關于在SMP模式下同樣的輸入文件每次提交計算結果不同問題的說明

2016-11-15 by:CAE仿真在線來源:互聯網

剛才看到一個帖子,講了在SMP模式下每次提交計算結果不同的情況,我看了一下dyna手冊中的×CONTROL_PARALLEL關鍵字,特將手冊翻譯如下,翻譯不準的地方請同志們指正。
對于任何給定的問題,當結果一致性選項關閉時(此選項在*control_parallel中),如CONST=2,同樣的任務在不同的時間提交,即使CPU數是一樣的或者CPU數是不一樣的,我們會看到結果會有細微的差別,對照節點加速度常常會顯示出較大范圍的差異;然而,需要指出的是,加速度計結果常常顯示出不明顯的變化是因為加速度計的平滑效應通常是與剛體節點相聯系的,精度問題,并不是數字模擬中的新問題,而是碰撞仿真的固有問題,因為在這一類問題中在壓縮載荷的作用下結構的分歧是很普遍的。這個問題可以很輕易的用一個完美的截面為四邊形的薄壁管來演示,在一個壓縮載荷下,典型情況是,每次在一個CPU上運行,且每次只做一個很小的修改(如單元或者沙漏公式),每次的運行結果都會有很多的不同,這一點可以從管子的最終形狀上看出,同樣的,如果同樣的問題運行在不同品牌的電腦上也會出現。如果同樣的問題在多處理器及其上提交時,即使提交計算的文件沒有修改,但是所得出的結果卻變化很大。這個問題是由于隨機的數字截斷(或者說圓整)作為“完美”梁問題的引發器,因為每次計算匯總(CONST=2)是按照不同的次序的,數字截斷也是隨機的,一致性判據CONST=1提供了每次同一(或相近)結果,無論是在SMP模式下使用一個、兩個或者更多的處理器,因為在這種情況下,要求所有相關的全局向量計算在一個精確的順序下而與處理器無關。當檢測結果一致性時,應該比較節點位移或單元應力。(不同批次提交結果)NODOUT和ELOUT文件中的數字應該是一致的,然后,GLSTAT,SEFORC和許多其他ASCII文件應該是不一致的,因為工程量在這些文件中總計是并行的因為效率的原因,并且總計操作的順序并不是強制的。使用這個選項的最大缺點是至少多15%CPU消耗,如果PARA=1并且使用2個或更多處理器時CPU消耗會少許多。除非PARA判據是打開的(對于非向量處理器),并行縮放比例是起反作用的。一致性判據不作用于MPP模式。

下面手手冊上的原文。
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.

注:在971R5.0中已經廢棄了這個卡片,

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