On systems with kernels compiled with built in support for cpusets, all processes are attached to a cpuset, and cpusets are always present. If a system supports cpusets, then it will have the entry nodev cpuset in the file /proc/filesystems. By mounting the cpuset filesystem (see the EXAMPLES section below), the administrator can configure the cpusets on a system to control the processor and memory placement of processes on that system. By default, if the cpuset configuration on a system is not modified or if the cpuset filesystem is not even mounted, then the cpuset mechanism, though present, has no effect on the system's behavior.
A cpuset defines a list of CPUs and memory nodes.
The CPUs of a system include all the logical processing units on which a process can execute, including, if present, multiple processor cores within a package and Hyper-Threads within a processor core. Memory nodes include all distinct banks of main memory; small and SMP systems typically have just one memory node that contains all the system's main memory, while NUMA (non-uniform memory access) systems have multiple memory nodes.
Cpusets are represented as directories in a hierarchical pseudo-filesystem, where the top directory in the hierarchy (/dev/cpuset) represents the entire system (all online CPUs and memory nodes) and any cpuset that is the child (descendant) of another parent cpuset contains a subset of that parent's CPUs and memory nodes. The directories and files representing cpusets have normal filesystem permissions.
Every process in the system belongs to exactly one cpuset. A process is confined to run only on the CPUs in the cpuset it belongs to, and to allocate memory only on the memory nodes in that cpuset. When a process fork(2)s, the child process is placed in the same cpuset as its parent. With sufficient privilege, a process may be moved from one cpuset to another and the allowed CPUs and memory nodes of an existing cpuset may be changed.
When the system begins booting, a single cpuset is defined that includes all CPUs and memory nodes on the system, and all processes are in that cpuset. During the boot process, or later during normal system operation, other cpusets may be created, as subdirectories of this top cpuset, under the control of the system administrator, and processes may be placed in these other cpusets.
Cpusets are integrated with the sched_setaffinity(2) scheduling affinity mechanism and the mbind(2) and set_mempolicy(2) memory-placement mechanisms in the kernel. Neither of these mechanisms let a process make use of a CPU or memory node that is not allowed by that process's cpuset. If changes to a process's cpuset placement conflict with these other mechanisms, then cpuset placement is enforced even if it means overriding these other mechanisms. The kernel accomplishes this overriding by silently restricting the CPUs and memory nodes requested by these other mechanisms to those allowed by the invoking process's cpuset. This can result in these other calls returning an error, if for example, such a call ends up requesting an empty set of CPUs or memory nodes, after that request is restricted to the invoking process's cpuset.
Typically, a cpuset is used to manage the CPU and memory-node confinement for a set of cooperating processes such as a batch scheduler job, and these other mechanisms are used to manage the placement of individual processes or memory regions within that set or job.
New cpusets are created using the mkdir(2) system call or the mkdir(1) command. The properties of a cpuset, such as its flags, allowed CPUs and memory nodes, and attached processes, are queried and modified by reading or writing to the appropriate file in that cpuset's directory, as listed below.
The pseudo-files in each cpuset directory are automatically created when the cpuset is created, as a result of the mkdir(2) invocation. It is not possible to directly add or remove these pseudo-files.
A cpuset directory that contains no child cpuset directories, and has no attached processes, can be removed using rmdir(2) or rmdir(1). It is not necessary, or possible, to remove the pseudo-files inside the directory before removing it.
The pseudo-files in each cpuset directory are small text files that may be read and written using traditional shell utilities such as cat(1), and echo(1), or from a program by using file I/O library functions or system calls, such as open(2), read(2), write(2), and close(2).
The pseudo-files in a cpuset directory represent internal kernel state and do not have any persistent image on disk. Each of these per-cpuset files is listed and described below.
In addition to the above pseudo-files in each directory below /dev/cpuset, each process has a pseudo-file, /proc/<pid>/cpuset, that displays the path of the process's cpuset directory relative to the root of the cpuset filesystem.
Also the /proc/<pid>/status file for each process has four added lines, displaying the process's Cpus_allowed (on which CPUs it may be scheduled) and Mems_allowed (on which memory nodes it may obtain memory), in the two formats Mask Format and List Format (see below) as shown in the following example:
Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff Cpus_allowed_list: 0-127 Mems_allowed: ffffffff,ffffffff Mems_allowed_list: 0-63
A cpuset that is mem_exclusive restricts kernel allocations for buffer cache pages and other internal kernel data pages commonly shared by the kernel across multiple users. All cpusets, whether mem_exclusive or not, restrict allocations of memory for user space. This enables configuring a system so that several independent jobs can share common kernel data, while isolating each job's user allocation in its own cpuset. To do this, construct a large mem_exclusive cpuset to hold all the jobs, and construct child, non-mem_exclusive cpusets for each individual job. Only a small amount of kernel memory, such as requests from interrupt handlers, is allowed to be placed on memory nodes outside even a mem_exclusive cpuset.
