XFEL

Decoupling Architecture for All-to-all Computation

Outline

  • SACLA XFEL and the K Computer
  • Analyzing Diffraction Images
  • Load Balancing and Minimizing I/O
  • Decoupling Architecture
  • Evaluation
  • Summary

SACLA and the K Computer

  • Data Acquisition: SACLA (XFEL Facility)
    XFEL: X-ray Free Electron Laser
  • Data Processing: the K computer
  • To Analyze 3D Structure of Particles

Analyzing Diffraction Images (1/2)

  • Orientations of target particles are uncontrollable
    • Correlation (FFT) is the clue of viewpoint => Clustering
    • All-to-all computation is needed O(N2) !
  • Diffraction image contains quantum noise
    • 100 images must be averaged. 1 million images must be shot !!

Analyzing Diffraction Images (2/2)

  • 2-D Cartesian coordinate image is converted to Polar coordinate image (coaxial rings)
  • Each ring is FFTed
  • Two images are compared (correlation), ring by ring
  • Finally, resulting "a correlation line" indicating they are close enough
High-speed classification of coherent X-ray diffraction patterns on the K computer for high-resolution single biomolecule imaging
(A. Tokuhisa, et al.), In Journal of Synchrotron Radiation, volume 20, 2013.

Fact Sheet

  • Data Acquisition (SACLA)
    • max. 30 images/sec (depending on the kind of target particle)
    • 20MB/image (3Å resolution) (depending on resolution)
    • 1 million images for one particle analysis
      • quantum noise and all possible orientations yielding 20TB in 10 hours !
  • Data Transfer (from SACLA to "K")
    • Gfarm copy tool (gfpcopy) takes 20 hours to copy 20TB data
  • Data Processing ("K")
    • O(N2) , but can be reduced to O(K·N) K is the number of clustering groups

Load Balancing and Minimizing I/O

  • Load must be balanced
  • Each data file must be read only once
    Read data are passed to neighbors at every step
    • #steps is (N+1)/2 (N is #chunks or #nodes)

Stage1 and Stage2


I/O Optimization

  • Parallel file system can be a bottleneck
  • Maximizing parallelism up to #nodes
    Utilizing the K file system
    • File staging system, and
    • "Rank-local" file system

I/O Performance

  • Computation time
    • 5Å 0.06 Sec/2images
    • 3Å 0.45 Sec/2images
  • Communication time
    • Hidden
  • Read Time
  • Computation time dominates ("K", 80K nodes)
    • 5Å 1 hour
    • 3Å 24 hours

Decoupling Architecture

  • Motivation
    • Generalizing "all-to-all computation"
      • Lowering the threshold to use the K computer
  • Decoupling Architecture
    • Decoupling the kernel code and the other system programing staff
      • by using the glibc popen() function

popen

  • popen creates a subprocess, and
  • returns a file pointer of a pipe which is connected with the stdout or stdin of the subprocess

Carp

  • pCarp
    • Parallel version for production run
  • sCarp
    • Sequential version for kernel development and debugging

Code Skeleton

  • Users do not care about
    • Programming Language
    • MPI
    • Load Balance
    • I/O

pCarp Performance

  • Original Program
    • 240 Sec.
  • pCarp + Sequential Programs
    • 430 Sec. (+190 Sec.)
      • Pipe Transfer: 159 GBytes
      • Pipe BW: 1 GB/s
      • Most of the overhead comes from the transfer via the pipe (160 Sec.) !!

Pipe Performance


Summary

  • Decoupling Architecture
    • Users do not to worry about the parallel programing (I/O, communication, loadbalancing, ...)
    • Users can write their programs in any programing languages

Future Work

  • [ps]Carp improvement - in the future
    • Increasing the pipe buffer size in the Linux kernel
      • fcntl( fd, F_SETPIPE_SZ )
        • Linux 2.6.35~, or
    • Developing new "popen" implementation to improve the bandwidth
  • Having applications other than XFEL

XFEL in the World

http://en.wikipedia.org/wiki/XFEL#X-ray_FELs