Testing Efficiency of Parallel IO Software

1
Testing Efficiency of Parallel I/O Software

• Weikuan Yu, Jeffrey Vetter
• December 6, 2006

2
Testing Parallel IO at ORNL

• Earlier analysis of scientific codes running at
  ORNL/LCF
• Most users have limited I/O capability in their
  applications because they have not had access to
  portable, widespread PIO
• Seldom direct use of MPI-IO
• Little use of high-level IO middleware PnetCDF
  or HDF5
• Large variance in performance of parallel IO
  using different software stacks (as demonstrated
  in VH1 experiments)
• Ongoing work
• Collect application IO access pattern and Lustre
  server IO traces, with tau, craypat, mpiP, etc
• Testing other parallel IO components over Lustre
• Analysis, benchmarking and optimization of data
  intensive scientific codes

3
Parallel IO Optimization at ORNL

• Parallel IO over Lustre
• A new file system still relies on a generic ADIO
  implementation
• Generations of platforms at ORNL demands
  efficient parallel IO
• Performance with Jaguar
• Good read/write bandwidth for large shared single
  file
• Not scalable for small read/write and parallel IO
  management operations (metadata)
• Approaches for Optimizations
• Providing a specific, ADIO implementation
  well-tuned for Lustre
• Investigating parameters for adjusting striping
  pattern
• Exploited Lustre file joining
• Regular files can be joined in place
• Split writing and hierarchical striping
• Developed a prototype on an 80-node Linux cluster
• Paper submitted to CCGrid 2006, available if
  interested

4
Some Characteristics of Lustre IO Performance

• Performance can be significantly affected by
  stripe width
• Need to introduce flexibilities in striping
  pattern
• Exploit file joining for growing stripe width
  with increasing file size

5
Explore Lustre File Joining

• Split writing
• Create/write a shared file as multiple small
  files, aka subfiles
• Temporary structure to hold file attributes
• Subfiles joined at the closing time

open
File Attributes
read/write
Subfiles
close
Joined file
Diagram of Split Writing
6
Hierarchical Striping

• Hierarchical striping
• Create another level of striping pattern for
  subfiles
• Allow maximum coverage of Lustre storage targets
• Mitigate the impact of striping overhead

Diagram of Hierarchical Striping (HS) (HS width
N1 HS size Sw)
subfile 0
subfile 1
subfile n
ost0
ost1
ost2
ost3
ost2n
ost2n1
S1
S
0
1
nS1
nS
S2
S3
2
3
nS2
nS3
S-2
S-1
2S-2
2S-1
nS-2
nS-1
(Stripe width 2 Stripe size w)
7
Evaluation
Table 1 Scalability of Management Operations
Table 2 Performance of Collective Read/Write

• Write/Read performance improved dramatically for
  new files
• Read/Write of an existing join file is not well
  performing due to a non-optimized IO path for a
  join file in Lustre

• Scalability of file open and file resize improved
  dramatically

8
Results on Scientific Benchmarks MPI-Tile-IO
and BT/IO

• IO Pattern as represented by BT-IO can be
  improved if the number of iterations is small. It
  may help if an arbitrary number of files can be
  joined.
• Write Performance in MPI-Tile-IO can be improved
  dramatically
• Read performance in MPI-Tile-IO cannot be
  improved by file joining because reading an
  existing join file does not perform well

9
Conclusions

• Parallel IO over Lustre
• Split writing can improve metadata management
  operations
• Stripe overhead can be mitigated with careful
  augmentations of stripe width
• Lustre file joining
• Race conditions when joining files multiple
  processes
• Low read/write performance on an existing file
• Not possible for arbitrary hierarchical striping
  because limited number of files can be joined
• Need improvement before its production usage
  parallel IO
• Next Steps
• Continue optimization of parallel IO at ORNL,
• Adapting the earlier techniques to liblustre on
  XT3/XT4
• Develop/Exploit other features, group locks and
  dynamic stripe width
• Adapting parallel I/O and parallel FS to wide
  area collaborative science with other IO
  protocols such as pNFS and Logistical Networking