Re: [petsc-users] Using PETSc GPU backend

[Barry Smith]

  
   Cho,

    We currently have a crappy API for turning on GPU support, and our 
documentation is misleading in places. 

    People constantly say "to use GPU's with PETSc you only need to use 
-mat_type aijcusparse (for example)" This is incorrect.

 This does not work with code that uses the convenience Mat constructors such 
as MatCreateAIJ(), MatCreateAIJWithArrays etc. It only works if you use the 
constructor approach of MatCreate(), MatSetSizes(), MatSetFromOptions(), 
MatXXXSetPreallocation(). ...  Similarly you need to use VecCreate(), 
VecSetSizes(), VecSetFromOptions() and -vec_type cuda

   If you use DM to create the matrices and vectors then you can use 
-dm_mat_type aijcusparse -dm_vec_type cuda

   Sorry for the confusion.

   Barry




> On Jul 15, 2023, at 8:03 AM, Matthew Knepley <knep...@gmail.com> wrote:
> 
> On Sat, Jul 15, 2023 at 1:44 AM Ng, Cho-Kuen <c...@slac.stanford.edu 
> <mailto:c...@slac.stanford.edu>> wrote:
>> Matt,
>> 
>> After inserting 2 lines in the code:
>> 
>>   ierr = MatCreate(PETSC_COMM_WORLD,&A);CHKERRQ(ierr);                       
>>   ierr = MatSetFromOptions(A);CHKERRQ(ierr);
>>   ierr = MatCreateAIJ(PETSC_COMM_WORLD,mlocal,mlocal,m,n,
>>                       d_nz,PETSC_NULL,o_nz,PETSC_NULL,&A);;CHKERRQ(ierr);
>> 
>> "There are no unused options." However, there is no improvement on the GPU 
>> performance.
> 
> 1. MatCreateAIJ() sets the type, and in fact it overwrites the Mat you 
> created in steps 1 and 2. This is detailed in the manual.
> 
> 2. You should replace MatCreateAIJ(), with MatSetSizes() before 
> MatSetFromOptions().
> 
>   THanks,
> 
>     Matt
>  
>> Thanks,
>> Cho
>> 
>> From: Matthew Knepley <knep...@gmail.com <mailto:knep...@gmail.com>>
>> Sent: Friday, July 14, 2023 5:57 PM
>> To: Ng, Cho-Kuen <c...@slac.stanford.edu <mailto:c...@slac.stanford.edu>>
>> Cc: Barry Smith <bsm...@petsc.dev <mailto:bsm...@petsc.dev>>; Mark Adams 
>> <mfad...@lbl.gov <mailto:mfad...@lbl.gov>>; petsc-users@mcs.anl.gov 
>> <mailto:petsc-users@mcs.anl.gov> <petsc-users@mcs.anl.gov 
>> <mailto:petsc-users@mcs.anl.gov>>
>> Subject: Re: [petsc-users] Using PETSc GPU backend
>>  
>> On Fri, Jul 14, 2023 at 7:57 PM Ng, Cho-Kuen <c...@slac.stanford.edu 
>> <mailto:c...@slac.stanford.edu>> wrote:
>> I managed to pass the following options to PETSc using a GPU node on 
>> Perlmutter.
>> 
>>     -mat_type aijcusparse -vec_type cuda -log_view -options_left
>> 
>> Below is a summary of the test using 4 MPI tasks and 1 GPU per task.
>> 
>> o #PETSc Option Table entries:
>>    -log_view
>>    -mat_type aijcusparse
>>    -options_left
>>    -vec_type cuda
>>    #End of PETSc Option Table entries
>>    WARNING! There are options you set that were not used!
>>    WARNING! could be spelling mistake, etc!
>>    There is one unused database option. It is:
>>    Option left: name:-mat_type value: aijcusparse
>> 
>> The -mat_type option has not been used. In the application code, we use
>> 
>>     ierr = MatCreateAIJ(PETSC_COMM_WORLD,mlocal,mlocal,m,n,
>>              d_nz,PETSC_NULL,o_nz,PETSC_NULL,&A);;CHKERRQ(ierr);
>> 
>> 
>> If you create the Mat this way, then you need MatSetFromOptions() in order 
>> to set the type from the command line.
>> 
>>   Thanks,
>> 
>>      Matt
>>  
>> o The percent flops on the GPU for KSPSolve is 17%.
>> 
>> In comparison with a CPU run using 16 MPI tasks, the GPU run is an order of 
>> magnitude slower. How can I improve the GPU performance?
>> 
>> Thanks,
>> Cho
>> From: Ng, Cho-Kuen <c...@slac.stanford.edu <mailto:c...@slac.stanford.edu>>
>> Sent: Friday, June 30, 2023 7:57 AM
>> To: Barry Smith <bsm...@petsc.dev <mailto:bsm...@petsc.dev>>; Mark Adams 
>> <mfad...@lbl.gov <mailto:mfad...@lbl.gov>>
>> Cc: Matthew Knepley <knep...@gmail.com <mailto:knep...@gmail.com>>; 
>> petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov> 
>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>>
>> Subject: Re: [petsc-users] Using PETSc GPU backend
>>  
>> Barry, Mark and Matt,
>> 
>> Thank you all for the suggestions. I will modify the code so we can pass 
>> runtime options.
>> 
>> Cho
>> From: Barry Smith <bsm...@petsc.dev <mailto:bsm...@petsc.dev>>
>> Sent: Friday, June 30, 2023 7:01 AM
>> To: Mark Adams <mfad...@lbl.gov <mailto:mfad...@lbl.gov>>
>> Cc: Matthew Knepley <knep...@gmail.com <mailto:knep...@gmail.com>>; Ng, 
