32
votes
Accepted
Why would you need frameworks like MPI when you can multi-task using threads?
There is one real and one practical reason.
First, MPI was developed at a time when machines had exactly one processor core and when we wanted to couple different machines. It is today used on ...
- 52.4k
12
votes
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What is "good" parallel scaling?
Good is a relative term, and it will depend on the nature of the problem, the nature of the algorithm, and properties of the hardware involved. The only absolute reference point is ideal scaling (100%...
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10
votes
Communication overhead in supercomputing
A long standing favorite benchmark in high performance computing has been the HPLinpack benchmark, which measures the speed of a computer system in floating point operations per second while solving a ...
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10
votes
Accepted
Writing a parallel version of an algorithm. Only which someparts are worth distributing
If the operation is as trivial as you say, and each node has all the information necessary to carry out the operation, then the communication will be substantially more expensive than recomputing ...
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9
votes
Why would you need frameworks like MPI when you can multi-task using threads?
Wolfgang Bangerth's answer is totally correct, and I only want to add one practical aspect.
Portability across hardware
Let's say you write a research code from scratch. You have a powerful multi-...
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6
votes
MPI+OpenMP Scalability
No. You need to test it to that scale, especially if you have MPI calls within OpenMP regions.
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6
votes
How does one write MPI-implementation-independent code?
Bill Barth already gave great advice. My advice is to not read the MPICH or OpenMPI documents. Read the MPI standard instead. As far as standards are concerned, it's actually quite readable. ...
- 52.4k
6
votes
Accepted
How does one write MPI-implementation-independent code?
Always use the correct types as specified by the standard. MPI_Comm for your communicators not int unless you're in Fortran. Etc., etc.
This should be relatively easy. What problems are you really ...
- 10.9k
6
votes
Communication overhead in supercomputing
The honest answer is that we don't know. The answer depends heavily on what is actually being run and what code the user has written. As Brian Borchers points out, there's a big difference between two ...
- 10.9k
6
votes
Why my parallel code using MPI is much slower than the serial one?
The first thing you need to ask yourself: is your problem big enough that the overhead of MPI messaging is less than the work that you save. Your problem size is 10k which is small, but on the other ...
- 1,305
5
votes
Distributed (MPI) matrix matrix multiplication
You say that you want an MPI version. Then you need to study the literature, as the distributed memory variant of matrix-matrix product are not a simple parallellization of the sequential version.
...
- 1,305
5
votes
Writing a parallel version of an algorithm. Only which someparts are worth distributing
An addition to @Daniel Shapero's answer.
It might also be important to know if there are computations that can lead to different results depending on which machine they are launched (or just vary ...
- 8,542
5
votes
CPU usage when a MPI rank waits during a blocking communication
It depends on the communication settings you use for MPI. In blocking communication, MPI has three wait modes.
Aggressive busy wait. This is a kind of default mode. Open MPI, at least, uses this when ...
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4
votes
Accepted
How do I make sparse solvers to accept custom matvec function insted of matrix?
PETSc does this. See the documentation for MatShell and Section 3.3 in the manual.
- 839
4
votes
Accepted
Reference Suggestions for MPI
The two books that come to mind are Using MPI and Using Advanced MPI by Gropp and Lusk. They were also released in a 3rd edition last year, so they are very up to date. As others have said, the ...
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4
votes
Accepted
MapReduce with MPI question
As Wolfgang observed, you are basically transposing a connectivity matrix: every process $i$ knows row $i$ that states which $j$ values you're sending to. What you're interested in is the columns: the ...
- 1,305
4
votes
Accepted
How do I interpret the results from MPI_Wtime()?
That depends on your goal.
For one process it's obvious that $t$ is the wall-time of the entire program launch.
For two processes, you got $t_1$ and $t_2$ which correspond for the walltime on ...
- 8,542
4
votes
Causes for different results for different number of nodes in MPI
There is no way for anyone to tell you where the issue comes from without access to the actual code. But here are common reasons why the output (legitimately) differs for different numbers of ...
- 52.4k
3
votes
Accepted
How does MPI differentiate between two computers?
The standard is purposefully silent on these issues preferring to leave such issue to library implementers to figure out. There's no need to standardize such mechanisms. Most network hardware has a ...
- 10.9k
3
votes
Reference Suggestions for MPI
First, you should make sure that you understand the fundamentals of parallel processing first. It's very hard to write good MPI code if you don't understand the principles of distributed computing. ...
- 2,096
3
votes
Any recommendations for unit-testing frameworks compatible with code/libraries that use MPI?
The Teuchos Unit test harness in Trilinos natively supports unit tests that use MPI. Things like controlling output from multiple processes and aggregating pass/fail over all processes is automatic. ...
3
votes
Is it possible to configure Linux OpenMPI v1.6.5 to use multiple working directories on a single node?
Instead of launching the executable, why don't you launch a script that changes directories into the one you want and then execs your program? OpenMPI sets ...
- 10.9k
3
votes
Accepted
All eigenpairs of large sparse symmetric matrix
To emphasize what the comments have said, if you want to produce the dense 52728x52728 matrix containing the eigenvectors, there is no point to computing using the sparse input. Use a dense input ...
- 2,096
3
votes
Accepted
General understanding of Intel MKL, threads and MPI
Nothing stops you from decomposing the problem up yourself and feeding the relevant partitioned data into MKL sequentially, or even in parallel. It will work as long as you avoid data races, but you ...
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3
votes
MPI data broadcast or not in C
The question I think you're asking, based on your code, is:
A. Better to have each node do redundant work
B. Better to have one node do the work distribute the results.
The answer is usually A.
To ...
- 3,586
3
votes
Accepted
Send Petsc sequential matrix to another MPI rank
I don't think PETSc supports it. PETSc really thinks in parallel, so it converts at most between distributed matrices and sequential matrices through submatrix taking operations. I would MatGetArray ...
- 1,305
3
votes
Affecting parts of an array outside MPI process efficiently
The basic model of MPI is "two-sided communication": you have a sender who knows where to send, and a receiver who knows from where to expect something. In your description that is not the ...
- 1,305
3
votes
Recommended CPU for CFD running parallel, FDS (Fire Dynamics Simulator)
Here are some generic recommendations on HPC workstation and server node procurement.
First, figure out if you need ECC memory or not, since that determines whether you buy Intel Core and AMD Ryzen ...
- 2,096
3
votes
How does a blocking MPI all to all synchronization work?
Your theory 1 is correct. Why do you think all ranks "release" at the same time? They don't. Why would this even be necessary? Your mental model of MPI is incorrect. Processes run completely ...
- 1,305
2
votes
Accepted
Is it valid MPI-3 to nest non-blocking collectives?
Yes.
The reason I asked was that with a particular implementation (cray-mpich 7.0.4), in a particular environment on a particular system, the second MPI_Wait call ...
- 839
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