Intel vs. GCC for the OpenMP vector triad: Barrier shootout!

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Posted on by Georg Hager

We use the Sch√∂nauer Vector Triad for most of our microbenchmarking. It’s a simple benchmark that everyone can write. It looks quite simple when parallelized with OpenMP:

double precision, dimension(N) :: a,b,c,d
! initialization etc. omitted
s = walltime()
!$omp parallel private(R,i)
do R=1,NITER
!$omp do
  do i=1,N
    a(i) = b(i) + c(i) * d(i)
  enddo
!$omp end do
enddo
!$omp end parallel
e=walltime()
MFlops = R*N/(e-s)/1.e6

There are some details that are necessary to make it work as intended; you can read all about this in our book [1]. Usually we choose NITER for every N so that the runtime is a couple hundred milliseconds (so it can be measured accurately), and report performance for N ranging from small to large. The performance of the vector triad is determined by the data transfers, even when the data is in the L1 cache. In the parallel case we can additionally see the usual multicore bandwidth bottleneck(s).

The OpenMP parallelization adds its own overhead, of course. As it turns out, it is mostly concentrated in the implicit barrier at the end of the workshared loop in this case. So, when looking at the performance of the OpenMP code vs. N, we usually see that using more threads slows down the code if N is too small. We can even calculate the barrier overhead from this (again, the book will tell you the gory details).

The barrier overhead varies across compilers and compiler versions, and it depends on the positions of the threads in the machine (e.g., sharing caches or not). You can certainly measure it directly with a suitable microbenchmark [2], but it is quite interesting to see the impact directly in the vector triad performance data.

vtriad_Lima_icpc_vs_gcc

Here we see the OpenMP vector triad performance on one “Intel Xeon Westmere” socket (6 cores) running at about 2.8 GHz, comparing a reasonably current Intel compiler with g++ 4.7.0. With the Intel compiler the sequential code achieves “best possible” performance within the L1 cache (4 flops in 3 cycles). With OpenMP turned on you cannot see this, of course, since the barrier overhead dominates for loop lengths below a couple of 1000s.

Looking at the results for the two compilers we see that GCC has not learned anything over the last five years (this is for how long we have been comparing compilers in terms of OpenMP barrier overhead): The barrier takes roughly a factor of 20 longer with gcc than with the Intel compiler. Comparing with the ECM performance model [3] for the vector triad we see that the Intel compiler’s barrier is fast enough to at least get near the performance limit in the L2 cache (blue dashed line). Both compilers are on par where it’s easy, i.e., in L3 cache and memory, where the loop is so long that the overhead is negligible.

Note that the bad performance of g++ in this benchmark is not due to some “magic” compiler option that I’ve missed. It’s the devastatingly slow OpenMP barrier. For reference, these are the compiler options I have used:

icpc -openmp -Ofast -xHOST -fno-alias ...
g++ -fopenmp -O3 -msse4.2 -fargument-noalias-global ...

In conclusion, the GCC OpenMP barrier is still slooooow. If you have “short” loops to parallelize, GCC is not for you. Of course you might be able to work around such problems (mutilating a popular saying from one of the Great Old Ones: “If synchronization is the problem, don’t synchronize!”), but it’s still good to be aware of them.

If you are interested in concrete numbers you can take a look at any of our recent tutorials [4], where we always include some recent barrier measurements with current compilers.

[1] G. Hager and G. Wellein: Introduction to High Performance Computing for Scientists and Engineers. CRC Press, 2010.

[2] The EPCC OpenMP Microbenchmarks.

[3] G. Hager, J. Treibig, J. Habich, and G. Wellein: Exploring performance and power properties of modern multicore chips via simple machine models. Computation and Concurrency: Practice and Experience, DOI: 10.1002/cpe.3180 (2013), Preprint: arXiv:1208.2908

[4] My Tutorials blog page

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2 comments on “Intel vs. GCC for the OpenMP vector triad: Barrier shootout!

  1. Jeff Hammond says:

    Lots of stuff happens in the thread pool creation that only happens once. Why not run the benchmark with a loop around the parallel region that skips the first time in the timer so that the one-time startup cost isn’t included? Furthermore, why not split the cost of reconciling the thread team from the actual barrier as follows? There are a bunch of checks that occur in the parallel region startup that should be amortized away in a sensible code.


    double precision, dimension(N) :: a,b,c,d
    ! initialization etc. omitted
    !$omp parallel private(R,i)
    s = walltime()
    !$omp barrier
    do R=1,NITER
    !$omp do
    do i=1,N
    a(i) = b(i) + c(i) * d(i)
    enddo
    !$omp barrier
    enddo
    !$omp end parallel
    e=walltime()
    !$omp end parallel
    MFlops = R*N/(e-s)/1.e6

  2. Georg Hager says:

    The major part of the OpenMP overhead in a workshared loop is the implicit barrier at the end. It is true that there is some initial overhead for setting up the thread team; the real benchmark repeats the whole thing many times (using an outer loop not shown above) to determine NITER, so that this particular overhead is not measured. It is clear that in a real application code one may or may not do things to avoid different kinds of overhead, which may be applicable to the benchmark, too. E.g., one may add a “nowait” clause to the loop, preventing the barrier. This would reduce the overhead very much, but it would defeat the purpose of the benchmark: to show the impact of the implicit barrier. On the other hand, one could pull the parallel directive inside and use “parallel do” on the loop. This would strongly increase the overhead.

    I could just as well post the raw numbers from the EPCC benchmarks, but I think it is more impressive to see how much the barrier overhead hurts performance for a simple loop. Looking at performance vs. loop length also provides some insight into where the bottlenecks are: OpenMP overhead for small N, memory bandwidth for large N, and almost perfect scaling in between (hopefully).

    Btw, your code above has two barriers (one implicit at the end of the do loop, and on explicit). Did it get mixed up?

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