GB to 8GB. Does this mean something goes wrong with my
>>code or the first 7GB is only allocated without actually writing the
>>data.
>
>The file must be allocated up to the Nth bit before that bit can be 
>written.  One (likely) possibility is that process 7 writes to the 
>file fairly early, meaning that the file is (sparsely) allocated up 
>through that process's contribution.  Whenever process 8 gets a 
>turn, the remainder of the file is allocated and written.
>
>You can see this working for yourself; test total writes (for 8 
>processes) of 2GB/4GB/8GB, and you should see essentially linear 
>scaling.


-- 



====================================================

Richard Hedges
Scalable I/O Project
Development Environment Group - Livermore Computing
Lawrence Livermore National Laboratory
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