# Report error when 1 processor but 2 or more is OK

**URL:** <https://community.freefem.org/t/report-error-when-1-processor-but-2-or-more-is-ok/3921>\
**Category:** General Discussion\
**Created:** [May 17, 2025, 6:32am UTC](https://community.freefem.org/t/report-error-when-1-processor-but-2-or-more-is-ok/3921 "2025-05-17T06:32:53Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zxss](https://avatars.discourse-cdn.com/v4/letter/z/bc8723/32.png) [@zxss](https://community.freefem.org/u/zxss)\
**Post date:** [May 17, 2025, 6:32am UTC](https://community.freefem.org/t/report-error-when-1-processor-but-2-or-more-is-ok/3921/1 "2025-05-17T06:32:53Z")

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Dear all,  
Here is my parallel code  
[try\_movemesh.edp](https://community.freefem.org/uploads/short-url/k7c7xB8XOsz0uDOoQhld6XmFrof.edp) (4.0 KB)  
[movemesh.mesh](https://community.freefem.org/uploads/short-url/2becceKoPCxwmIim5bMcuxy6sb9.mesh) (82.9 KB)

I find when I run it with 2 or more processors(ff-mpirun -n 2 try\_movemesh.edp -wg -v 0  
), it can work well.  
However, when I choose only one processor(ff-mpirun -n 1 try\_movemesh.edp -wg -v 0  
), it report an error as

 ![Screenshot from 2025-05-17 07-32-15](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/0/0a4421375b67c94e885949b0e9af50e567482c7c.png)  
Does anyone know why?

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**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\
**Post date:** [May 18, 2025, 10:46am UTC](https://community.freefem.org/t/report-error-when-1-processor-but-2-or-more-is-ok/3921/2 "2025-05-18T10:46:20Z")

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I think you should set `penal=-1.e-8` in order to prevent having a singular matrix (pressure not unique because of addition of a constant). When there are several processes the linear system is solved by approximations that eventually do not “see” that the matrix is singular.

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**Author:** ![zxss](https://avatars.discourse-cdn.com/v4/letter/z/bc8723/32.png) [@zxss](https://community.freefem.org/u/zxss)\
**Post date:** [May 18, 2025, 12:59pm UTC](https://community.freefem.org/t/report-error-when-1-processor-but-2-or-more-is-ok/3921/3 "2025-05-18T12:59:46Z")

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Yes, it works. Many thanks, professor.