H5P_SET_DXPL_MPIO_CHUNK_OPT_RATIO sets a threshold for the use of collective I/O based on the ratio of processes with collective access to a dataset with chunked storage. The decision whether to use collective I/O is made on a per-chunk basis.
The library will calculate the percentage of the total number of processes, the ratio, that hold selections in each chunk. If that percentage is greater than the threshold set in
percent_proc_per_chunk, the library will do collective I/O for this chunk; otherwise, independent I/O will be done for the chunk.