Re: [eigen] 3.1.4 & 3.0.7 on Friday?

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On Thu, 1 Aug 2013, Rhys Ulerich wrote:

Running 'EIGEN_SEED=1375367469
./build/test/product_trmm_21' causes a repeatable failure for all
compilers tested.

I cannot reproduce. Does it fails for this seed only?

It failed on several seeds for me.  Using 'while
./build/test/product_trmm_21 ; do echo WORKED ; done' will let your
CPU crank on it until either you find one that breaks or you get bored
of waiting.

I can reproduce it, with the same seed, both for the new 3.1.4 and the current dev branch.

I haven't looked at it in detail yet, but the test compares two ways of computing A - B, where A is a given matrix and B is a product involving a triangular matrix. For some reason, there is a lot of cancellation going on, even though these are big random matrices. The norm of A and B are both about 300, but the norm of A - B is 0.019 and norm of the difference of computing A - B with both methods is 6e-5 (here, norm is the Kronecker norm, the sqrt of sum of squares). The test compares 6e-5 with 0.019 and fails, but arguably it should compare it with 300 and succeed.

So my guess is that the failure is nothing serious, but it's a tad preliminary at the moment, and I need to stop now.


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