关于在SMP模式下同样的输入文件每次提交计算结果不同问题的说明

hiluckzj

刚才看到一个帖子，讲了在SMP模式下每次提交计算结果不同的情况，我看了一下dyna手册中的×CONTROL_PARALLEL关键字，特将手册翻译如下，翻译不准的地方请同志们指正。
     对于任何给定的问题，当结果一致性选项关闭时（此选项在*control_parallel中），如CONST=2，同样的任务在不同的时间提交，即使CPU数是一样的或者CPU数是不一样的，我们会看到结果会有细微的差别，对照节点加速度常常会显示出较大范围的差异；然而，需要指出的是，加速度计结果常常显示出不明显的变化是因为加速度计的平滑效应通常是与刚体节点相联系的，精度问题，并不是数字模拟中的新问题，而是碰撞仿真的固有问题，因为在这一类问题中在压缩载荷的作用下结构的分歧是很普遍的。这个问题可以很轻易的用一个完美的截面为四边形的薄壁管来演示，在一个压缩载荷下，典型情况是，每次在一个CPU上运行，且每次只做一个很小的修改（如单元或者沙漏公式），每次的运行结果都会有很多的不同，这一点可以从管子的最终形状上看出，同样的，如果同样的问题运行在不同品牌的电脑上也会出现。如果同样的问题在多处理器及其上提交时，即使提交计算的文件没有修改，但是所得出的结果却变化很大。这个问题是由于随机的数字截断（或者说圆整）作为“完美”梁问题的引发器，因为每次计算汇总（CONST=2）是按照不同的次序的，数字截断也是随机的，一致性判据CONST=1提供了每次同一（或相近）结果，无论是在SMP模式下使用一个、两个或者更多的处理器，因为在这种情况下，要求所有相关的全局向量计算在一个精确的顺序下而与处理器无关。当检测结果一致性时，应该比较节点位移或单元应力。（不同批次提交结果）NODOUT和ELOUT文件中的数字应该是一致的，然后，GLSTAT,SEFORC和许多其他ASCII文件应该是不一致的，因为工程量在这些文件中总计是并行的因为效率的原因，并且总计操作的顺序并不是强制的。使用这个选项的最大缺点是至少多15%CPU消耗，如果PARA=1并且使用2个或更多处理器时CPU消耗会少许多。除非PARA判据是打开的（对于非向量处理器），并行缩放比例是起反作用的。一致性判据不作用于MPP模式。

hiluckzj

下面手手册上的原文。
For any given problem with the consistency option off, i.e., CONST=2, slight differences in results are seen when running the same job multiple times with the same number of processors and also when varying the number of processors. Comparisons of nodal accelerations often show wide discrepancies; however, it is worth noting that the results of accelerometers often show insignificant variations due to the smoothing effect of the accelerometers which are generally attached to nodal rigid bodies. The accuracy issues are not new and are inherent in numerical simulations of automotive crash and impact problems where structural bifurcations under compressive loads are common. This problem can be easily demonstrated by using a perfectly square thin-walled tubular beam of uniform cross section under a compressive load. Typically, every run on one processor that includes a minor input change (i.e., element or hourglass formulation) will produces dramatically different results in terms of the final shape, and, likewise, if the same problem is again run on a different brand of computer. If the same problem is run on multiple processors the results can vary dramatically from run to run WITH NO INPUT CHANGE. The problem here is due to the randomness of numerical round-off which acts as a trigger in a “perfect” beam. Since summations with (CONST=2) occur in a different order from run to run, the round-off is also random. The consistency flag,CONST=1, provides for identical results (or nearly so) whether one, two, or more processors are used while running in the shared memory parallel (SMP) mode. This is done by requiring that all contributions to global vectors be summed in a precise order independently of the number of processors used. When checking for consistent results,nodal displacements or element stresses should be compared. The NODOUT and ELOUT files should be digit to digit identical. However, the GLSTAT, SECFORC, and
many of the other ASCII files will not be identical since the quantities in these files are
summed in parallel for efficiency reasons and the ordering of summation operations are not enforced. The biggest drawback of this option is the CPU cost penalty which is at least 15 percent if PARA=0 and is much less if PARA=1 and 2 or more processors are used. Unless the PARA flag is on (for non-vector processors), parallel scaling is
adversely affected. The consistency flag does not apply to MPP parallel.