path: root/erts/doc/src/erts_alloc.xml

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  <header>
    <copyright>
      <year>2002</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>erts_alloc</title>
    <prepared>Rickard Green</prepared>
    <docno>1</docno>
    <date>03-06-11</date>
    <rev>1</rev>
    <file>erts_alloc.xml</file>
  </header>
  <lib>erts_alloc</lib>
  <libsummary>An Erlang Run-Time System internal memory allocator library.</libsummary>
  <description>
    <p><c>erts_alloc</c> is an Erlang Run-Time System internal memory
      allocator library. <c>erts_alloc</c> provides the Erlang
      Run-Time System with a number of memory allocators.</p>
  </description>

  <section>
    <title>Allocators</title>
    <marker id="allocators"></marker>
    <p>Currently the following allocators are present:</p>
    <taglist>
      <tag><c>temp_alloc</c></tag>
      <item>Allocator used for temporary allocations.</item>
      <tag><c>eheap_alloc</c></tag>
      <item>Allocator used for Erlang heap data, such as Erlang process heaps.</item>
      <tag><c>binary_alloc</c></tag>
      <item>Allocator used for Erlang binary data.</item>
      <tag><c>ets_alloc</c></tag>
      <item>Allocator used for ETS data.</item>
      <tag><c>driver_alloc</c></tag>
      <item>Allocator used for driver data.</item>
      <tag><c>literal_alloc</c></tag>
      <item>Allocator used for constant terms in Erlang code.</item>
      <tag><c>sl_alloc</c></tag>
      <item>Allocator used for memory blocks that are expected to be
       short-lived.</item>
      <tag><c>ll_alloc</c></tag>
      <item>Allocator used for memory blocks that are expected to be
       long-lived, for example Erlang code.</item>
      <tag><c>fix_alloc</c></tag>
      <item>A fast allocator used for some frequently used
       fixed size data types.</item>
      <tag><c>exec_alloc</c></tag>
      <item>Allocator used by hipe for native executable code
      on specific architectures (x86_64).</item>
      <tag><c>std_alloc</c></tag>
      <item>Allocator used for most memory blocks not allocated via any of
       the other allocators described above.</item>
      <tag><c>sys_alloc</c></tag>
      <item>This is normally the default <c>malloc</c> implementation
       used on the specific OS.</item>
      <tag><c>mseg_alloc</c></tag>
      <item>A memory segment allocator. <c>mseg_alloc</c> is used by other
       allocators for allocating memory segments and is currently only
       available on systems that have the <c>mmap</c> system
       call. Memory segments that are deallocated are kept for a
       while in a segment cache before they are destroyed. When
       segments are allocated, cached segments are used if possible
       instead of creating new segments.  This in order to reduce
       the number of system calls made.</item>
    </taglist>
    <p><c>sys_alloc</c> and <c>literal_alloc</c> are always enabled and
      cannot be disabled. <c>exec_alloc</c> is only available if it is needed
      and cannot be disabled. <c>mseg_alloc</c> is always enabled if it is
      available and an allocator that uses it is enabled. All other
      allocators can be <seealso marker="#M_e">enabled or disabled</seealso>.
      By default all allocators are enabled.
      When an allocator is disabled, <c>sys_alloc</c> is used instead of
      the disabled allocator.</p>
    <p>The main idea with the <c>erts_alloc</c> library is to separate
      memory blocks that are used differently into different memory
      areas, and by this achieving less memory fragmentation. By
      putting less effort in finding a good fit for memory blocks that
      are frequently allocated than for those less frequently
      allocated, a performance gain can be achieved.</p>
  </section>

