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Lecture 22: OpenMP Work Sharing

Lecture Summary

  • Last time
    • OpenMP: Tasks, variable scoping, synchronization (barrier & critical constructs)
  • Today
    • Wrap up synchronization
    • OpenMP rules of thumb
    • Parallel computing w/ OpenMP: NUMA aspects & how caches come into play

Synchronization

  • The atomic directive
    • A guarded memory access operation
    • Can only protect a single assignment
    • Applies only to simple update of memory
    • Is a special case of a critical section with significantly less overhead due to atomicity
  • The reduction construct (see example down below)
    • Local copy of sum for each thread engaged in the reduction is private
      • Each local sum is initialized to the identity operand associated with the operator that comes into play. In this case, we have "+", so the init value is 0.
    • All local copies of sum are added together and stored in a "global" variable
    • #pragma omp for reduction(op:list)
      • The variables in list will be shared in the enclosing parallel region
  • The simd directive
    • #pragma omp for simd reduction(+:sum)
The atomic directive
The reduction construct

Performance Issues

  • Common causes are:
    • Too much sequential code in your app
      • Seek to reduce amount of execution time where only one thread executes code
    • Too much communication
      • Difficult to pin down costly memory operations
    • Load imbalance
      • One thread gets too much work, while others idle waiting for it
      • For OpenMP for, one can use schedule(runtime)
        • Example: setenv OMP_SCHEDULE "dynamic,5"
    • Synchronization
      • Barriers can be expensive
      • Avoid them using
        • Careful use of the nowait clause
        • Parallelize at the outermost level possible
        • Use critical or atomic
        • Use other OpenMP facilities like reduce
    • Compiler (non-)optimizations
      • Sometimes the addition of parallel directives can prevent the compiler from performing sequential optimization
      • Symptom: parallel code running with 1 thread has longer execution and higher instruction count than sequential code

NUMA

  • Up to this point, we have been using the Symmetric Multi-Processing (SMP) model and we haven't been concerned about the mechanics of shared memory access
  • In today's servers/clusters, nodes have many CPUs, each with many cores (this is called multi-socket configurations, as opposed to one chip per motherboard), and not all memory access are equal
  • NUMA: Non-uniform memory access
    • Cost of memory access depends on which memory bank stores your data
  • The NUMA factor: the ratio between the largest and shortest average amount of time for a thread running on a particular core to reach data in memory
    • A low NUMA factor is desirable (not much of a difference which bank data is stored)
    • Numa factor = 1: SMP system
    • Accessing memory outside a NUMA node: 20% slowdown for reads, 30% slowdown for writes
  • NUMA aspects where OS comes into play
    • When a thread mallocs memory, how should this memory be allocated
    • Affinity: How the runtime/OS assigns a thread to a certain core
      • OMP_PROC_BIND: Allows you to dictate a distribution policy
        • master: Collocate threads with the master thread
        • close: Place threads close to the master in the places list
          • Useful if code is compute-bound and don't do many trips to main memory
          • Reduce synchronization costs (single, barrier, etc.)
        • spread (default): Spread out thread as much as possible
          • Useful if code is memory-bound as it improves aggregate system memory bandwidth
        • false: Set no binding
        • true: lock thread to a core
      • OMP_PLACES: Allows you to control locations. OMP_PLACES can assume one of these values
        • threads: Hardware thread, assuming hyper threading is on
        • cores: Core
        • sockets: Node (socket)
        • A place list: Defined by user, explicitly referencing the underlying hardware of the machine
      • An extensive list of examples can be found in the slides
OMP_PLACES usage. The CPU ids can be found through `numactl -H` or `lscpu`