MEMORYALLOCATORS(9)    NetBSD Kernel Developer's Manual    MEMORYALLOCATORS(9)

     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
         address space

   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
     setup beforehand.

     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 <>
     YAMAMOTO Takashi <>
     Taylor R Campbell <>

NetBSD 9.0                     October 28, 2017                     NetBSD 9.0

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