Copyright (C) 2004 BULL SA.
Written by Simon.Derr@bull.net
-Portions Copyright (c) 2004 Silicon Graphics, Inc.
+Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
Modified by Paul Jackson <pj@sgi.com>
+Modified by Christoph Lameter <clameter@sgi.com>
CONTENTS:
=========
1.1 What are cpusets ?
1.2 Why are cpusets needed ?
1.3 How are cpusets implemented ?
- 1.4 How do I use cpusets ?
+ 1.4 What are exclusive cpusets ?
+ 1.5 What does notify_on_release do ?
+ 1.6 What is memory_pressure ?
+ 1.7 What is memory spread ?
+ 1.8 How do I use cpusets ?
2. Usage Examples and Syntax
2.1 Basic Usage
2.2 Adding/removing cpus
allocate a page on a node that is not allowed in the requesting tasks
mems_allowed vector.
-If a cpuset is cpu or mem exclusive, no other cpuset, other than a direct
-ancestor or descendent, may share any of the same CPUs or Memory Nodes.
-
User level code may create and destroy cpusets by name in the cpuset
virtual file system, manage the attributes and permissions of these
cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
But larger systems, which benefit more from careful processor and
memory placement to reduce memory access times and contention,
and which typically represent a larger investment for the customer,
-can benefit from explictly placing jobs on properly sized subsets of
+can benefit from explicitly placing jobs on properly sized subsets of
the system.
This can be especially valuable on:
and a database), or
* NUMA systems running large HPC applications with demanding
performance characteristics.
+ * Also cpu_exclusive cpusets are useful for servers running orthogonal
+ workloads such as RT applications requiring low latency and HPC
+ applications that are throughput sensitive
These subsets, or "soft partitions" must be able to be dynamically
adjusted, as the job mix changes, without impacting other concurrently
-executing jobs.
+executing jobs. The location of the running jobs pages may also be moved
+when the memory locations are changed.
The kernel cpuset patch provides the minimum essential kernel
mechanisms required to efficiently implement such subsets. It
1.3 How are cpusets implemented ?
---------------------------------
-Cpusets provide a Linux kernel (2.6.7 and above) mechanism to constrain
-which CPUs and Memory Nodes are used by a process or set of processes.
+Cpusets provide a Linux kernel mechanism to constrain which CPUs and
+Memory Nodes are used by a process or set of processes.
The Linux kernel already has a pair of mechanisms to specify on which
CPUs a task may be scheduled (sched_setaffinity) and on which Memory
- A cpuset may be marked exclusive, which ensures that no other
cpuset (except direct ancestors and descendents) may contain
any overlapping CPUs or Memory Nodes.
+ Also a cpu_exclusive cpuset would be associated with a sched
+ domain.
- You can list all the tasks (by pid) attached to any cpuset.
The implementation of cpusets requires a few, simple hooks
into the rest of the kernel, none in performance critical paths:
- - in main/init.c, to initialize the root cpuset at system boot.
+ - in init/main.c, to initialize the root cpuset at system boot.
- in fork and exit, to attach and detach a task from its cpuset.
- in sched_setaffinity, to mask the requested CPUs by what's
allowed in that tasks cpuset.
- in sched.c migrate_all_tasks(), to keep migrating tasks within
the CPUs allowed by their cpuset, if possible.
+ - in sched.c, a new API partition_sched_domains for handling
+ sched domain changes associated with cpu_exclusive cpusets
+ and related changes in both sched.c and arch/ia64/kernel/domain.c
- in the mbind and set_mempolicy system calls, to mask the requested
Memory Nodes by what's allowed in that tasks cpuset.
- - in page_alloc, to restrict memory to allowed nodes.
+ - in page_alloc.c, to restrict memory to allowed nodes.
- in vmscan.c, to restrict page recovery to the current cpuset.
In addition a new file system, of type "cpuset" may be mounted,
- cpus: list of CPUs in that cpuset
- mems: list of Memory Nodes in that cpuset
+ - memory_migrate flag: if set, move pages to cpusets nodes
- cpu_exclusive flag: is cpu placement exclusive?
- mem_exclusive flag: is memory placement exclusive?
