On Oct 1, 2011, at 9:22 PM, Dave Nystrom wrote: > Hi Barry, > > I've sent a couple more emails on this topic. What I am trying to do at the > moment is to figure out how to have a problem run on only one gpu if it will > fit in the memory of that gpu. Back in April when I had built petsc-dev with > Cuda 3.2, petsc would only use one gpu if you had multiple gpus on your > machine. In order to use multiple gpus for a problem, one had to use > multiple threads with a separate thread assigned to control each gpu. But > Cuda 4.0 has, I believe, made that transparent and under the hood. So now > when I run a small example problem such as > src/ksp/ksp/examples/tutorials/ex2f.F with an 800x800 problem, it gets > partitioned to run on both of the gpus in my machine. The result is a very > large performance hit because of communication back and forth from one gpu to > the other via the cpu.
How do you know there is lots of communication from the GPU to the CPU? In the -log_summary? Nope because PETSc does not manage anything like that (that is one CPU process using both GPUs). > So this problem with a 3200x3200 grid runs 5x slower > now than it did with Cuda 3.2. I believe if one is programming down at the > cuda level, it is possible to have a smaller problem run on only one gpu so > that there is communication only between the cpu and gpu and only at the > start and end of the calculation. > > To me, it seems like what is needed is a petsc option to specify the number > of gpus to run on that can somehow get passed down to the cuda level through > cusp and thrust. I fear that the short term solution is going to have to be > for me to pull one of the gpus out of my desktop system but it would be nice > if there was a way to tell petsc and friends to just use one gpu when I want > it to. > > If necessary, I can send a couple of log files to demonstrate what I am > trying to describe regarding the performance hit. I am not convinced that the poor performance you are getting now has anything to do with using both GPUs. Please run a PETSc program with the command -cuda_show_devices What are the choices? You can then pick one of them and run with -cuda_set_device integer Does this change things? Barry > > Thanks, > > Dave > > Barry Smith writes: >> Dave, >> >> We have no mechanism in the PETSc code for a PETSc single CPU process to >> use two GPUs at the same time. However you could have two MPI processes >> each using their own GPU. >> >> The one tricky part is you need to make sure each MPI process uses a >> different GPU. We currently do not have a mechanism to do this assignment >> automatically. I think it can be done with cudaSetDevice(). But I don't >> know the details, sending this to petsc-dev at mcs.anl.gov where more people >> may know. >> >> PETSc-folks, >> >> We need a way to have this setup automatically. >> >> Barry >> >> On Oct 1, 2011, at 5:43 PM, Dave Nystrom wrote: >> >>> I'm running petsc on a machine with Cuda 4.0 and 2 gpus. This is a desktop >>> machine with a single processor. I know that Cuda 4.0 has support for >>> running on multiple gpus but don't know if petsc uses that. But suppose I >>> have a problem that will fit in the memory for a single gpu. Will petsc run >>> the problem on a single gpu or does it split it between the 2 gpus and incur >>> the communication overhead of copying data between the two gpus? >>> >>> Thanks, >>> >>> Dave >>> >>