# Can CMAQ only run with less than a thousand processors?
**URL:** https://forum.cmascenter.org/t/can-cmaq-only-run-with-less-than-a-thousand-processors/6118
**Category:** CMAQ
**Created:** [December 30, 2025, 5:53pm UTC](https://forum.cmascenter.org/t/can-cmaq-only-run-with-less-than-a-thousand-processors/6118 "2025-12-30T17:53:14Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@vkrmravi](https://forum.cmascenter.org/u/vkrmravi)
#### Post date: [December 30, 2025, 5:53pm UTC](https://forum.cmascenter.org/t/can-cmaq-only-run-with-less-than-a-thousand-processors/6118/1 "2025-12-30T17:53:14Z")
Hello
I am running CMAQv5.5 built using gcc on a cray machine. I’m testing the optimal configuration (e.g., domain decomposition for my HPC), for CONUS 12US1 domain using 2022v1 EMP data. In the process I set up CMAQ to run with more than 1000 processors:
`@ NPCOL = 46; @ NPROW = 30`
This fails almost immediately and I see that no log files are created for processor ID exceeding 999 (last two shown below):
`CTM_LOG_999.v55_gcc_IE_12US1_cb6r5_ae7_aq_m3dry_20220101`
`'CTM_LOG_***.v55_gcc_IE_12US1_cb6r5_ae7_aq_m3dry_20220101'`
Is that an expected behavior and CMAQ is set to run with less than 1000 processors total (which I’m guessing based on the log file name structure) or am I doing something wrong here? I’ve another ongoing run which is progressing fine with 900 processors (for testing).
Thank you for your insights.
PS: I recall we had a similar issue with MCIP but with the domain specified such that number of rows (or columns) exceeded 999 resulted in MCIP failing.
PS2: In my testing, a 7x increase in the number of processors resulted in a 50% reduction in run time.
PS3: my simulation run time has essentially flattened beyond 750 processors.
---
### Author:  [@nash.skipper](https://forum.cmascenter.org/u/nash.skipper)
#### Post date: [January 6, 2026, 8:52pm UTC](https://forum.cmascenter.org/t/can-cmaq-only-run-with-less-than-a-thousand-processors/6118/2 "2026-01-06T20:52:50Z")
There should be a workaround for this. The names for the CTM\_LOG files are created in [setup\_logdev.F](https://github.com/USEPA/CMAQ/blob/main/CCTM/src/util/util/setup_logdev.F). To correctly generate CTM\_LOG file names when running with more than 1000 processors, you can edit [this line](https://github.com/USEPA/CMAQ/blob/9bd3734176479c2e49139fea98e1d5e8a16170e3/CCTM/src/util/util/setup_logdev.F#L90) from:
`WRITE ( CMYPE, '(I3.3)' ) MYPE`
to:
`WRITE ( CMYPE, '(I4.4)' ) MYPE`
Then also update a [line in RUNTIME\_VARS.F](https://github.com/USEPA/CMAQ/blob/9bd3734176479c2e49139fea98e1d5e8a16170e3/CCTM/src/util/util/RUNTIME_VARS.F#L87) from:
`CHARACTER( 3 ) :: CMYPE = "" ! Processor Number`
to:
`CHARACTER( 4 ) :: CMYPE = "" ! Processor Number`
Optionally, you can also update [wrsubdmap.f](https://github.com/USEPA/CMAQ/blob/9bd3734176479c2e49139fea98e1d5e8a16170e3/CCTM/src/PARIO/wrsubdmap.f#L40) to properly format the processor numbers that get printed to the log file to describe the processor to subdomain mapping. The change would be from:
```auto
write( *,* )
write( *,* ) ' -=- MPP Processor-to-Subdomain Map -=-'
write( *,'(A,I3)' ) ' Number of Processors = ',nprocs
write( *,* ) ' ____________________________________________________'
write( *,* ) ' | |'
write( *,* ) ' |' // title // ' |'
write( *,* ) ' | __________________________________________________ |'
write( *,* ) ' | |'
do i = 1, nprocs
write( *,1003 ) i-1, ncols_pe(i), colsx_pe(1,i), colsx_pe(2,i),
& nrows_pe(i), rowsx_pe(1,i), rowsx_pe(2,i)
end do
write( *,* ) ' | __________________________________________________ |'
write( *,* )
1003 format(' |', i3, 5x, i4, 3x, i4, ':', i4,
& 7x, i4, 3x, i4, ':', i4, ' |')
```
to:
```auto
write( *,* )
write( *,* ) ' -=- MPP Processor-to-Subdomain Map -=-'
write( *,'(A,I4)' ) ' Number of Processors = ',nprocs
write( *,* ) ' ____________________________________________________'
write( *,* ) ' | |'
write( *,* ) ' |' // title // ' |'
write( *,* ) ' | __________________________________________________ |'
write( *,* ) ' | |'
do i = 1, nprocs
write( *,1003 ) i-1, ncols_pe(i), colsx_pe(1,i), colsx_pe(2,i),
& nrows_pe(i), rowsx_pe(1,i), rowsx_pe(2,i)
end do
write( *,* ) ' | __________________________________________________ |'
write( *,* )
1003 format(' |', i4, 5x, i4, 3x, i4, ':', i4,
& 7x, i4, 3x, i4, ':', i4, ' |')
```
Besides these formatting issues, I am not aware of anything else in the code that prevents CMAQ from running with more than 1000 processors.
One final thing is that our sample run scripts usually use `CTM_LOG_???` to find log files, so this would need to be updated to `CTM_LOG_????` for the run scripts to behave as expected with the above changes to the source code.
---
### Author:  [@vkrmravi](https://forum.cmascenter.org/u/vkrmravi)
#### Post date: [January 6, 2026, 11:09pm UTC](https://forum.cmascenter.org/t/can-cmaq-only-run-with-less-than-a-thousand-processors/6118/3 "2026-01-06T23:09:43Z")
thanks, Nash! I’ll give this a try.