# OpenMP and FORTRAN **URL:** \ **Category:** Help\ **Created:** [April 10, 2024, 12:01pm UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807 "2024-04-10T12:01:33Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![JohnCampbell](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@JohnCampbell](https://fortran-lang.discourse.group/u/JohnCampbell)\ **Post date:** [April 13, 2024, 7:35am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/21 "2024-04-13T07:35:49Z")
> [@suki](#): > > Should I be using ifort instead of gfortran? No, Gfortran is very good. I think the problem is your 12th Gen Intel® Core™ i5-1235U. Google search finds The i5–1235U is **faster in tasks that use only 1–2 CPU cores or threads, such as photo editing**. This is because the i5–1235U only has 2 high-performance CPU cores, its other 8 cores are much weaker “efficiency” cores.23 Dec 2023. It is surprising there are only 2 performance cores, but 8 efficiency cores ! Then there is the problem with cooling with any recent Intel laptop. ---
**Author:** ![suki](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@suki](https://fortran-lang.discourse.group/u/suki)\ **Post date:** [April 13, 2024, 7:59am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/23 "2024-04-13T07:59:18Z")
Yes John, you are absolutely on the dot; it generates a lot of heat for which I have a cooling fan underneath. Suki ---
**Author:** ![PierU](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/pieru/32/1848_2.png) [@PierU](https://fortran-lang.discourse.group/u/PierU)\ **Post date:** [April 13, 2024, 8:09am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/24 "2024-04-13T08:09:24Z")
> [@JohnCampbell](#): > > This is because the i5–1235U only has 2 high-performance CPU cores, its other 8 cores are much weaker “efficiency” cores. Indeed. Not only the speed-up is reduced once Efficiency cores are used, but I think that OpenMP runtimes do not handle well (and maybe can’t handle well) the mix between performance and efficiency cores. Moreover, this CPU has 10 physical cores (not 12) + 2 logical cores. When running with 12 threads the logical cores are used, but logical cores generally do not help speeding-up such computations. Given all of this, the limited speed-up is not surprising. > [@JohnCampbell](#): > > It is surprising there are only 2 performance cores, but 8 efficiency cores ! > Then there is the problem with cooling with any recent Intel laptop. Both are linked: more performance cores would result in more heating, and ultimately in automatic throttling (frequency downclocking) of the performance cores to reduce the temperature. ---
**Author:** ![PierU](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/pieru/32/1848_2.png) [@PierU](https://fortran-lang.discourse.group/u/PierU)\ **Post date:** [April 13, 2024, 8:18am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/25 "2024-04-13T08:18:11Z")
On CPUs with Efficiency Cores, it would be worth trying the `schedule(nonmonotonic:dynamic)` clause in `!$omp do` . The `dynamic` schedule can help when the workload is not balanced between the threads (and maybe when the threads do not run on equally performant cores). The problem is that this schedule has more overheads than the default `static` one, but with the recent `nonmonotonic` modifier the overheads get very limited. But I’m not sure that gfortran supports it right now (recent versions of ifx do). ---
**Author:** ![suki](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@suki](https://fortran-lang.discourse.group/u/suki)\ **Post date:** [April 13, 2024, 8:19am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/26 "2024-04-13T08:19:59Z")
Oops! ```auto mojo run num_cpu_cores.mojo Number of physical cores: 10 Number of logical cores: 12 Number of performance cores: 10 ``` Is this mojo script: ```auto from sys.info import num_physical_cores, num_logical_cores, num_performance_cores fn main(): print('Number of physical cores:',num_physical_cores()) print('Number of logical cores:',num_logical_cores()) print('Number of performance cores:', num_performance_cores()) ``` being pedantic or wrong? ---
**Author:** ![PierU](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/pieru/32/1848_2.png) [@PierU](https://fortran-lang.discourse.group/u/PierU)\ **Post date:** [April 13, 2024, 8:23am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/27 "2024-04-13T08:23:37Z")
> [@suki](#): > > Number of performance cores: 10 Definitely wrong (at least if your CPU is really an [i5 1235U](https://www.intel.com/content/www/us/en/products/sku/226266/intel-core-i51235u-processor-12m-cache-up-to-4-40-ghz-with-ipu/specifications.html)) ---
**Author:** ![suki](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@suki](https://fortran-lang.discourse.group/u/suki)\ **Post date:** [April 13, 2024, 8:28am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/28 "2024-04-13T08:28:15Z")
Comes from here: > **[info | Modular Docs](https://docs.modular.com/mojo/stdlib/sys/info#num_performance_cores)** > > Implements methods for querying the host target info. ```auto lscpu|head Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Address sizes: 39 bits physical, 48 bits virtual Byte Order: Little Endian CPU(s): 12 On-line CPU(s) list: 0-11 Vendor ID: GenuineIntel Model name: 12th Gen Intel(R) Core(TM) i5-1235U CPU family: 6 Model: 154 ``` ---
