151 lines
8.6 KiB
ReStructuredText
151 lines
8.6 KiB
ReStructuredText
.. _numa:
|
|
|
|
Started Nov 1999 by Kanoj Sarcar <kanoj@sgi.com>
|
|
|
|
=============
|
|
What is NUMA?
|
|
=============
|
|
|
|
This question can be answered from a couple of perspectives: the
|
|
hardware view and the Linux software view.
|
|
|
|
From the hardware perspective, a NUMA system is a computer platform that
|
|
comprises multiple components or assemblies each of which may contain 0
|
|
or more CPUs, local memory, and/or IO buses. For brevity and to
|
|
disambiguate the hardware view of these physical components/assemblies
|
|
from the software abstraction thereof, we'll call the components/assemblies
|
|
'cells' in this document.
|
|
|
|
Each of the 'cells' may be viewed as an SMP [symmetric multi-processor] subset
|
|
of the system--although some components necessary for a stand-alone SMP system
|
|
may not be populated on any given cell. The cells of the NUMA system are
|
|
connected together with some sort of system interconnect--e.g., a crossbar or
|
|
point-to-point link are common types of NUMA system interconnects. Both of
|
|
these types of interconnects can be aggregated to create NUMA platforms with
|
|
cells at multiple distances from other cells.
|
|
|
|
For Linux, the NUMA platforms of interest are primarily what is known as Cache
|
|
Coherent NUMA or ccNUMA systems. With ccNUMA systems, all memory is visible
|
|
to and accessible from any CPU attached to any cell and cache coherency
|
|
is handled in hardware by the processor caches and/or the system interconnect.
|
|
|
|
Memory access time and effective memory bandwidth varies depending on how far
|
|
away the cell containing the CPU or IO bus making the memory access is from the
|
|
cell containing the target memory. For example, access to memory by CPUs
|
|
attached to the same cell will experience faster access times and higher
|
|
bandwidths than accesses to memory on other, remote cells. NUMA platforms
|
|
can have cells at multiple remote distances from any given cell.
|
|
|
|
Platform vendors don't build NUMA systems just to make software developers'
|
|
lives interesting. Rather, this architecture is a means to provide scalable
|
|
memory bandwidth. However, to achieve scalable memory bandwidth, system and
|
|
application software must arrange for a large majority of the memory references
|
|
[cache misses] to be to "local" memory--memory on the same cell, if any--or
|
|
to the closest cell with memory.
|
|
|
|
This leads to the Linux software view of a NUMA system:
|
|
|
|
Linux divides the system's hardware resources into multiple software
|
|
abstractions called "nodes". Linux maps the nodes onto the physical cells
|
|
of the hardware platform, abstracting away some of the details for some
|
|
architectures. As with physical cells, software nodes may contain 0 or more
|
|
CPUs, memory and/or IO buses. And, again, memory accesses to memory on
|
|
"closer" nodes--nodes that map to closer cells--will generally experience
|
|
faster access times and higher effective bandwidth than accesses to more
|
|
remote cells.
|
|
|
|
For some architectures, such as x86, Linux will "hide" any node representing a
|
|
physical cell that has no memory attached, and reassign any CPUs attached to
|
|
that cell to a node representing a cell that does have memory. Thus, on
|
|
these architectures, one cannot assume that all CPUs that Linux associates with
|
|
a given node will see the same local memory access times and bandwidth.
|
|
|
|
In addition, for some architectures, again x86 is an example, Linux supports
|
|
the emulation of additional nodes. For NUMA emulation, linux will carve up
|
|
the existing nodes--or the system memory for non-NUMA platforms--into multiple
|
|
nodes. Each emulated node will manage a fraction of the underlying cells'
|
|
physical memory. NUMA emluation is useful for testing NUMA kernel and
|
|
application features on non-NUMA platforms, and as a sort of memory resource
|
|
management mechanism when used together with cpusets.
|
|
[see Documentation/cgroup-v1/cpusets.txt]
|
|
|
|
For each node with memory, Linux constructs an independent memory management
|
|
subsystem, complete with its own free page lists, in-use page lists, usage
|
|
statistics and locks to mediate access. In addition, Linux constructs for
|
|
each memory zone [one or more of DMA, DMA32, NORMAL, HIGH_MEMORY, MOVABLE],
|
|
an ordered "zonelist". A zonelist specifies the zones/nodes to visit when a
|
|
selected zone/node cannot satisfy the allocation request. This situation,
|
|
when a zone has no available memory to satisfy a request, is called
|
|
"overflow" or "fallback".
