# Mpiallreduce probelm
**URL:** https://community.freefem.org/t/mpiallreduce-probelm/1290
**Category:** General Discussion
**Created:** [October 28, 2021, 3:43pm UTC](https://community.freefem.org/t/mpiallreduce-probelm/1290 "2021-10-28T15:43:44Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@zcwang6](https://community.freefem.org/u/zcwang6)
#### Post date: [October 28, 2021, 3:43pm UTC](https://community.freefem.org/t/mpiallreduce-probelm/1290/1 "2021-10-28T15:43:44Z")
Hello there, a quick question about MPIALLREDUCE; I used this function in the following script
mpiComm Comm(mpiCommWorld, 0, 0);
real nubl = mpirank;
real nubg=0;
mpiAllReduce(nubg, nubl, mpiCommWorld, mpiSUM);
cout \<\< “finnal SUM " \<\< nubg \<\< " elements” \<\< " mpi "\<\< mpirank \<\< " has " \<\< nubl \<\< endl;
real[int] S1(4),S2(4),S(4);
real MP;
MP = mpirank\*2;
S = [0,0,0,0];
S2 = [MP,MP,MP,MP];
mpiAllReduce(S,S2, mpiCommWorld, mpiSUM);
cout \<\< S \<\< endl;
the data types are real and real[int]; it seems mpiALLReduce reinitialized my data; But I believe this function works fine when I am doing domain decomposition method;
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### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [October 28, 2021, 3:55pm UTC](https://community.freefem.org/t/mpiallreduce-probelm/1290/2 "2021-10-28T15:55:01Z")
You are switching all parameters, it is `mpiAllReduce(in, out...)`, not `mpiAllReduce(out, in...)`.
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### Author:  [@zcwang6](https://community.freefem.org/u/zcwang6)
#### Post date: [October 28, 2021, 4:01pm UTC](https://community.freefem.org/t/mpiallreduce-probelm/1290/3 "2021-10-28T16:01:43Z")
damn it, Thx for pointing it out; 😂