Since each node has its own memory in a distributed memory system, there is no such thing as "global variable" that can be accessed by all processes. So you need to use MPI to scatter the input from rank 0 process to all the other processes explicitly.
From: dtustud...@hotmail.com To: us...@open-mpi.org List-Post: users@lists.open-mpi.org Date: Mon, 12 Jul 2010 20:33:04 -0600 Subject: Re: [OMPI users] OpenMPI load data to multiple nodes thanks very much !!! May I use global variable to do that ? It means that all nodes have the same global variable, such as globalVector. In the initialization, only node 0 load data from files and assign values to the globalVector. After that, all other nodes can get the same data by accessing the globalVector. Does it make sense ? Any help is appreciated. Jack July 12 2010 > Date: Mon, 12 Jul 2010 21:44:34 -0400 > From: g...@ldeo.columbia.edu > To: us...@open-mpi.org > Subject: Re: [OMPI users] OpenMPI load data to multiple nodes > > Hi Jack/Jinxu > > Jack Bryan wrote: > > Dear All, > > > > I am working on a multi-computer Open MPI cluster system. > > > > If I put some data files in /home/mypath/folder, is it possible that all > > non-head nodes can access the files in the folder ? > > > > Yes, possible, for instance, if the /home/mypath/folder directory is > NFS mounted on all nodes/computers. > Otherwise, if all disks and directories are local to each computer, > you need to copy the input files to the local disks before you > start, and copy the output files back to your login computer after the > program ends. > > > I need to load some data to some nodes, if all nodes can access the > > data, I do not need to load them to each node one by one. > > > > If multiple nodes access the same file to get data, is there conflict ? > > > > To some extent. > The OS (on the computer where the file is located) > will do the arbitration on which process gets the hold of the file at > each time. > If you have 1000 processes, this means a lot of arbitration, > and most likely contention. > Even for two processes only, if the processes are writing data to a > single file, this won't ensure that they write > the output data in the order that you want. > > > For example, > > > > fopen(myFile) by node 1, at the same time fopen(myFile) by node 2. > > > > Is it allowed to do that on MPI cluster without conflict ? > > > > I think MPI won't have any control over this. > It is up to the operational system, and depends on > which process gets its "fopen" request to the OS first, > which is not a deterministic sequence of events. > That is not a clean technique. > > You could instead: > > 1) Assign a single process, say, rank 0, > to read and write data from/to the file(s). > Then use, say, MPI_Scatter[v] and MPI_Gather[v], > to distribute and collect the data back and forth > between that process (rank 0) and all other processes. > > That is an old fashioned but very robust technique. > It avoids any I/O conflict or contention among processes. > All the data flows across the processes via MPI. > The OS receives I/O requests from a single process (rank 0). > > Besides MPI_Gather/MPI_Scatter, look also at MPI_Bcast, > if you need to send the same data to all processes, > assuming the data is being read by a single process. > > 2) Alternatively, you could use the MPI I/O functions, > if your files are binary. > > I hope it helps, > Gus Correa > > > Any help is appreciated. > > > > Jinxu Ding > > > > July 12 2010 > > > > ------------------------------------------------------------------------ > > The New Busy think 9 to 5 is a cute idea. Combine multiple calendars > > with Hotmail. Get busy. > > <http://www.windowslive.com/campaign/thenewbusy?tile=multicalendar&ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_5> > > > > > > ------------------------------------------------------------------------ > > > > _______________________________________________ > > users mailing list > > us...@open-mpi.org > > http://www.open-mpi.org/mailman/listinfo.cgi/users > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users The New Busy is not the old busy. Search, chat and e-mail from your inbox. Get started. _________________________________________________________________ Your E-mail and More On-the-Go. Get Windows Live Hotmail Free. https://signup.live.com/signup.aspx?id=60969
