[MPICH] slow IOR when using fileview

[Yu, Weikuan]

> If the independent 
> access is used instead, I don't know why each write is divided into 512 KB 
> chunks and locking is ever needed to guaranteed the atomic access of the 
> 10 MB contiguous file range. For this particular access pattern, ROMIO 
> should not do read-modify-write at all.

512KB is the default buffer size for data sieving. So with 512KB buffer size, each process is only able to write out 512KB data in each call of ADIOI_GEN_WriteStrided. For 10MB, this results in 20 iterations of write_all(), 40 fcntl() total. crayPat indicates that fcntl() takes 88% of the total Wall clock time with fileview, 0% w/o fileview.

--Weikuan