What are CPUSETS ?

• Web Servers running multiple instances of the same web application.
• Servers running different applications (for instance, a web server and a database).
• HPC applications, especially in NUMA machines.

CPUSETS allow to:

• create sets of CPUs on the system, and bind applications to them
• provide a way to create sets of cpus *inside* a set of cpus : hence a system administrator can partition a system among users, and users can partition their partition among their applications.
• The same applies to memory : the memory used by a cpuset can be restricted to some of the nodes of a NUMA system

Typical Usage

• CPU-bound applications : Many applications (as it is often the case for HPC apps) use to have a "one process on one processor" policy. They can use sched_setaffinity() to do so, but what if we have to run several such apps at the same time ? One can do this by creating a cpuset for each app.
• Web Serving : A server containing a web server (apache for instance) and a database (MySQL, Oracle) may want to have one part of the machine dedicated for each task. A cpuset can be created for each task. If the server is used to run two instances of this web-system, cpusets can be used to split the system in two, and then in each partition create one area for the web server and one area for the database.
• User administration : an administrator may give fixed numbers of CPUs to some classes of users and leave the rest to other users, for instance. He can do so by creating cpusets and assigning them to users.
• Critical applications : processors inside strict areas may not be used by other areas. Thus, a critical application (real time ...) may be run inside an area and be sure that other processes won't use its CPU. This implies that others applications won't be able to lower its reactivity. This can be done by creating a cpuset for the critical application, and another for all the other tasks.

• migration: Change on the fly the execution area of a whole set of processes (to give more resources to a critical application, for example). When you change the CPU list of a cpusets, all processes that belong to this cpuset will be migrated, if necessary, to stay inside the CPU list.
• virtualization: translate the masks of CPUs given to sched_setaffinity() so they stay inside the set of CPUs. With this mechanism, processors are virtualized, for the use of sched_setaffinity() and /proc information. Thus, any former application using this syscall to bind processes to processors will work with virtual CPUs without any change. A new file is added in each cpuset, in the cpuset filesystem, to choose whether a cpuset is virtualized, or not.