How come i get different output answers with the same matlab version, the same code installed on two different computers?

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Siyu on 21 May 2014
Edited: Ba Mo on 4 Oct 2020
Hi all,
I have written a quite long code to simulate dam break problem, in which a matrix needs to be solved out to get the pressure contour for the water. and i got a problem when i run the code on my desktop and laptop for the comparison purpose. the results obtained from these two computers agree well at a few beginning steps. after around 200 steps, desktop one has the singularity problem when solving the matrix while laptop can keep running. eventually, laptop one stops at 900 steps also due to singularity problem.
I compared the two results one step by one step and found that at first a few steps, the results are same. after a certain timestep, there are small differences in two results. the results are the same up to first 15 digits. it's only different on the 16th digit like 1620.66808491866 with 1620.66808491869. and then the error is accumulating, and become bigger and bigger.
I am pretty sure the codes i am using are the same, the version of matlab are the same, but how come it gives me different answers. Can anyone answer my questions? Thanks in advance.
Ryan Zhao
Ryan Zhao on 22 Oct 2015
Actually I had similiar experience. But my case is that, changing the version of Matlab (from 64-bit to 32-bit version) so that the two PCs has the same compiler, same matlab codes. But this does not make a difference. Still, my code can run on only one computer. Could you tell me how did you solve this question eventually? Thank you.

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Accepted Answer

Mahdi on 26 May 2014
I am assuming that your problem is ill-conditioned or is very sensitive and depends greatly on the numbers from each step (or the steps before). I believe that the reason that you're getting different results is because you're probably using 32-bit version of MATLAB on one machine and 64-bit on the other.
If you install the same version on both computers (32 on both), I think you will get the same results. Put simply, this change happens because of the number of digits/computations each computer is able to store.

More Answers (3)

Walter Roberson
Walter Roberson on 22 Oct 2015
Round off error can be different from run to run in some cases:
  • Different number of cores when using explicit parallelization
  • Different order that results were available in when using explicit parallelization
  • With sufficiently large matrices and some patterns of operations, MATLAB may call into highly optimized libraries that use parallelization; with different number of cores or different orders that the cores report in, the results can be different
  • When the CPU manufacturers differ, in particular Intel vs AMD, the set of available instructions for high performance operations can differ
  • On MS Windows, Intel makes available the MLK library for high performance mathematics; AMD provides a similar library that is not identical
  • Within any given CPU manufacturer, instruction sets and instruction timings differ between model lines, affecting the order that cores report results
  • Within any given model line of a CPU manufacturer, different designs (e.g. i3 vs i5 vs i7) of the architecture have different instruction timings, affecting the order that cores report results
  • Within any one design of a CPU (e.g. i3 vs i5 vs i7), different releases have different microkernels, some of which will be improvements in speed and some of which will be bug fixes
  • Within any one design and release of a CPU (e.g., one release of i7), different manufacturering techniques are used, such as copper vs aluminum, 7 micro vs 11 micro; these can result in different instruction timings, affecting the order that different cores report in
  • For any one design an implementing technology, the devices are sometimes constructed to shut down or enable portions of the device depending upon load and device temperature, in order to lower device power drain and manage thermal load; this can affect timings and so the order that different cores report in
  • different GPUs manufacturers can result in different round off
  • different GPUs models within one manufacturer have different memory size and numbers of processing units, resulting in work being dispatched in different orders and gathered in different orders
  • The system has to keep processing whatever else is going on, so there are going to be interrupts and process switching that affect the order that results are returned in
I talk about the order that results are returned in because it has to be remembered that floating point operations are not transitive or distributive. 0.1 + 0.2 - 0.3 might not have the same result as 0.1 - 0.3 + 0.2. Especially for matrix inversions, this can make a really big relative difference. (This is one of the reasons why matrix inversions should be avoided.) When you are using multiple cores to do summations, the segment produced by any one core might be identical between runs, but the order the values are added in can vary due to chance circumstances, leading to overall results that differ.
Alan Bindemann
Alan Bindemann on 16 Jan 2020
We have a Simulink model, coded as an S-Function subsystem, running on two machines, each with Intel processors (One i5 and another a Xeon). When the S-Function is fed identical inputs on each machine we get different results. The differences are initially small, and then grow to noticable levels over time due to the accumulation of errors internal to the S-Function.
It was our understanding that, once coded as an S-Function, different Intel processors would still produce identical floating point results.
Can someone give additional details as to why S-Functions (and models coded out to .dll's) might give different results? We thought that once things were reduced to x86 instructions, we could expect repeatable results, regardless of the processor being used.

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Peter Monk
Peter Monk on 19 Sep 2018
Edited: Peter Monk on 19 Sep 2018
I also see a problem running on macs with osx 10.13.6 and matlab 2018a running on an i7 or i5 based machine. This code gives different results
format long; A=hilb(7); x_exact=.1*(1:7)'; b=A*x_exact; x_comp=A\b; resid=norm(A*x_comp-b,1)
On my i7 machine "resid" is non-zero (true) whereas on the i5 machines "resid=0" which is misleading since x_exact and x_comp differ in the 10th decimal place or so.

Ba Mo
Ba Mo on 4 Oct 2020
Edited: Ba Mo on 4 Oct 2020
facing the same problem, running on two HPCs (clusters). problem is, the better more powerful HPC returns better results; but it's always crowded.
i dont face singularities or runtime errors; but the output of a stochastic optimization problem differs by 1%-2%. This 1%-2% becomes noticeable when your objective function is in the order of 10^8

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