April 2018 - petsc-dev - mailman3.cels.anl.gov

Re: [petsc-dev] getting on with the nested timers
by Matthew Knepley 05 Apr '18

05 Apr '18

On Thu, Apr 5, 2018 at 7:50 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> wrote: > ??? In my original email I mention that page and state that the > command line option is working. > > > Were you expecting me to understand from your reply that the > option :ascii_xml somehow translates to > PetscViewerPushFormat(viewer,"PETSC_VIEWER_ASCII_XML")? > Yes. The format for viewer input is -foo_view type:filename:format:mode where any argument can be omitted so that it takes the default. The defaults are type - ascii filename - stdout format - FORMAT_DEFAULT mode - read Thanks, Matt > Chris > ------------------------------ > *From:* Matthew Knepley <knepley(a)gmail.com> > *Sent:* Thursday, April 05, 2018 1:35 PM > *To:* Klaij, Christiaan > *Cc:* Smith, Barry F.; petsc-dev(a)mcs.anl.gov; Koos Huijssen > *Subject:* Re: [petsc-dev] getting on with the nested timers > > On Thu, Apr 5, 2018 at 7:31 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> > wrote: > >> Matt, >> >> I guess you meant to send this link: >> >> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/ >> Viewer/PetscViewerPushFormat.html >> > No, I meant to send the link I sent. In the section Options Database Keys, > it gives you the format. > > Matt > >> >> If so, it doesn't say anything on XML either. But after some >> trial and error I found the format to >> be "PETSC_VIEWER_ASCII_XML", so problem solved. >> >> Summarizing the problem from a user perspective: >> >> 1) The manual page of PetscLogView mentions PetscViewerASCIIOpen, >> which mentions PetscViewerPushFormat, I should have figured that >> out by myself, sorry. >> >> 2) The manual page of PetscLogView mentions PetscLogNestedBegin, >> which doesn't have a manual entry. >> >> 3) The manual page of PetscViewerPushFormat doesn't mention the >> format PETSC_VIEWER_ASCII_XML, although it is available. >> >> Chris >> >> dr. ir. Christiaan Klaij | Senior Researcher | Research & Development >> MARIN | T +31 317 49 33 44 | C.Klaij(a)marin.nl | www.marin.nl >> >> [image: LinkedIn] <https://www.linkedin.com/company/marin> [image: >> YouTube] <http://www.youtube.com/marinmultimedia> [image: Twitter] >> <https://twitter.com/MARIN_nieuws> [image: Facebook] >> <https://www.facebook.com/marin.wageningen> >> MARIN news: Comfort draft for semi-submersible yachts >> <http://www.marin.nl/web/News/News-items/Comfort-draft-for-semisubmersible-y…> >> >> ------------------------------ >> *From:* Matthew Knepley <knepley(a)gmail.com> >> *Sent:* Thursday, April 05, 2018 12:50 PM >> *To:* Klaij, Christiaan >> *Cc:* Smith, Barry F.; petsc-dev(a)mcs.anl.gov; Koos Huijssen >> *Subject:* Re: [petsc-dev] getting on with the nested timers >> >> On Thu, Apr 5, 2018 at 6:15 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> >> wrote: >> >>> Barry, >>> >>> I'm still trying to use petsc's nested logging with our code. I >>> have it working for 3.8.3 with the command line option, for >>> example: >>> >>> mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml >>> >>> This gives the expected performance.xml file. How to get the same >>> without command line options? So far I have: >>> >>> CALL PetscLogDefaultBegin(ierr) >>> ... >>> CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) >>> CALL PetscLogView(viewer,ierr) >>> >>> which gives the regular log in the performance.xml file. >>> >>> I guess I should replace PetscLogDefaultBegin by >>> PetscLogNestedBegin as stated in the manual page of PetscLogView. >>> >>> But there's no manual page for PetscLogNestedBegin and I didn't >>> find anything like PetscViewerASCIIOpenXML either. >>> >> >> http://www.mcs.anl.gov/petsc/petsc-master/docs/manualpages/P >> rofiling/PetscLogView.html >> >> You need to push the ASCII_XML format before viewing. >> >> Thanks, >> >> Matt >> >> >>> Chris >>> >>> >>> dr. ir. Christiaan Klaij | Senior Researcher | Research & Development >>> MARIN | T +31 317 49 33 44 | mailto:[email protected] | >>> http://www.marin.nl >>> >>> MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10-nove >>> mber-naar-de-open-dag-in-Wageningen.htm >>> >>> >> >> >> -- >> 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.caam.rice.edu/~mk51/> >> >> > > > -- > 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.caam.rice.edu/~mk51/> > -- 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.caam.rice.edu/~mk51/>

