Virtual machine (VM) instances in the Research Cloud are available in “Standard” sizes:

  • Small (1 core), Medium (2 cores), Large (4 cores), Extra-large or XL (8 cores), XXL (16 cores)

  • Per core, a VM gets 4 GB of memory (RAM) and 30 GB of On-Instance Storage (On-Instance secondary drive).

  • Example: an XL VM instance has 8 processing cores, 32 GB RAM and 240 GB of On-Instance Storage.

You may be unsure how many CPU cores you need for your instance: Which flavor to choose?

You will have to take a closer look at your application to find out how much you can benefit from several cores.

Some applications are single-threaded, which means they only use one core anyway. In this case, you will not benefit from launching an instance with several cores.
However many applications are multi-threaded, and support a certain number of threads (you may regard a thread as a working process that can run on its own core). The documentation of your application should reveal more information about that.

Example:
Say your application supports 6 threads. You may then select up to the same number of cores — with 6 cores, the application will run at maximum performance. If you choose more cores, you will not experience any performance gains. It is ok to select a lower number of cores: say, you choose 2 cores while your application supports 6 threads. In this case, the workload of the 6 threads will be distributed across the 2 cores, and your application will run one third of its maximum performance (2 cores / 6 threads).

image

Use of OpenMP

OpenMP is an API that can be used for multi-thread handling on shared memory systems.

image

A computer (or a virtual machine) with several CPUs is a shared memory system when all CPUs access the same memory.

OpenMP uses a set of compiler directives (within the C code) that generate a multi-threaded version of your code at compile time.

You can think of OpenMP as a slightly more automated way to create multi-threaded programs than when low-level thread libraries as pthread were used. As such, also OpenMP programs gain maximum performance when it is given the same amount of cores as it uses threads (the cores may also be distributed over several CPUs of the same machine).