[petsc-users] Different precision from MatAssembly/MatView

Sun Mar 15 12:16:38 CDT 2026

On Sun, Mar 15, 2026 at 12:58 PM Noam T. <dontbugthedevs at proton.me> wrote:

> The results above of the SNES output were using the flags
>
> -ksp_type fgmres -pc_type lu
>

This is very easy to check. Multiply the solution by your matrix and
subtract the RHS. It must satisfy the tolerance you have for the solver. If
not, there is some other problem, but I imagine it does, which is what we
mean by the solution.

To answer the first question, there is no reduction in precision in
MatSetValues(). I am sure if you carry out the addition by hand you will
see that the result is within 1ulp as it should be from the standard.

  Thanks,

     Matt

> Thanks
> On Sunday, March 15th, 2026 at 5:12 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
> On Sun, Mar 15, 2026 at 11:19 AM Noam T. via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> A bit more information:
>>
>> Looking at the solution of the system of equations, knowing the exact RHS
>> vector:
>> ---
>> [-0.00197, 0.00203, -0.00197]
>> ---
>>
>> SNES gives the solution:
>> ---
>> 3.316262462871E-03, -4.189244774965E-03, 2.468317119413E-03
>> ---
>>
>> which is indeed (closer to) the solution for the jacobian shown in
>> MatView (the "lower precision" one). The "exact" solution would be:
>> ---
>> 3.282607248309093b-3, -4.241572005990172b-3,, 2.425030835360701b-3
>> ---
>>
>> which is already different form the second decimal digit.
>>
>
> Are you using an exact solution method, like LU?
>
> Thanks,
>
> Matt
>
>>
>> ***
>> Requesting a low precision to have SNES do a few iterations, shows that
>> the built jacobian seems to deviate more and more form the computed one
>> after some iterations, e.g.:
>>
>> Computed (printed from in-code):
>> ---
>> 0.536488494840 0.066474939660 0.198813816284
>> 0.066474939660 0.529482312614 -0.002835445071
>> 0.198813816284 -0.002835445071 0.530589055263
>> ---
>>
>> MatView:
>> ---
>> row 0: (0, 0.538491) (1, 0.0662019) (2, 0.198808)
>> row 1: (0, 0.0662019) (1, 0.526981) (2, -0.00257554)
>> row 2: (0, 0.198808) (1, -0.00257554) (2, 0.529883)
>> ---
>>
>> Only one digit is correct/the same in entries [1,2], [2,1] and [2,2].
>>
>> Thank you.
>> Noam
>>
>>
>> On Sunday, March 15th, 2026 at 3:29 PM, Noam T. <dontbugthedevs at proton.me>
>> wrote:
>>
>> Hello,
>>
>> Looking at the assembly of the Jacobian using MatView, I noticed values
>> were somewhat different from the computed ones in the jacobian function
>> (DMSNESSetJacobian). For the test, I used a single Q1 element, with 4
>> quadrature points, so the exact Jacobian matrix can be computed
>> analytically.
>>
>> In the jacobian function, after looping through all quadrature points,
>> printing the matrix to stdout shows:
>>
>> ----
>> 0.531360000000 0.066670000000 0.197333333327
>> 0.066670000000 0.535360000000 -0.003333333327
>> 0.197333333327 -0.003333333327 0.527413333333
>> ----
>>
>> (showing only the 3 free DoFs). The 2x2 submatrix in [0:1] has exactly
>> four/five nonzero digits, as shown above. The rest of the elements have a
>> periodic 3 ending (a couple of digits at the end are off, but that's fine).
>>
>> This same matrix is the one added to the global jacobian during the
>> quadrature loop, with "DMPlexMatSetClosure". After looping, calling
>> "MatAssemblyBegin/End", then MatView:
>>
>> ----
>> Mat Object: 1 MPI process
>> type: seqaij
>> row 0: (0, 0.531387) (1, 0.0666733) (2, 0.197338)
>> row 1: (0, 0.0666733) (1, 0.535387) (2, -0.0033385)
>> row 2: (0, 0.197338) (1, -0.0033385) (2, 0.52744)
>> ----
>>
>> Values are close, but definitely not the same as computed. Are these
>> values from "MatView" the ones actually stored in the matrix? It seems
>> there is some precision loss in the process.
>>
>> Interestingly, computing the jacobian through finite differences, shows
>> the same result for the "hand-coded" jacobian, whereas the finite
>> differences ones is the "exact" one (same as first one above):
>>
>> ----
>> ||J - Jfd||_F/||J||_F = 4.90736e-05, ||J - Jfd||_F = 4.74265e-05
>> Hand-coded Jacobian ----------
>> Mat Object: 1 MPI process
>> type: seqaij
>> row 0: (0, 0.531387) (1, 0.0666733) (2, 0.197338)
>> row 1: (0, 0.0666733) (1, 0.535387) (2, -0.0033385)
>> row 2: (0, 0.197338) (1, -0.0033385) (2, 0.52744)
>> Finite difference Jacobian ----------
>> Mat Object: 1 MPI process
>> type: seqaij
>> row 0: (0, 0.53136) (1, 0.06667) (2, 0.197333)
>> row 1: (0, 0.06667) (1, 0.53536) (2, -0.00333333)
>> row 2: (0, 0.197333) (1, -0.00333333) (2, 0.527413)
>> Hand-coded minus finite-difference Jacobian with tolerance 1e-05
>> ----------
>> Mat Object: 1 MPI process
>> type: seqaij
>> row 0: (0, 2.66554e-05)
>> row 1: (1, 2.66554e-05)
>> row 2: (2, 2.6558e-05)
>> ----
>>
>> The main code is in Fortran, using double precision variables, in case it
>> is relevant.
>>
>> Thank you.
>>
>> Noam
>>
>>
>>
>>
>
> --
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> -- Norbert Wiener
>
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-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

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