This enables configuring a system so that several independent jobs can share common kernel data, such as filesystem pages, while isolating each job's user allocation in its own cpuset. To do this, construct a large hardwall cpuset to hold all the jobs, and construct child cpusets for each individual job which are not hardwall cpusets.
The default value of notify_on_release in the root cpuset at system boot is disabled (0). The default value of other cpusets at creation is the current value of their parent's notify_on_release setting.
The command /sbin/cpuset_release_agent is invoked, with the name (/dev/cpuset relative path) of the to-be-released cpuset in argv.
The usual contents of the command /sbin/cpuset_release_agent is simply the shell script:
#!/bin/sh rmdir /dev/cpuset/$1
This enables batch managers that are monitoring jobs running in dedicated cpusets to efficiently detect what level of memory pressure that job is causing.
This is useful both on tightly managed systems running a wide mix of submitted jobs, which may choose to terminate or reprioritize jobs that are trying to use more memory than allowed on the nodes assigned them, and with tightly coupled, long-running, massively parallel scientific computing jobs that will dramatically fail to meet required performance goals if they start to use more memory than allowed to them.
This mechanism provides a very economical way for the batch manager to monitor a cpuset for signs of memory pressure. It's up to the batch manager or other user code to decide what action to take if it detects signs of memory pressure.
Unless memory pressure calculation is enabled by setting the pseudo-file /dev/cpuset/cpuset.memory_pressure_enabled, it is not computed for any cpuset, and reads from any memory_pressure always return zero, as represented by the ASCII string "0\n". See the WARNINGS section, below.
A per-cpuset, running average is employed for the following reasons:
The memory_pressure of a cpuset is calculated using a per-cpuset simple digital filter that is kept within the kernel. For each cpuset, this filter tracks the recent rate at which processes attached to that cpuset enter the kernel direct reclaim code.
The kernel direct reclaim code is entered whenever a process has to satisfy a memory page request by first finding some other page to repurpose, due to lack of any readily available already free pages. Dirty filesystem pages are repurposed by first writing them to disk. Unmodified filesystem buffer pages are repurposed by simply dropping them, though if that page is needed again, it will have to be reread from disk.
The cpuset.memory_pressure file provides an integer number representing the recent (half-life of 10 seconds) rate of entries to the direct reclaim code caused by any process in the cpuset, in units of reclaims attempted per second, times 1000.
If the per-cpuset Boolean flag file cpuset.memory_spread_page is set, then the kernel will spread the filesystem buffers (page cache) evenly over all the nodes that the faulting process is allowed to use, instead of preferring to put those pages on the node where the process is running.
If the per-cpuset Boolean flag file cpuset.memory_spread_slab is set, then the kernel will spread some filesystem-related slab caches, such as those for inodes and directory entries, evenly over all the nodes that the faulting process is allowed to use, instead of preferring to put those pages on the node where the process is running.
The setting of these flags does not affect the data segment (see brk(2)) or stack segment pages of a process.
By default, both kinds of memory spreading are off and the kernel prefers to allocate memory pages on the node local to where the requesting process is running. If that node is not allowed by the process's NUMA memory policy or cpuset configuration or if there are insufficient free memory pages on that node, then the kernel looks for the nearest node that is allowed and has sufficient free memory.
When new cpusets are created, they inherit the memory spread settings of their parent.
Setting memory spreading causes allocations for the affected page or slab caches to ignore the process's NUMA memory policy and be spread instead. However, the effect of these changes in memory placement caused by cpuset-specified memory spreading is hidden from the mbind(2) or set_mempolicy(2) calls. These two NUMA memory policy calls always appear to behave as if no cpuset-specified memory spreading is in effect, even if it is. If cpuset memory spreading is subsequently turned off, the NUMA memory policy most recently specified by these calls is automatically reapplied.
Both cpuset.memory_spread_page and cpuset.memory_spread_slab are Boolean flag files. By default, they contain "0", meaning that the feature is off for that cpuset. If a "1" is written to that file, that turns the named feature on.
Cpuset-specified memory spreading behaves similarly to what is known (in other contexts) as round-robin or interleave memory placement.