- tasks: list of tasks (by pid) attached to that cpuset
+ - notify_on_release flag: run /sbin/cpuset_release_agent on exit?
+ - memory_pressure: measure of how much paging pressure in cpuset
+
+In addition, the root cpuset only has the following file:
+ - memory_pressure_enabled flag: compute memory_pressure?
New cpusets are created using the mkdir system call or shell
command. The properties of a cpuset, such as its flags, allowed
to represent the cpuset hierarchy provides for a familiar permission
and name space for cpusets, with a minimum of additional kernel code.
-1.4 How do I use cpusets ?
+
+1.4 What are exclusive cpusets ?
+--------------------------------
+
+If a cpuset is cpu or mem exclusive, no other cpuset, other than
+a direct ancestor or descendent, may share any of the same CPUs or
+Memory Nodes.
+
+A cpuset that is cpu_exclusive has a scheduler (sched) domain
+associated with it. The sched domain consists of all CPUs in the
+current cpuset that are not part of any exclusive child cpusets.
+This ensures that the scheduler load balancing code only balances
+against the CPUs that are in the sched domain as defined above and
+not all of the CPUs in the system. This removes any overhead due to
+load balancing code trying to pull tasks outside of the cpu_exclusive
+cpuset only to be prevented by the tasks' cpus_allowed mask.
+
+A cpuset that is mem_exclusive restricts kernel allocations for
+page, buffer and other data 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,
+such as file system pages, while isolating each jobs 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 typical kernel memory,
+such as requests from interrupt handlers, is allowed to be taken
+outside even a mem_exclusive cpuset.
+
+
+1.5 What does notify_on_release do ?
+------------------------------------
+
+If the notify_on_release flag is enabled (1) in a cpuset, then whenever
+the last task in the cpuset leaves (exits or attaches to some other
+cpuset) and the last child cpuset of that cpuset is removed, then
+the kernel runs the command /sbin/cpuset_release_agent, supplying the
+pathname (relative to the mount point of the cpuset file system) of the
+abandoned cpuset. This enables automatic removal of abandoned 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 parents notify_on_release setting.
+
+
+1.6 What is memory_pressure ?
+-----------------------------
+The memory_pressure of a cpuset provides a simple per-cpuset metric
+of the rate that the tasks in a cpuset are attempting to free up in
+use memory on the nodes of the cpuset to satisfy additional memory
+requests.
+
+This enables batch managers 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 re-prioritize 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 to do about it and
+take action.
+
+==> Unless this feature is enabled by writing "1" to the special file
+ /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
+ code of __alloc_pages() for this metric reduces to simply noticing
+ that the cpuset_memory_pressure_enabled flag is zero. So only
+ systems that enable this feature will compute the metric.
+
+Why a per-cpuset, running average:
+
+ Because this meter is per-cpuset, rather than per-task or mm,
+ the system load imposed by a batch scheduler monitoring this
+ metric is sharply reduced on large systems, because a scan of
+ the tasklist can be avoided on each set of queries.
+
+ Because this meter is a running average, instead of an accumulating
+ counter, a batch scheduler can detect memory pressure with a
+ single read, instead of having to read and accumulate results
+ for a period of time.
+
+ Because this meter is per-cpuset rather than per-task or mm,
+ the batch scheduler can obtain the key information, memory
+ pressure in a cpuset, with a single read, rather than having to
+ query and accumulate results over all the (dynamically changing)
+ set of tasks in the cpuset.
+
+A per-cpuset simple digital filter (requires a spinlock and 3 words
+of data per-cpuset) is kept, and updated by any task attached to that
+cpuset, if it enters the synchronous (direct) page reclaim code.
+
+A per-cpuset file provides an integer number representing the recent
+(half-life of 10 seconds) rate of direct page reclaims caused by
+the tasks in the cpuset, in units of reclaims attempted per second,
+times 1000.
+
+
+1.7 What is memory spread ?
+---------------------------
+There are two boolean flag files per cpuset that control where the
+kernel allocates pages for the file system buffers and related in
+kernel data structures. They are called 'memory_spread_page' and
+'memory_spread_slab'.
+
+If the per-cpuset boolean flag file 'memory_spread_page' is set, then
+the kernel will spread the file system buffers (page cache) evenly
+over all the nodes that the faulting task is allowed to use, instead
+of preferring to put those pages on the node where the task is running.