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Re: [petsc-dev] getting on with the nested timers
by Klaij, Christiaan 05 Apr '18

05 Apr '18

??? In my original email I mention that page and state that the command line option is working. Were you expecting me to understand from your reply that the option :ascii_xml somehow translates to PetscViewerPushFormat(viewer,"PETSC_VIEWER_ASCII_XML")? Chris ________________________________ From: Matthew Knepley <knepley(a)gmail.com> Sent: Thursday, April 05, 2018 1:35 PM To: Klaij, Christiaan Cc: Smith, Barry F.; petsc-dev(a)mcs.anl.gov; Koos Huijssen Subject: Re: [petsc-dev] getting on with the nested timers On Thu, Apr 5, 2018 at 7:31 AM, Klaij, Christiaan <C.Klaij(a)marin.nl<mailto:[email protected]>> wrote: Matt, I guess you meant to send this link: http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Viewer/PetscVie… No, I meant to send the link I sent. In the section Options Database Keys, it gives you the format. Matt If so, it doesn't say anything on XML either. But after some trial and error I found the format to be "PETSC_VIEWER_ASCII_XML", so problem solved. Summarizing the problem from a user perspective: 1) The manual page of PetscLogView mentions PetscViewerASCIIOpen, which mentions PetscViewerPushFormat, I should have figured that out by myself, sorry. 2) The manual page of PetscLogView mentions PetscLogNestedBegin, which doesn't have a manual entry. 3) The manual page of PetscViewerPushFormat doesn't mention the format PETSC_VIEWER_ASCII_XML, although it is available. Chris dr. ir. Christiaan Klaij | Senior Researcher | Research & Development MARIN | T +31 317 49 33 44 | C.Klaij(a)marin.nl<mailto:[email protected]> | www.marin.nl<http://www.marin.nl> [LinkedIn]<https://www.linkedin.com/company/marin> [YouTube] <http://www.youtube.com/marinmultimedia> [Twitter] <https://twitter.com/MARIN_nieuws> [Facebook] <https://www.facebook.com/marin.wageningen> MARIN news: Comfort draft for semi-submersible yachts<http://www.marin.nl/web/News/News-items/Comfort-draft-for-semisubmersible-y…> ________________________________ From: Matthew Knepley <knepley(a)gmail.com<mailto:[email protected]>> Sent: Thursday, April 05, 2018 12:50 PM To: Klaij, Christiaan Cc: Smith, Barry F.; petsc-dev(a)mcs.anl.gov<mailto:[email protected]>; Koos Huijssen Subject: Re: [petsc-dev] getting on with the nested timers On Thu, Apr 5, 2018 at 6:15 AM, Klaij, Christiaan <C.Klaij(a)marin.nl<mailto:[email protected]>> wrote: Barry, I'm still trying to use petsc's nested logging with our code. I have it working for 3.8.3 with the command line option, for example: mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml This gives the expected performance.xml file. How to get the same without command line options? So far I have: CALL PetscLogDefaultBegin(ierr) ... CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) CALL PetscLogView(viewer,ierr) which gives the regular log in the performance.xml file. I guess I should replace PetscLogDefaultBegin by PetscLogNestedBegin as stated in the manual page of PetscLogView. But there's no manual page for PetscLogNestedBegin and I didn't find anything like PetscViewerASCIIOpenXML either. http://www.mcs.anl.gov/petsc/petsc-master/docs/manualpages/Profiling/PetscL… You need to push the ASCII_XML format before viewing. Thanks, Matt Chris dr. ir. Christiaan Klaij | Senior Researcher | Research & Development MARIN | T +31 317 49 33 44 | mailto:[email protected]<mailto:[email protected]> | http://www.marin.nl MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10-november-naar-de-op… -- 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.caam.rice.edu/~mk51/> -- 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.caam.rice.edu/~mk51/>

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Re: [petsc-dev] getting on with the nested timers
by Matthew Knepley 05 Apr '18