Cpuset-specified memory spreading can provide substantial performance improvements for jobs that:
Without this policy, the memory allocation across the nodes in the job's cpuset can become very uneven, especially for jobs that might have just a single thread initializing or reading in the data set.
When memory migration is enabled in a cpuset, if the mems setting of the cpuset is changed, then any memory page in use by any process in the cpuset that is on a memory node that is no longer allowed will be migrated to a memory node that is allowed.
Furthermore, if a process is moved into a cpuset with memory_migrate enabled, any memory pages it uses that were on memory nodes allowed in its previous cpuset, but which are not allowed in its new cpuset, will be migrated to a memory node allowed in the new cpuset.
The relative placement of a migrated page within the cpuset is preserved during these migration operations if possible. For example, if the page was on the second valid node of the prior cpuset, then the page will be placed on the second valid node of the new cpuset, if possible.
The algorithmic cost of load balancing and its impact on key shared kernel data structures such as the process list increases more than linearly with the number of CPUs being balanced. For example, it costs more to load balance across one large set of CPUs than it does to balance across two smaller sets of CPUs, each of half the size of the larger set. (The precise relationship between the number of CPUs being balanced and the cost of load balancing depends on implementation details of the kernel process scheduler, which is subject to change over time, as improved kernel scheduler algorithms are implemented.)
The per-cpuset flag sched_load_balance provides a mechanism to suppress this automatic scheduler load balancing in cases where it is not needed and suppressing it would have worthwhile performance benefits.
By default, load balancing is done across all CPUs, except those marked isolated using the kernel boot time "isolcpus=" argument. (See Scheduler Relax Domain Level, below, to change this default.)
This default load balancing across all CPUs is not well suited to the following two situations:
When the per-cpuset flag sched_load_balance is enabled (the default setting), it requests load balancing across all the CPUs in that cpuset's allowed CPUs, ensuring that load balancing can move a process (not otherwise pinned, as by sched_setaffinity(2)) from any CPU in that cpuset to any other.
When the per-cpuset flag sched_load_balance is disabled, then the scheduler will avoid load balancing across the CPUs in that cpuset, except in so far as is necessary because some overlapping cpuset has sched_load_balance enabled.
So, for example, if the top cpuset has the flag sched_load_balance enabled, then the scheduler will load balance across all CPUs, and the setting of the sched_load_balance flag in other cpusets has no effect, as we're already fully load balancing.
Therefore in the above two situations, the flag sched_load_balance should be disabled in the top cpuset, and only some of the smaller, child cpusets would have this flag enabled.
When doing this, you don't usually want to leave any unpinned processes in the top cpuset that might use nontrivial amounts of CPU, as such processes may be artificially constrained to some subset of CPUs, depending on the particulars of this flag setting in descendant cpusets. Even if such a process could use spare CPU cycles in some other CPUs, the kernel scheduler might not consider the possibility of load balancing that process to the underused CPU.
On small systems, such as those with just a few CPUs, immediate load balancing is useful to improve system interactivity and to minimize wasteful idle CPU cycles. But on large systems, attempting immediate load balancing across a large number of CPUs can be more costly than it is worth, depending on the particular performance characteristics of the job mix and the hardware.
The exact meaning of the small integer values of sched_relax_domain_level will depend on internal implementation details of the kernel scheduler code and on the non-uniform architecture of the hardware. Both of these will evolve over time and vary by system architecture and kernel version.
As of this writing, when this capability was introduced in Linux 2.6.26, on certain popular architectures, the positive values of sched_relax_domain_level have the following meanings.
The sched_relax_domain_level value of zero (0) always means don't perform immediate load balancing, hence that load balancing is done only periodically, not immediately when a CPU becomes available or another task becomes runnable.
The sched_relax_domain_level value of minus one (-1) always means use the system default value. The system default value can vary by architecture and kernel version. This system default value can be changed by kernel boot-time "relax_domain_level=" argument.
In the case of multiple overlapping cpusets which have conflicting sched_relax_domain_level values, then the highest such value applies to all CPUs in any of the overlapping cpusets. In such cases, the value minus one (-1) is the lowest value, overridden by any other value, and the value zero (0) is the next lowest value.
This format displays each 32-bit word in hexadecimal (using ASCII characters "0" - "9" and "a" - "f"); words are filled with leading zeros, if required. For masks longer than one word, a comma separator is used between words. Words are displayed in big-endian order, which has the most significant bit first. The hex digits within a word are also in big-endian order.
The number of 32-bit words displayed is the minimum number needed to display all bits of the bit mask, based on the size of the bit mask.