>> Cho-Kuen <c...@slac.stanford.edu <mailto:c...@slac.stanford.edu>>; 
>> petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov> 
>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>>
>> Subject: Re: [petsc-users] Using PETSc GPU backend
>>  
>> 
>>   Note that options like -mat_type aijcusparse  -vec_type cuda only work if 
>> the program is set up to allow runtime swapping of matrix and vector types. 
>> If you have a call to MatCreateMPIAIJ() or other specific types then then 
>> these options do nothing but because Mark had you use -options_left the 
>> program will tell you at the end that it did not use the option so you will 
>> know.
>> 
>>> On Jun 30, 2023, at 9:30 AM, Mark Adams <mfad...@lbl.gov 
>>> <mailto:mfad...@lbl.gov>> wrote:
>>> 
>>> PetscCall(PetscInitialize(&argc, &argv, NULL, help)); gives us the args and 
>>> you run:
>>> 
>>> a.out -mat_type aijcusparse -vec_type cuda -log_view -options_left
>>> 
>>> Mark
>>> 
>>> On Fri, Jun 30, 2023 at 6:16 AM Matthew Knepley <knep...@gmail.com 
>>> <mailto:knep...@gmail.com>> wrote:
>>> On Fri, Jun 30, 2023 at 1:13 AM Ng, Cho-Kuen via petsc-users 
>>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>> wrote:
>>> Mark,
>>> 
>>> The application code reads in parameters from an input file, where we can 
>>> put the PETSc runtime options. Then we pass the options to 
>>> PetscInitialize(...). Does that sounds right?
>>> 
>>> PETSc will read command line argument automatically in PetscInitialize() 
>>> unless you shut it off.
>>> 
>>>   Thanks,
>>> 
>>>     Matt
>>>  
>>> Cho
>>> From: Ng, Cho-Kuen <c...@slac.stanford.edu <mailto:c...@slac.stanford.edu>>
>>> Sent: Thursday, June 29, 2023 8:32 PM
>>> To: Mark Adams <mfad...@lbl.gov <mailto:mfad...@lbl.gov>>
>>> Cc: petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov> 
>>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>>
>>> Subject: Re: [petsc-users] Using PETSc GPU backend
>>>  
>>> Mark,
>>> 
>>> Thanks for the information. How do I put the runtime options for the 
>>> executable, say, a.out, which does not have the provision to append 
>>> arguments? Do I need to change the C++ main to read in the options?
>>> 
>>> Cho
>>> From: Mark Adams <mfad...@lbl.gov <mailto:mfad...@lbl.gov>>
>>> Sent: Thursday, June 29, 2023 5:55 PM
>>> To: Ng, Cho-Kuen <c...@slac.stanford.edu <mailto:c...@slac.stanford.edu>>
>>> Cc: petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov> 
>>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>>
>>> Subject: Re: [petsc-users] Using PETSc GPU backend
>>>  
>>> Run with options: -mat_type aijcusparse -vec_type cuda -log_view 
>>> -options_left
>>> 
>>> The last column of the performance data (from -log_view) will be the 
>>> percent flops on the GPU. Check that that is > 0.
>>> 
>>> The end of the output will list the options that were used and options that 
>>> were _not_ used (if any). Check that there are no options left.
>>> 
>>> Mark
>>> 
>>> On Thu, Jun 29, 2023 at 7:50 PM Ng, Cho-Kuen via petsc-users 
>>> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>> wrote:
>>> I installed PETSc on Perlmutter using "spack install petsc+cuda+zoltan" and 
>>> used it by "spack load petsc/fwge6pf". Then I compiled the application code 
>>> (purely CPU code) linking to the petsc package, hoping that I can get 
>>> performance improvement using the petsc GPU backend. However, the timing 
>>> was the same using the same number of MPI tasks with and without GPU 
>>> accelerators. Have I missed something in the process, for example, setting 
>>> up PETSc options at runtime to use the GPU backend?
>>> 
>>> Thanks,
>>> Cho
>>> 
>>> 
>>> -- 
>>> What most experimenters take for granted before they begin their 
>>> experiments is infinitely more interesting than any results to which their 
>>> experiments lead.
>>> -- Norbert Wiener
>>> 
>>> https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
>> 
>> 
>> 
>> -- 
>> What most experimenters take for granted before they begin their experiments 
>> is infinitely more interesting than any results to which their experiments 
>> lead.
>> -- Norbert Wiener
>> 
>> https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments 
> is infinitely more interesting than any results to which their experiments 
> lead.
> -- Norbert Wiener
> 
> https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>