05 Apr '18

On Thu, Apr 5, 2018 at 7:31 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> wrote: > Matt, > > I guess you meant to send this link: > > http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Viewer/ > PetscViewerPushFormat.html > No, I meant to send the link I sent. In the section Options Database Keys, it gives you the format. Matt > > If so, it doesn't say anything on XML either. But after some > trial and error I found the format to > be "PETSC_VIEWER_ASCII_XML", so problem solved. > > Summarizing the problem from a user perspective: > > 1) The manual page of PetscLogView mentions PetscViewerASCIIOpen, > which mentions PetscViewerPushFormat, I should have figured that > out by myself, sorry. > > 2) The manual page of PetscLogView mentions PetscLogNestedBegin, > which doesn't have a manual entry. > > 3) The manual page of PetscViewerPushFormat doesn't mention the > format PETSC_VIEWER_ASCII_XML, although it is available. > > Chris > > dr. ir. Christiaan Klaij | Senior Researcher | Research & Development > MARIN | T +31 317 49 33 44 | C.Klaij(a)marin.nl | www.marin.nl > > [image: LinkedIn] <https://www.linkedin.com/company/marin> [image: > YouTube] <http://www.youtube.com/marinmultimedia> [image: Twitter] > <https://twitter.com/MARIN_nieuws> [image: Facebook] > <https://www.facebook.com/marin.wageningen> > MARIN news: Comfort draft for semi-submersible yachts > <http://www.marin.nl/web/News/News-items/Comfort-draft-for-semisubmersible-y…> > > ------------------------------ > *From:* Matthew Knepley <knepley(a)gmail.com> > *Sent:* Thursday, April 05, 2018 12:50 PM > *To:* Klaij, Christiaan > *Cc:* Smith, Barry F.; petsc-dev(a)mcs.anl.gov; Koos Huijssen > *Subject:* Re: [petsc-dev] getting on with the nested timers > > On Thu, Apr 5, 2018 at 6:15 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> > wrote: > >> Barry, >> >> I'm still trying to use petsc's nested logging with our code. I >> have it working for 3.8.3 with the command line option, for >> example: >> >> mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml >> >> This gives the expected performance.xml file. How to get the same >> without command line options? So far I have: >> >> CALL PetscLogDefaultBegin(ierr) >> ... >> CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) >> CALL PetscLogView(viewer,ierr) >> >> which gives the regular log in the performance.xml file. >> >> I guess I should replace PetscLogDefaultBegin by >> PetscLogNestedBegin as stated in the manual page of PetscLogView. >> >> But there's no manual page for PetscLogNestedBegin and I didn't >> find anything like PetscViewerASCIIOpenXML either. >> > > http://www.mcs.anl.gov/petsc/petsc-master/docs/manualpages/ > Profiling/PetscLogView.html > > You need to push the ASCII_XML format before viewing. > > Thanks, > > Matt > > >> Chris >> >> >> dr. ir. Christiaan Klaij | Senior Researcher | Research & Development >> MARIN | T +31 317 49 33 44 | mailto:[email protected] | >> http://www.marin.nl >> >> MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10-nove >> mber-naar-de-open-dag-in-Wageningen.htm >> >> > > > -- > 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.caam.rice.edu/~mk51/> > > -- 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.caam.rice.edu/~mk51/>

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Re: [petsc-dev] getting on with the nested timers
by Klaij, Christiaan 05 Apr '18

05 Apr '18

Matt, I guess you meant to send this link: http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Viewer/PetscVie… If so, it doesn't say anything on XML either. But after some trial and error I found the format to be "PETSC_VIEWER_ASCII_XML", so problem solved. Summarizing the problem from a user perspective: 1) The manual page of PetscLogView mentions PetscViewerASCIIOpen, which mentions PetscViewerPushFormat, I should have figured that out by myself, sorry. 2) The manual page of PetscLogView mentions PetscLogNestedBegin, which doesn't have a manual entry. 3) The manual page of PetscViewerPushFormat doesn't mention the format PETSC_VIEWER_ASCII_XML, although it is available. Chris dr. ir. Christiaan Klaij | Senior Researcher | Research & Development MARIN | T +31 317 49 33 44 | C.Klaij(a)marin.nl<mailto:[email protected]> | www.marin.nl<http://www.marin.nl> [LinkedIn]<https://www.linkedin.com/company/marin> [YouTube] <http://www.youtube.com/marinmultimedia> [Twitter] <https://twitter.com/MARIN_nieuws> [Facebook] <https://www.facebook.com/marin.wageningen> MARIN news: Comfort draft for semi-submersible yachts<http://www.marin.nl/web/News/News-items/Comfort-draft-for-semisubmersible-y…> ________________________________ From: Matthew Knepley <knepley(a)gmail.com> Sent: Thursday, April 05, 2018 12:50 PM To: Klaij, Christiaan Cc: Smith, Barry F.; petsc-dev(a)mcs.anl.gov; Koos Huijssen Subject: Re: [petsc-dev] getting on with the nested timers On Thu, Apr 5, 2018 at 6:15 AM, Klaij, Christiaan <C.Klaij(a)marin.nl<mailto:[email protected]>> wrote: Barry, I'm still trying to use petsc's nested logging with our code. I have it working for 3.8.3 with the command line option, for example: mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml This gives the expected performance.xml file. How to get the same without command line options? So far I have: CALL PetscLogDefaultBegin(ierr) ... CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) CALL PetscLogView(viewer,ierr) which gives the regular log in the performance.xml file. I guess I should replace PetscLogDefaultBegin by PetscLogNestedBegin as stated in the manual page of PetscLogView. But there's no manual page for PetscLogNestedBegin and I didn't find anything like PetscViewerASCIIOpenXML either. http://www.mcs.anl.gov/petsc/petsc-master/docs/manualpages/Profiling/PetscL… You need to push the ASCII_XML format before viewing. Thanks, Matt Chris dr. ir. Christiaan Klaij | Senior Researcher | Research & Development MARIN | T +31 317 49 33 44 | mailto:[email protected]<mailto:[email protected]> | http://www.marin.nl MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10-november-naar-de-op… -- 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.caam.rice.edu/~mk51/>