Examples of the Mask Format:
00000001 # just bit 0 set 40000000,00000000,00000000 # just bit 94 set 00000001,00000000,00000000 # just bit 64 set 000000ff,00000000 # bits 32-39 set 00000000,000e3862 # 1,5,6,11-13,17-19 set
A mask with bits 0, 1, 2, 4, 8, 16, 32, and 64 set displays as:
Examples of the List Format:
For instance, a process can put itself in some other cpuset (than its current one) if it can write the tasks file for that cpuset. This requires execute permission on the encompassing directories and write permission on the tasks file.
An additional constraint is applied to requests to place some other process in a cpuset. One process may not attach another to a cpuset unless it would have permission to send that process a signal (see kill(2)).
A process may create a child cpuset if it can access and write the parent cpuset directory. It can modify the CPUs or memory nodes in a cpuset if it can access that cpuset's directory (execute permissions on the each of the parent directories) and write the corresponding cpus or mems file.
There is one minor difference between the manner in which these permissions are evaluated and the manner in which normal filesystem operation permissions are evaluated. The kernel interprets relative pathnames starting at a process's current working directory. Even if one is operating on a cpuset file, relative pathnames are interpreted relative to the process's current working directory, not relative to the process's current cpuset. The only ways that cpuset paths relative to a process's current cpuset can be used are if either the process's current working directory is its cpuset (it first did a cd or chdir(2) to its cpuset directory beneath /dev/cpuset, which is a bit unusual) or if some user code converts the relative cpuset path to a full filesystem path.
In theory, this means that user code should specify cpusets using absolute pathnames, which requires knowing the mount point of the cpuset filesystem (usually, but not necessarily, /dev/cpuset). In practice, all user level code that this author is aware of simply assumes that if the cpuset filesystem is mounted, then it is mounted at /dev/cpuset. Furthermore, it is common practice for carefully written user code to verify the presence of the pseudo-file /dev/cpuset/tasks in order to verify that the cpuset pseudo-filesystem is currently mounted.
echo 19 > cpuset.mems
failed because memory node 19 was not allowed (perhaps the current system does not have a memory node 19), then the echo command might not display any error. It is better to use the /bin/echo external command to change cpuset file settings, as this command will display write(2) errors, as in the example:
If hot-plug functionality is used to remove all the CPUs that are currently assigned to a cpuset, then the kernel will automatically update the cpus_allowed of all processes attached to CPUs in that cpuset to allow all CPUs. When memory hot-plug functionality for removing memory nodes is available, a similar exception is expected to apply there as well. In general, the kernel prefers to violate cpuset placement, rather than starving a process that has had all its allowed CPUs or memory nodes taken offline. User code should reconfigure cpusets to refer only to online CPUs and memory nodes when using hot-plug to add or remove such resources.
A few kernel-critical, internal memory-allocation requests, marked GFP_ATOMIC, must be satisfied immediately. The kernel may drop some request or malfunction if one of these allocations fail. If such a request cannot be satisfied within the current process's cpuset, then we relax the cpuset, and look for memory anywhere we can find it. It's better to violate the cpuset than stress the kernel.
The possible errno settings and their meaning when set on a failed cpuset call are as listed below.
For example, the following sequence of commands will set up a cpuset named "Charlie", containing just CPUs 2 and 3, and memory node 1, and then attach the current shell to that cpuset.
$ mkdir /dev/cpuset $ mount -t cpuset cpuset /dev/cpuset $ cd /dev/cpuset $ mkdir Charlie $ cd Charlie $ /bin/echo 2-3 > cpuset.cpus $ /bin/echo 1 > cpuset.mems $ /bin/echo $$ > tasks # The current shell is now running in cpuset Charlie # The next line should display '/Charlie' $ cat /proc/self/cpuset
The following sequence of commands accomplishes this.
$ cd /dev/cpuset $ mkdir beta $ cd beta $ /bin/echo 16-19 > cpuset.cpus $ /bin/echo 8-9 > cpuset.mems $ /bin/echo 1 > cpuset.memory_migrate $ while read i; do /bin/echo $i; done < ../alpha/tasks > tasks
The above should move any processes in alpha to beta, and any memory held by these processes on memory nodes 2-3 to memory nodes 8-9, respectively.
Notice that the last step of the above sequence did not do:
$ cp ../alpha/tasks tasks
The while loop, rather than the seemingly easier use of the cp(1) command, was necessary because only one process PID at a time may be written to the tasks file.
The same effect (writing one PID at a time) as the while loop can be accomplished more efficiently, in fewer keystrokes and in syntax that works on any shell, but alas more obscurely, by using the -u (unbuffered) option of sed(1):