**Author:** ![JohnCampbell](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@JohnCampbell](https://fortran-lang.discourse.group/u/JohnCampbell)\ **Post date:** [April 13, 2024, 10:16am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/29 "2024-04-13T10:16:59Z")
@suki Given you have reported your program performance is topping out at 2 threads, this appears to be consistent with the google thread of only 2 performance cores, not “Number of performance cores: 10” I have not had any experience with using “efficiency” cores for OpenMP, but assuming their instruction excludes avx instructions, I would think if using threads with such an imbalanced performance would exclude most of the computation types I have. You could possibly try adding !$OMP& SCHEDULE (DYNAMIC) that is assuming Gfortran -fopenmp even tries to use the other cores. I have asked questions of if Gfortran or OpenMP implimentations utilise “efficiency” cores but never received a clear answer. !$OMP PARALLEL DO can be more sensitive to threads of different performance, so I don’t intend to try. I currently use a Ryzen 5900X where I have turned off “2-way simultaneous multithreading”, as I get no benefit from the extra threads. On my Intel 8700K, I also do not use hyper-threading, although most of my calculations are with large arrays where memory bandwidth might be the problem. For multi-threaded computation, a desktop with adequate cooling could be a much better option. The AMD (nonX) processors running at a slower clock rate look to be a much better solution than an Intel room heater ! I wonder where the glossy marketed processors are taking us. ---
**Author:** ![PierU](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/pieru/32/1848_2.png) [@PierU](https://fortran-lang.discourse.group/u/PierU)\ **Post date:** [April 13, 2024, 10:42am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/30 "2024-04-13T10:42:30Z")
> [@JohnCampbell](#): > > The AMD (nonX) processors running at a slower clock rate look to be a much better solution than an Intel room heater ! I wonder where the glossy marketed processors are taking us. The Intel approach makes sense in a general-purpose laptop (e.g. not targeted at HPC), where the cooling capabilities are limited by the size of the laptop. The performance cores can handle peak computations demands for a short period of time, and the efficiency cores handle all background processes that are generally not very demanding. Your Ryzen 5900X has a 105W TDP, which is much too high for most of the laptops. Apple has here an edge with its Apple Silicon chips, which have a much better efficiency (flops/W) then the current x86 chips. A Macbook Pro can get better performances than equivalent x86 laptops, and without heating that much. ---
**Author:** ![suki](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@suki](https://fortran-lang.discourse.group/u/suki)\ **Post date:** [April 13, 2024, 11:01am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/31 "2024-04-13T11:01:43Z")
On a pseudo-server-grade PC we assembled it performs quite well doubling speed (again only till 4 performance cores): ```auto $lscpu|head Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Address sizes: 39 bits physical, 48 bits virtual Byte Order: Little Endian CPU(s): 8 On-line CPU(s) list: 0-7 Vendor ID: GenuineIntel Model name: Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz CPU family: 6 Model: 94 $mojo run num_cpu_cores.mojo Number of physical cores: 4 Number of logical cores: 8 Number of performance cores: 4 $gfortran ... Total run in seconds: 1 thread 16.3750 2 threads 8.3750 4 threads 4.4375 8 threads 3.8750 ``` ---
**Author:** ![suki](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@suki](https://fortran-lang.discourse.group/u/suki)\ **Post date:** [April 13, 2024, 4:01pm UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/32 "2024-04-13T16:01:31Z")
It looks like horses for courses: [https://github.com/Sukii/vortex\_simulations/tree/main/benchmark](https://github.com/Sukii/vortex_simulations/tree/main/benchmark) With mojo SIMD arrays doing better on 12th Gen Interl-5 12-core Laptop CPUs, while Fortran OpenMP doing well i7 6700 8-core PC CPUs Suki ---
**Author:** ![JohnCampbell](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@JohnCampbell](https://fortran-lang.discourse.group/u/JohnCampbell)\ **Post date:** [October 6, 2024, 3:36am UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/33 "2024-10-06T03:36:40Z")
> [@JohnCampbell](#): > > Significantly, at STOP, the program reports a “Note”, but does not exit. > If I batch a sequence of programs to run, having a program failing to exit could be a significant issue. I have now identified that this problem is probably due to call omp\_set\_num\_threads () This is occurring on my windows 10 PC and was introduced in Gfortran Ver 11.3.0 and later versions. It does not occur in Gfortran Ver 11.1.0 and earlier versions. I am not sure if this is in Gfortran or the windows thread managment library for [equation.com](http://equation.com)’s implementation of Gfortran. This is a code that I have to demonstrate the problem. My batch test is ```auto call set_gcc 11.1.0 set program=test4 del %program%.exe gfortran %program%.f90 -O3 -march=native -fopenmp -o %program%.exe %program% ``` A simple reproducer below which exhibits the problem if “call omp\_set\_num\_threads (4)” is included. ```auto program test ! small reproducer version of OpenMP program hanging on Win 10 / Gfortran 11.3.0 ! gfortran test.f90 -O3 -march=native -fopenmp -o test.exe use iso_fortran_env implicit none integer, parameter :: num = 1000 real :: A(num) real :: RA integer :: i write (*,*) 'Vern : ',compiler_version () write (*,*) 'Opts : ',compiler_options () call omp_set_num_threads (4) ! this causes problem for Gfortran 11.3.0 + write ( *,*) 'Test n=',num A = 1 ra = 0 !$OMP PARALLEL DO private (i) shared (A), REDUCTION (+: RA) do i = 1, size(A) RA = RA + A(i)**2 end do !$OMP END PARALLEL DO RA = sqrt (RA) print*,RA,' OpenMP', sqrt(real(num)) ! problem is demonstrated if peogram hangs here and does not exit end program test ``` If others can test to identify if it occurs with other OS or other Gfortran implementations, I would be interested to know. ---
**Author:** ![MattAl](https://avatars.discourse-cdn.com/v4/letter/m/6a8cbe/32.png) [@MattAl](https://fortran-lang.discourse.group/u/MattAl)\ **Post date:** [December 9, 2024, 12:18pm UTC](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807/34 "2024-12-09T12:18:57Z")
Not sure if it’s useful, but I had no failures on: - Centos7: gcc `4.8.5`, `6.3.0`, `7.1.0`, `8.2.0`, `10.2.0`, `12.1.0` - Rocky9: gcc `11.4.1`, `12.2.0`, `14.2.0` [Previous page](https://fortran-lang.discourse.group/t/openmp-and-fortran/7807.md?page=1)