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MEMORYALLOCATORS(9) NetBSD Kernel Developer's Manual MEMORYALLOCATORS(9)
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memoryallocators -- introduction to kernel memory allocators
The NetBSD kernel provides several memory allocators, each with different
characteristics and purpose. This document summarizes the main differ-
ences between them.
You should use the kmem(9) allocator for all allocations unless you have
special needs that it does not provide, such as:
· use from interrupt handlers
· a minimum reserved number of allocations
· a maximum usable number of allocations
· costly object initialization that can be reused
· allocating resources other than pageable RAM-backed kernel virtual
The Kmem Allocator
The kmem(9) allocator is main general purpose allocator in the kernel.
It was modelled after an interface of the same name implemented in
kmem(9) is fast and requires no setup. It cannot be used from interrupt
Internally, kmem(9) is implemented using a collection of pool caches for
common small allocation sizes, so there is no performance benefit to
using a pool cache if you have no other needs.
The Pool Allocator
The pool(9) allocator is a fixed-size memory allocator which requires
setup to initialize a shared pool.
A pool can be configured with a low-water mark to reserve a minimum num-
ber of objects available, a high-water mark to bound the maximum number
of objects in reserve, and a hard limit to bound on the maximum number of
objects in use.
pool_get(9) can be used to allocate memory in interrupt context for
objects that have been reserved in advance, with the possibility of fail-
ure if there are none.
By default, pool(9) allocates pageable RAM-backed kernel virtual address
space from the same backing store as kmem(9), but it can be configured to
allocate any kind of resource with a custom allocator.
The Pool Cache Allocator
The pool cache allocator is a per-CPU cache on top of pool(9) for fixed-
size memory allocations that may occur in interrupt context requiring
The per-CPU cache makes allocation much cheaper -- no interprocessor syn-
chronization in the fast case -- at the cost of potentially caching some
extra resources on one CPU that cannot be used by another.
In addition to all the features of a pool like a custom backing alloca-
tor, a pool cache also supports a constructor and destructor routine for
when objects are drawn from the shared pool in case the per-CPU cache is
empty, or returned to it when the cache is full. This can reduce the
cost of reusable initialization and finalization, or associate objects
with CPU-local resources.
The UVM Kernel Memory Allocator
The uvm_km(9) API is a low-level memory allocator for page-aligned kernel
virtual address space in multiples of PAGE_SIZE, with wired RAM backing,
pageable RAM backing, or backing to be supplied by the caller with
The VMEM Allocator API
The vmem(9) API is a general address space allocator. It is used inter-
nally by kmem(9), pool(9), uvm(9), and other kernel subsystems and device
drivers to allocate regions of various kinds of address spaces. Inter-
nally, it allocates large chunks of the address space and uses a
pool_cache(9) to draw small allocations out of them.
The Extent Manager
The extent(9) API manages and allocates constrained regions of an address
space. The extent manager is optimized for simplicity, not speed, and is
available early at boot. NetBSD uses extent(9) to reserve regions of I/O
port and memory spaces to prevent drivers from using the same device reg-
isters or bus memory.
bus_space(9), extent(9), intro(9), kmem(9), pool(9), pool_cache(9),
uvm(9), uvm_km(9), vmem(9)
Elad Efrat <elad@NetBSD.org>
YAMAMOTO Takashi <yamt@NetBSD.org>
Taylor R Campbell <riastradh@NetBSD.org>
NetBSD 9.0 October 28, 2017 NetBSD 9.0