+
+If the per-cpuset boolean flag file 'memory_spread_slab' is set,
+then the kernel will spread some file system related slab caches,
+such as for inodes and dentries evenly over all the nodes that the
+faulting task is allowed to use, instead of preferring to put those
+pages on the node where the task is running.
+
+The setting of these flags does not affect anonymous data segment or
+stack segment pages of a task.
+
+By default, both kinds of memory spreading are off, and memory
+pages are allocated on the node local to where the task is running,
+except perhaps as modified by the tasks NUMA mempolicy or cpuset
+configuration, so long as sufficient free memory pages are available.
+
+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 tasks NUMA mempolicy and be spread
+instead. Tasks using mbind() or set_mempolicy() calls to set NUMA
+mempolicies will not notice any change in these calls as a result of
+their containing tasks memory spread settings. If memory spreading
+is turned off, then the currently specified NUMA mempolicy once again
+applies to memory page allocations.
+
+Both 'memory_spread_page' and '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, then that turns
+the named feature on.
+
+The implementation is simple.
+
+Setting the flag 'memory_spread_page' turns on a per-process flag
+PF_SPREAD_PAGE for each task that is in that cpuset or subsequently
+joins that cpuset. The page allocation calls for the page cache
+is modified to perform an inline check for this PF_SPREAD_PAGE task
+flag, and if set, a call to a new routine cpuset_mem_spread_node()
+returns the node to prefer for the allocation.
+
+Similarly, setting 'memory_spread_cache' turns on the flag
+PF_SPREAD_SLAB, and appropriately marked slab caches will allocate
+pages from the node returned by cpuset_mem_spread_node().
+
+The cpuset_mem_spread_node() routine is also simple. It uses the
+value of a per-task rotor cpuset_mem_spread_rotor to select the next
+node in the current tasks mems_allowed to prefer for the allocation.
+
+This memory placement policy is also known (in other contexts) as
+round-robin or interleave.
+
+This policy can provide substantial improvements for jobs that need
+to place thread local data on the corresponding node, but that need
+to access large file system data sets that need to be spread across
+the several nodes in the jobs cpuset in order to fit. Without this
+policy, especially for jobs that might have one thread reading in the
+data set, the memory allocation across the nodes in the jobs cpuset
+can become very uneven.
+
+
+1.8 How do I use cpusets ?
--------------------------
In order to minimize the impact of cpusets on critical kernel
impacting the scheduler code in the kernel with a check for changes
in a tasks processor placement.
-There is an exception to the above. If hotplug funtionality is used
+Normally, once a page is allocated (given a physical page
+of main memory) then that page stays on whatever node it
+was allocated, so long as it remains allocated, even if the
+cpusets memory placement policy 'mems' subsequently changes.
+If the cpuset flag file 'memory_migrate' is set true, then when
+tasks are attached to that cpuset, any pages that task had
+allocated to it on nodes in its previous cpuset are migrated
+to the tasks new cpuset. The relative placement of the 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.
+
+Also if 'memory_migrate' is set true, then if that cpusets
+'mems' file is modified, pages allocated to tasks in that
+cpuset, that were on nodes in the previous setting of 'mems',
+will be moved to nodes in the new setting of 'mems.'
+Pages that were not in the tasks prior cpuset, or in the cpusets
+prior 'mems' setting, will not be moved.
+
+There is an exception to the above. If hotplug 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
tasks attached to CPUs in that cpuset to allow all CPUs. When memory
# The next line should display '/Charlie'
cat /proc/self/cpuset
-In the case that a change of cpuset includes wanting to move already
-allocated memory pages, consider further the work of IWAMOTO
-Toshihiro <iwamoto@valinux.co.jp> for page remapping and memory
-hotremoval, which can be found at:
-
- http://people.valinux.co.jp/~iwamoto/mh.html
-
-The integration of cpusets with such memory migration is not yet
-available.
-
In the future, a C library interface to cpusets will likely be
available. For now, the only way to query or modify cpusets is
via the cpuset file system, using the various cd, mkdir, echo, cat,
git://git.onelab.eu / linux-2.6.git / blobdiff