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Re: [petsc-dev] getting on with the nested timers
by Matthew Knepley 05 Apr '18

05 Apr '18

On Thu, Apr 5, 2018 at 6:15 AM, Klaij, Christiaan <C.Klaij(a)marin.nl> wrote: > Barry, > > I'm still trying to use petsc's nested logging with our code. I > have it working for 3.8.3 with the command line option, for > example: > > mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml > > This gives the expected performance.xml file. How to get the same > without command line options? So far I have: > > CALL PetscLogDefaultBegin(ierr) > ... > CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) > CALL PetscLogView(viewer,ierr) > > which gives the regular log in the performance.xml file. > > I guess I should replace PetscLogDefaultBegin by > PetscLogNestedBegin as stated in the manual page of PetscLogView. > > But there's no manual page for PetscLogNestedBegin and I didn't > find anything like PetscViewerASCIIOpenXML either. > http://www.mcs.anl.gov/petsc/petsc-master/docs/manualpages/Profiling/PetscL… You need to push the ASCII_XML format before viewing. Thanks, Matt > Chris > > > dr. ir. Christiaan Klaij | Senior Researcher | Research & Development > MARIN | T +31 317 49 33 44 | mailto:[email protected] | http://www.marin.nl > > MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10- > november-naar-de-open-dag-in-Wageningen.htm > > -- 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.caam.rice.edu/~mk51/>

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getting on with the nested timers
by Klaij, Christiaan 05 Apr '18

05 Apr '18

Barry, I'm still trying to use petsc's nested logging with our code. I have it working for 3.8.3 with the command line option, for example: mpirun -n 3 ./refresco -log_view :performance.xml:ascii_xml This gives the expected performance.xml file. How to get the same without command line options? So far I have: CALL PetscLogDefaultBegin(ierr) ... CALL PetscViewerASCIIOpen(MPI_COMM_WORLD,'performance.xml',viewer,ierr) CALL PetscLogView(viewer,ierr) which gives the regular log in the performance.xml file. I guess I should replace PetscLogDefaultBegin by PetscLogNestedBegin as stated in the manual page of PetscLogView. But there's no manual page for PetscLogNestedBegin and I didn't find anything like PetscViewerASCIIOpenXML either. Chris dr. ir. Christiaan Klaij | Senior Researcher | Research & Development MARIN | T +31 317 49 33 44 | mailto:[email protected] | http://www.marin.nl MARIN news: http://www.marin.nl/web/News/News-items/Kom-zaterdag-10-november-naar-de-op…

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[issue1595] Issues of limited number of MPI communicators when having many instances of hypre boomerAMG with Moose
by Rob Falgout hypre Tracker 04 Apr '18

04 Apr '18

Rob Falgout <rfalgout(a)llnl.gov> added the comment: 1. Yes. AMG coarsens recursively based on matrix entries, so you can't know a priori which ranks will be active by the time you get to the coarsest grid. 2. If you use the other direct solver option, it will be a little less performant. Ulrike can probably comment more quantitatively. The performance degradation will depend on the number of total tasks in the fine grid communicator. Another option that might well work is the iterative method option Ulrike mentioned. If it works for the systems in Moose, then it should be the best option to use. 4. I agree that this would be a mess to debug and to communicate clearly in a user manual. I also think that it is not a common use case. Implementation, and ensuring correctness in the future, especially in the face of new code written by hypre developers, sounds like a fair amount of work. If it really isn't a common use case, then this has to be weighed against the hundreds of other things we would like to do in hypre. Maybe this is easier than I am thinking and there is a bigger demand than I was aware of? I'm not trying to simply write this off. -Rob ____________________________________________ hypre Issue Tracker <hypre-support(a)llnl.gov> <http://cascb1.llnl.gov/hypre/issue1595> ____________________________________________