|
|
|
|
Because some nodes contain multiple zones containing different types of
|
|
memory, Linux must decide whether to order the zonelists such that allocations
|
|
fall back to the same zone type on a different node, or to a different zone
|
|
type on the same node. This is an important consideration because some zones,
|
|
such as DMA or DMA32, represent relatively scarce resources. Linux chooses
|
|
a default Node ordered zonelist. This means it tries to fallback to other zones
|
|
from the same node before using remote nodes which are ordered by NUMA distance.
|
|
|
|
By default, Linux will attempt to satisfy memory allocation requests from the
|
|
node to which the CPU that executes the request is assigned. Specifically,
|
|
Linux will attempt to allocate from the first node in the appropriate zonelist
|
|
for the node where the request originates. This is called "local allocation."
|
|
If the "local" node cannot satisfy the request, the kernel will examine other
|
|
nodes' zones in the selected zonelist looking for the first zone in the list
|
|
that can satisfy the request.
|
|
|
|
Local allocation will tend to keep subsequent access to the allocated memory
|
|
"local" to the underlying physical resources and off the system interconnect--
|
|
as long as the task on whose behalf the kernel allocated some memory does not
|
|
later migrate away from that memory. The Linux scheduler is aware of the
|
|
NUMA topology of the platform--embodied in the "scheduling domains" data
|
|
structures [see Documentation/scheduler/sched-domains.txt]--and the scheduler
|
|
attempts to minimize task migration to distant scheduling domains. However,
|
|
the scheduler does not take a task's NUMA footprint into account directly.
|
|
Thus, under sufficient imbalance, tasks can migrate between nodes, remote
|
|
from their initial node and kernel data structures.
|
|
|
|
System administrators and application designers can restrict a task's migration
|
|
to improve NUMA locality using various CPU affinity command line interfaces,
|
|
such as taskset(1) and numactl(1), and program interfaces such as
|
|
sched_setaffinity(2). Further, one can modify the kernel's default local
|
|
allocation behavior using Linux NUMA memory policy.
|
|
[see Documentation/admin-guide/mm/numa_memory_policy.rst.]
|
|
|
|
System administrators can restrict the CPUs and nodes' memories that a non-
|
|
privileged user can specify in the scheduling or NUMA commands and functions
|
|
using control groups and CPUsets. [see Documentation/cgroup-v1/cpusets.txt]
|
|
|
|
On architectures that do not hide memoryless nodes, Linux will include only
|
|
zones [nodes] with memory in the zonelists. This means that for a memoryless
|
|
node the "local memory node"--the node of the first zone in CPU's node's
|
|
zonelist--will not be the node itself. Rather, it will be the node that the
|
|
kernel selected as the nearest node with memory when it built the zonelists.
|
|
So, default, local allocations will succeed with the kernel supplying the
|
|
closest available memory. This is a consequence of the same mechanism that
|
|
allows such allocations to fallback to other nearby nodes when a node that
|
|
does contain memory overflows.
|
|
|
|
Some kernel allocations do not want or cannot tolerate this allocation fallback
|
|
behavior. Rather they want to be sure they get memory from the specified node
|
|
or get notified that the node has no free memory. This is usually the case when
|
|
a subsystem allocates per CPU memory resources, for example.
|
|
|
|
A typical model for making such an allocation is to obtain the node id of the
|
|
node to which the "current CPU" is attached using one of the kernel's
|
|
numa_node_id() or CPU_to_node() functions and then request memory from only
|
|
the node id returned. When such an allocation fails, the requesting subsystem
|
|
may revert to its own fallback path. The slab kernel memory allocator is an
|
|
example of this. Or, the subsystem may choose to disable or not to enable
|
|
itself on allocation failure. The kernel profiling subsystem is an example of
|
|
this.
|
|
|
|
If the architecture supports--does not hide--memoryless nodes, then CPUs
|
|
attached to memoryless nodes would always incur the fallback path overhead
|
|
or some subsystems would fail to initialize if they attempted to allocated
|
|
memory exclusively from a node without memory. To support such
|
|
architectures transparently, kernel subsystems can use the numa_mem_id()
|
|
or cpu_to_mem() function to locate the "local memory node" for the calling or
|
|
specified CPU. Again, this is the same node from which default, local page
|
|
allocations will be attempted.
|