1 VMEM(9)                    Device Driver Interfaces                    VMEM(9)
   2 
   3 NAME
   4      vmem - virtual memory allocator
   5 
   6 DESCRIPTION
   7    Overview
   8      An address space is divided into a number of logically distinct pieces,
   9      or arenas: text, data, heap, stack, and so on.  Within these arenas we
  10      often subdivide further; for example, we use heap addresses not only for
  11      the kernel heap (kmem_alloc() space), but also for DVMA, bp_mapin(),
  12      /dev/kmem, and even some device mappings.
  13 
  14      The kernel address space, therefore, is most accurately described as a
  15      tree of arenas in which each node of the tree imports some subset of its
  16      parent.  The virtual memory allocator manages these arenas and supports
  17      their natural hierarchical structure.
  18 
  19    Arenas
  20      An arena is nothing more than a set of integers.  These integers most
  21      commonly represent virtual addresses, but in fact they can represent
  22      anything at all.  For example, we could use an arena containing the
  23      integers minpid through maxpid to allocate process IDs.  For uses of this
  24      nature, prefer id_space(9F) instead.
  25 
  26      vmem_create() and vmem_destroy() create and destroy vmem arenas.  In
  27      order to differentiate between arenas used for addresses and arenas used
  28      for identifiers, the VMC_IDENTIFIER flag is passed to vmem_create().
  29      This prevents identifier exhaustion from being diagnosed as general
  30      memory failure.
  31 
  32    Spans
  33      We represent the integers in an arena as a collection of spans, or
  34      contiguous ranges of integers.  For example, the kernel heap consists of
  35      just one span: [kernelheap, ekernelheap).  Spans can be added to an arena
  36      in two ways: explicitly, by vmem_add(); or implicitly, by importing, as
  37      described in Imported Memory below.
  38 
  39    Segments
  40      Spans are subdivided into segments, each of which is either allocated or
  41      free.  A segment, like a span, is a contiguous range of integers.  Each
  42      allocated segment [addr, addr + size) represents exactly one
  43      vmem_alloc(size) that returned addr.  Free segments represent the space
  44      between allocated segments.  If two free segments are adjacent, we
  45      coalesce them into one larger segment; that is, if segments [a, b) and
  46      [b, c) are both free, we merge them into a single segment [a, c).  The
  47      segments within a span are linked together in increasing-address order so
  48      we can easily determine whether coalescing is possible.
  49 
  50      Segments never cross span boundaries.  When all segments within an
  51      imported span become free, we return the span to its source.
  52 
  53    Imported Memory
  54      As mentioned in the overview, some arenas are logical subsets of other
  55      arenas.  For example, kmem_va_arena (a virtual address cache that
  56      satisfies most kmem_slab_create() requests) is just a subset of
  57      heap_arena (the kernel heap) that provides caching for the most common
  58      slab sizes.  When kmem_va_arena runs out of virtual memory, it imports
  59      more from the heap; we say that heap_arena is the vmem source for
  60      kmem_va_arena. vmem_create() allows you to specify any existing vmem
  61      arena as the source for your new arena.  Topologically, since every arena
  62      is a child of at most one source, the set of all arenas forms a
  63      collection of trees.
  64 
  65    Constrained Allocations
  66      Some vmem clients are quite picky about the kind of address they want.
  67      For example, the DVMA code may need an address that is at a particular
  68      phase with respect to some alignment (to get good cache coloring), or
  69      that lies within certain limits (the addressable range of a device), or
  70      that doesn't cross some boundary (a DMA counter restriction) -- or all of
  71      the above.  vmem_xalloc() allows the client to specify any or all of
  72      these constraints.
  73 
  74    The Vmem Quantum
  75      Every arena has a notion of `quantum', specified at vmem_create() time,
  76      that defines the arena's minimum unit of currency.  Most commonly the
  77      quantum is either 1 or PAGESIZE, but any power of 2 is legal.  All vmem
  78      allocations are guaranteed to be quantum-aligned.
  79 
  80    Relationship to the Kernel Memory Allocator
  81      Every kmem cache has a vmem arena as its slab supplier.  The kernel
  82      memory allocator uses vmem_alloc() and vmem_free() to create and destroy
  83      slabs.
  84 
  85 SEE ALSO
  86      id_space(9F), vmem_add(9F), vmem_alloc(9F), vmem_contains(9F),
  87      vmem_create(9F), vmem_walk(9F)
  88 
  89 
  90      Jeff Bonwick and Jonathan Adams, "Magazines and vmem: Extending the Slab
  91      Allocator to Many CPUs and Arbitrary Resources.", Proceedings of the 2001
  92      Usenix Conference, http://www.usenix.org/event/usenix01/bonwick.html.
  93 
  94 illumos                        January 18, 2017                        illumos