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Re: [petsc-dev] upcoming release and testing [v2]
by Satish Balay 04 Apr '18

04 Apr '18

On Wed, 4 Apr 2018, Satish Balay wrote: > I have the following branches in queue for master [via next-tmp - this build will start now] > > origin/dalcinl/fix-scatter-destroy > origin/dalcinl/fix-vecscatter-initpkg > origin/knepley/fix-cylinder-mesh > origin/rmills/feature-aijmkl-add-matptap-symmetric > origin/barry/removed-unneeded-mpi-comm-dup-boomeramg > origin/balay/fix-9bus-ex9busdmnetwork-chkstk-on-windows > origin/jed/futurize-stage-1 http://ftp.mcs.anl.gov/pub/petsc/nightlylogs/archive/2018/04/04/next-tmp.ht… next-tmp builds are clean [except for a few example timeouts] - so I merged them all to master. Satish

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Re: [petsc-dev] [issue1595] Issues of limited number of MPI communicators when having many instances of hypre boomerAMG with Moose
by Jed Brown 04 Apr '18

04 Apr '18

1. Are you saying that the ranks involved in the coarse solve depends on the matrix entries? 2. Okay, but this might degrade performance, right? 4. I think you are right that *if* all sends and receives have been posted (may be hard to guarantee if the user is using threads) and MPI_ANY_SOURCE is not used, then there will not be deadlock or incorrect message delivery. But the constraints above are hard to communicate to the user and to verify, and the consequences are so dire (non-deterministic deadlock, likely only seen at very large scale and especially subtle to debug because it would be caused by interaction of seemingly distant components, and users will blame the library) that libraries need to go out of their way to ensure that it doesn't happen. Rob Falgout hypre Tracker <hypre-support(a)llnl.gov> writes: > Rob Falgout <rfalgout(a)llnl.gov> added the comment: > > Hi All, > > Some comments and questions: > > 1. The Comm_create is used to create a subcommunicator that involves only the currently active MPI tasks so that the Allgather() will happen only over that subset. I don't think we can create this once, attach it to a parent communicator, and use it again in a different solve based on a different AMG setup (because the active tasks will likely be different). If the AMG setup is the same, the same communicator will be used. When the AMG Destroy() is called, that communicator is freed. > > 2. Ulrike has already provided a way to get around the Comm_create() by changing one of the AMG parameters. This is something that PETSc has control over and can do. > > 3. The idea of dup'ing a communicator, attaching it as an attribute to a parent communicator, and using it as hypre's internal communicator makes sense to me. I think it would require quite a bit of code changes to implement. My next comment is important to consider as well. > > 4. Regarding pending point-to-point messages, you are right that this could create problems without creating deadlock. I had not thought about this scenario. However, as long as all of the corresponding user send requests have also been issued (in a non-blocking manner), is there still really a problem here? MPI is supposed to guarantee message order, so any receives that hypre posts from the same task with the same tag should not interfere with an already posted (but not finalized) receive that the user has posted, right? > > -Rob > > ____________________________________________ > hypre Issue Tracker <hypre-support(a)llnl.gov> > <http://cascb1.llnl.gov/hypre/issue1595> > ____________________________________________

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Re: [petsc-dev] Nullspace with right preconditioning
by Jed Brown 04 Apr '18

04 Apr '18

Matthew Knepley <knepley(a)gmail.com> writes: > On Wed, Apr 4, 2018 at 3:23 PM, Smith, Barry F. <bsmith(a)mcs.anl.gov> wrote: > >> >> It may not be broken; it may just be misunderstood or incorrect approach. >> >> You'll need to explain why and where exactly you think it is broken. > > > I am solving Stokes. I will push the example for you to run tonight. > Basically, I put in a constant for the pressure > as the nullspace. If you run with gmres/left it gives back a pressure with > integral 0. If you run with gmres/right, > it gives back a pressure with a big integral. The code makes the Krylov basis orthogonal to the null space. With right preconditioning, the solution is x = P^{-1} y for some y in the Krylov space. I don't know how to easily make x orthogonal to the null space so you probably have to project it out after the solve. Note that discrete pressure has sum 0, but the continuous integral of pressure is generally not 0 (unless on a uniform grid). If you want the latter, you'll to do the explicit projection.

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