Issue with DESCINIT in Scalapack

OP1 — Thu, 04 Sep 2025 19:53:53 GMT

There is an issue with calls to DESCINIT in some circumstances. The program below illustrates the problem. As is, the program implements a manual workaround and will complete successfully. To exhibit the buggy behavior of DESCINIT, uncomment the three lines of the code that are identified as problematic.

PROGRAM SCALAPACK_DEMOS IMPLICIT NONE (TYPE, EXTERNAL) ! The intent of this program is to scatter a 2x2 matrix A ! defined on 1 process (1x1 BLACS process grid with context CONTEXT_A) ! to local 1x1 matrices B on 4 processes (2x2 process grid with context CONTEXT_B), ! using the scalapack routine PDGEMR2D. ! ! This program must be run as 4 processes/images. ! Appropriate linking to Blacs/Scalapack is necessary. INTEGER, PARAMETER :: DP = SELECTED_REAL_KIND(15, 307) EXTERNAL :: & BLACS_GET, & BLACS_PINFO, & BLACS_GRIDINIT, & DESCINIT, & PDGEMR2D INTEGER :: SYSTEM_CONTEXT, CONTEXT_A, CONTEXT_B ! BLACS CONTEXTS INTEGER :: MY_PROC, N_PROCS ! BLACS DATA INTEGER :: INFO ! SCALAPACK RETURN VALUE INTEGER, DIMENSION(9) :: DESC_A, DESC_B ! MATRIX DESCRIPTORS REAL(KIND = DP) :: & A(4), & ! Matrix to be scattered with meaningful values only on 1 process B(1) ! Space for local matrix on a 2x2 process grid after scatterinng CALL BLACS_GET(-1, 0, SYSTEM_CONTEXT) ! Get the BLACS System Context CALL BLACS_PINFO(MY_PROC, N_PROCS) ! Determine my process number MY_PROC and the number of processes N_PROCS in machine IF(N_PROCS /= 4) STOP ! Ensure code is being called with 4 processes CONTEXT_A = SYSTEM_CONTEXT CALL BLACS_GRIDINIT(CONTEXT_A, 'R', 1, 1) ! Initialize context for A with 1x1 process grid CONTEXT_B = SYSTEM_CONTEXT CALL BLACS_GRIDINIT(CONTEXT_B, 'R', 2, 2) ! Initialize context for B with on a 2x2 process grid B = -77.7_DP ! Assign garbage at locations where A will be scattered to CALL DESCINIT(DESC_B, 2, 2, 1, 1, 0, 0, CONTEXT_B, 1, INFO) ! Descriptor for B IF (CONTEXT_A /= -1) THEN A = [1.1_DP, 2.2_DP, 3.3_DP, 4.4_DP] ! Value on process in CONTEXT_A which to be scattered ELSE A = -999.0_DP ! Not a processes in CONTEXT_A. Assign garbage values END IF ! Create DESC_A ! Issue is below : ! The IF/ELSE below should not be necessary. ! Instead it should be possible to call DESCINIT directly on all processes. ! However if we try to do this we will get an error with INFO = -6 for processes not in CONTEXT_A. ! An error is issued internally because CONTEXT_A = -1 on those processes. ! But later PDGEMR2D demands DESC_A have -1 for context. So we have to create DESC_A manually. ! ! Uncomment the lines below to trigger the error above: ! !CALL DESCINIT(DESC_A, 2, 2, 1, 1, 0, 0, CONTEXT_A, 2, INFO) !WRITE(*,*) 'INFO for DESC_A ON PROC ', MY_PROC, ' = ', INFO !STOP IF (CONTEXT_A /= -1) THEN ! Fill only on master CALL DESCINIT(DESC_A, 2, 2, 1, 1, 0, 0, CONTEXT_A, 2, INFO) ELSE DESC_A = -2 ! Garbage value. Only entry for context matters on these processes DESC_A(2) = -1 ! Context END IF ! Scatter 2x2 A on 1 process to local 1x1 B's on 4 processes CALL PDGEMR2D(2, 2, A, 1, 1, DESC_A, B, 1, 1, DESC_B, CONTEXT_B) SYNC ALL WRITE(*,*) 'B on ', MY_PROC, ' = ', B(1) END PROGRAM SCALAPACK_DEMOS

Re:Issue with DESCINIT in Scalapack

Aleksandra_K — Tue, 09 Sep 2025 09:18:49 GMT

Hi,

Thank you for reporting the problem. I reproduced the issue and we will work on it internally. I'll update you here with any news.

Best regards,

Alex

Re:Issue with DESCINIT in Scalapack

Aleksandra_K — Thu, 11 Sep 2025 09:51:37 GMT

Hi,

The presented behavior is actually expected. If the current process is out of scope of the input context, it reports an error, see the reference implementation of the ScaLAPACK (netlib.org/scalapack/tools/descinit.f).

Regards,

Alex

Re:Issue with DESCINIT in Scalapack

Aleksandra_K — Tue, 16 Sep 2025 06:55:28 GMT

Hi,

Do you have any further questions on the topic or is everything clear?

Regards,

Alex

Re:Issue with DESCINIT in Scalapack

Aleksandra_K — Thu, 18 Sep 2025 08:10:08 GMT

Hi,

If you have any follow-up questions, please let us know. In case of no response within three days, this thread will no longer be monitored by Intel.

Re: Issue with DESCINIT in Scalapack

OP1 — Thu, 18 Sep 2025 14:05:48 GMT

@Aleksandra_K Thanks for clarifying the use of descinit! You may close this thread now.

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