MPI: cores or processors?

Question

Hi I am kind of MPI noob so please bear with me on this one. :)

Say I have an MPI program called foo.c and I run the executable with

mpirun -np 3 ./foo

Now this means the program will be run in parallel using 3 processors (1 process per processor). But since most processors today have more than one core, (take 2 cores per processor say) does this mean the program will be run on 3 cores or 3 processors?

Probably this has to do with my poor understanding of what the difference between a core and a processor really is so if you could also explain a little more that would be helpful.

Thank you.

Coray

If you cat /proc/cpuinfo on a Linux machine, you will see that each hyperthread is counted as a processor, coincidentally consistent with MPI's usage of "processor". – Flail

Answer 1

mpirun will execute a number of "processes" on the machine. The cpu or core where these processes are executed is operating-system dependent. On a N cpu machines with M cores on each cpu, you have room for N*M processes running at full speed.

But, typically:

• If you have multiple cores, each process will run on a separate core
• If you ask for more processes than the available core*cpus, everything will run, but with a lower efficiency (yes, you can run multi-process jobs on a single-cpu single-core machine...)
• If you are using a queuing system or a preconfigured MPI system for which a list of remote machines exists, the allocation will be distributed on the remote machines.

(Depending of the mpi implementation, there might be some options to force a specific cpu or core, but you should not need to worry about that).

Afton

Thanks for the answer it helped me too! – Whichsoever

Answer 2

Distribution of processes to cores and processors is handled by the operating system and the MPI implementation. Running on a desktop, the operating system will generally put each process on a different core, potentially redistributing processes during run-time. In larger systems such a s a supercomputer or a cluster, the distribution is handled by resource managers such as SLURM. However this happens, one or multiple processes will be assigned to each core.

Regarding hardware, a core can run only a single process at a time. Technologies such as hyper-threading allows multiple processes to share the resources of a single core. There are cases where two or more processes per core is optimal. For instance, if a processes is doing a large amount of file I/O another may take its place and do computation while the first is hung on a read or write.

In short, give MPI the number of processes you want to execute. Distribution of these processes is then handled transparent to the user. The number of processes that you use should be determined by requirements of the application (powers of 2, number of files to be read), the number of cores available, and the optimal number of processes per core for the application.

Harkins

Answer 3

The OS Scheduler will try to optimally allocate separate cores to your parallel application's processes in a multi core system OR to separate processors in multi processor system.

The interesting case is a multi-core multi cpu system. Again you can let the OS Scheduler do it for you , OR you can enforce the ( logical/physical) core affinity to your processes to bind them to a particular core.

Betts

Answer 4

The mpirun command uses a hostlist. If don't specify it, it will probably use "localhost" and run all your processes there. If you run 3 processes and you have a 4 core machine, you probably get good speedup because the OS will generally put them on different cores. If you only have two cores, then one core will get two processes.

The previous is not entirely true, since the OS is allowed to move processes, so you may want to use numactl to bind them to a core.

If you are on a multi-node cluster, then a well-setup mpi will generate a hostfile where each node appears as many times as it has cores. So on a 4 node cluster with 8 cores per node, you can request up to 32 processes and expect close to perfect speedup. (If your code and your algorithm allow that, of course.) Requesting 9 processes on that cluster may put 8 on one node and the 9th on another, which is of course not great for performance. You'd hope that your cluster software comes with an mpirun that spreads the processes out better than that.

Stenger

Answer 5

from performance view of MPI job,there are some explicit rule: 1) if you code is pure MPI code (BLAS is not tuned with openMP), turn off hyperthread and set the tasks number of job per node to the cores of node 2) if you code is MPI+openMP, you can set PPN (processes per node) to the cores of node and OMP_NUM_THEADS to the 2(if there are two hardware threads per core) 3) if you code is MPI+openMP and you cluster is huge then you can set PPN (processes per node) to 1 and OMP_NUM_THEADS to the logical CPU numbers to save the communication overhead

Seymore

Answer 6

In order to provide a useful framework I would consider this hierarchy:

• a motherboard can hold one or more chips/dice;
  • a chip/die can contain one or more cores (independent CPUs);
    • a CPU can work out one or more threads concurrently (the multithreading I know of consists of two threads)

In the early days, you had most often one motherboard with one chip with one CPU running one thread. Only one process at a time could be dealt with, and the attending hardware set was referred to as the processor. There was was one-to-one mapping between pieces of software (the task to run) and pieces of hardware (the device to run the task).

Process is definitely a software notion. 'Thread' is, cast quite simply, a specification of 'process' in the context of parallel concurrent computing. Nowadays processor can refer to a physical device as well as its extended processing capabilities (multithreading again, which to be sure is a technological implementation). For example, you can have machines with two chips on the motherboard, with four core/CPUs per chip, and with each core/CPUs running two threads concurrently. Then you would be able to run 2x4x2=16 processes (without oversubscription of resources, of courses).

The MPI syntax you quote addresses processes (option np), or threads if you like. The description part of man mpirun even refers to processes as 'slots' (for example, see the specs for the hostfile).

Slots indicate how many processes can potentially execute on a node.

This usage sounds like a legacy of that close correspondence between units of hardware and units of software that was standard back then. 'Slot' is originally a material/hardware feature, not unlike the term 'socket' that has undergone a similar change of semantics at times.

So indeed I feel quite some sympathy for your confusion. If you are a Linux user, you can visualize the report of cat /proc/cpuinfo. These lines refer to one processor named '2' out of four:

processor   : 2
...
physical id : 0
siblings    : 4
core id     : 2
cpu cores   : 4

They say that in this one machine I have gotten only one chip (since 'phyical id' takes only one value in the whole list, omitted), that this one chip as 4 'cpu cores' and that this one chip is running four siblings (4 threads, so there is no multithreading). In this case there are 4 processing elements, and 4 cpu cores.

In the example above with multithreading, you would see a listing for 16 processors, 2 values for 'physical id' (chips), 'cpu cores' equal to 4 (per chip) and `siblings' equal to 8 (per chip) since multithreading is enabled on that chip. In this case you have four times as many processors as cores.

Therefore, in this extended context, 'processor' indicates the machine's capability to work on a 'process', and this is what MPI and you want to use, regardeless of the number and feats of cores that can enable this. You only need to gain an overview of where these processing capabilities come from.

Another useful Linux command is then lscpu:

...
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
...

There 'socket' indeed is the physical connection in the motherboard where the chip is plugged into, so it is a byname of chip indeed. Indeed no multithreading here.

I am indebted to the discussions in this other post https://unix.stackexchange.com/q/146051/132913

Wood