  <section>
    <marker id="alloc_util"></marker>
    <title>The alloc_util framework</title>
    <p>Internally a framework called <c>alloc_util</c> is used for
      implementing allocators. <c>sys_alloc</c>, and
      <c>mseg_alloc</c> do not use this framework; hence, the
      following does <em>not</em> apply to them.</p>
    <p>An allocator manages multiple areas, called carriers, in which
      memory blocks are placed. A carrier is either placed in a
      separate memory segment (allocated via <c>mseg_alloc</c>), or in
      the heap segment (allocated via <c>sys_alloc</c>). Multiblock
      carriers are used for storage of several blocks. Singleblock
      carriers are used for storage of one block. Blocks that are
      larger than the value of the singleblock carrier threshold
      (<seealso marker="#M_sbct">sbct</seealso>) parameter are placed
      in singleblock carriers. Blocks that are smaller than the value
      of the <c>sbct</c> parameter are placed in multiblock
      carriers. Normally an allocator creates a "main multiblock
      carrier". Main multiblock carriers are never deallocated. The
      size of the main multiblock carrier is determined by the value
      of the <seealso marker="#M_mmbcs">mmbcs</seealso> parameter.</p>
    <p><marker id="mseg_mbc_sizes"></marker>Sizes of multiblock carriers
      allocated via <c>mseg_alloc</c> are
      decided based on the values of the largest multiblock carrier
      size (<seealso marker="#M_lmbcs">lmbcs</seealso>), the smallest
      multiblock carrier size (<seealso marker="#M_smbcs">smbcs</seealso>),
      and the multiblock carrier growth stages
      (<seealso marker="#M_mbcgs">mbcgs</seealso>) parameters. If
      <c>nc</c> is the current number of multiblock carriers (the main
      multiblock carrier excluded) managed by an allocator, the size
      of the next <c>mseg_alloc</c> multiblock carrier allocated by
      this allocator will roughly be
      <c><![CDATA[smbcs+nc*(lmbcs-smbcs)/mbcgs]]></c> when
      <c><![CDATA[nc <= mbcgs]]></c>,
      and <c>lmbcs</c> when <c><![CDATA[nc > mbcgs]]></c>. If the value of the
      <c>sbct</c> parameter should be larger than the value of the
      <c>lmbcs</c> parameter, the allocator may have to create
      multiblock carriers that are larger than the value of the
      <c>lmbcs</c> parameter, though.
      Singleblock carriers allocated via <c>mseg_alloc</c> are sized
      to whole pages.</p>
    <p>Sizes of carriers allocated via <c>sys_alloc</c> are
      decided based on the value of the <c>sys_alloc</c> carrier size
      (<seealso marker="#Muycs">ycs</seealso>) parameter. The size of
      a carrier is the least number of multiples of the value of the
      <c>ycs</c> parameter that satisfies the request.</p>
    <p>Coalescing of free blocks are always performed immediately.
      Boundary tags (headers and footers) in free blocks are used
      which makes the time complexity for coalescing constant.</p>
    <p><marker id="strategy"></marker>The memory allocation strategy
      used for multiblock carriers by an
      allocator is configurable via the <seealso marker="#M_as">as</seealso>
      parameter. Currently the following strategies are available:</p>
    <taglist>
      <tag>Best fit</tag>
      <item>
        <p>Strategy: Find the smallest block that satisfies the
          requested block size.</p>
        <p>Implementation: A balanced binary search tree is
          used. The time complexity is proportional to log N, where
          N is the number of sizes of free blocks.</p>
      </item>
      <tag>Address order best fit</tag>
      <item>
        <p>Strategy: Find the smallest block that satisfies the
          requested block size. If multiple blocks are found, choose
          the one with the lowest address.</p>
        <p>Implementation: A balanced binary search tree is
          used. The time complexity is proportional to log N, where
          N is the number of free blocks.</p>
      </item>
      <tag>Address order first fit</tag>
      <item>
        <p>Strategy: Find the block with the lowest address that satisfies the
          requested block size.</p>
        <p>Implementation: A balanced binary search tree is
          used. The time complexity is proportional to log N, where
          N is the number of free blocks.</p>
      </item>
      <tag>Address order first fit carrier best fit</tag>
      <item>
        <p>Strategy: Find the <em>carrier</em> with the lowest address that
	can satisfy the requested block size, then find a block within
	that carrier using the "best fit" strategy.</p>
        <p>Implementation: Balanced binary search trees are
          used. The time complexity is proportional to log N, where
          N is the number of free blocks.</p>
      </item>
      <tag>Address order first fit carrier address order best fit</tag>
      <item>
        <p>Strategy: Find the <em>carrier</em> with the lowest address that
	can satisfy the requested block size, then find a block within
	that carrier using the "adress order best fit" strategy.</p>
        <p>Implementation: Balanced binary search trees are
          used. The time complexity is proportional to log N, where
          N is the number of free blocks.</p>
      </item>
      <tag>Good fit</tag>
      <item>
        <p>Strategy: Try to find the best fit, but settle for the best fit
          found during a limited search.</p>
        <p>Implementation: The implementation uses segregated free
          lists with a maximum block search depth (in each list) in
          order to find a good fit fast. When the maximum block
          search depth is small (by default 3) this implementation
          has a time complexity that is constant. The maximum block
          search depth is configurable via the
          <seealso marker="#M_mbsd">mbsd</seealso> parameter.</p>
      </item>
      <tag>A fit</tag>
      <item>
        <p>Strategy: Do not search for a fit, inspect only one free
          block to see if it satisfies the request. This strategy is
          only intended to be used for temporary allocations.</p>
        <p>Implementation: Inspect the first block in a free-list.
          If it satisfies the request, it is used; otherwise, a new
          carrier is created. The implementation has a time
          complexity that is constant.</p>
	<p>As of erts version 5.6.1 the emulator will refuse to
	  use this strategy on other allocators than <c>temp_alloc</c>.
	  This since it will only cause problems for other allocators.</p>
      </item>
    </taglist>
    <p>Apart from the ordinary allocators described above a number of
       pre-allocators are used for some specific data types. These
       pre-allocators pre-allocate a fixed amount of memory for certain data
       types when the run-time system starts. As long as pre-allocated memory
       is available, it will be used. When no pre-allocated memory is
       available, memory will be allocated in ordinary allocators. These
       pre-allocators are typically much faster than the ordinary allocators,
       but can only satisfy a limited amount of requests.</p>
  </section>

  <section>
    <marker id="flags"></marker>
    <title>System Flags Effecting erts_alloc</title>
    <warning>
      <p>Only use these flags if you are absolutely sure what you are
        doing. Unsuitable settings may cause serious performance
        degradation and even a system crash at any time during
        operation.</p>
    </warning>
    <p>Memory allocator system flags have the following syntax:
      <c><![CDATA[+M<S><P> <V>]]></c>
      where <c><![CDATA[<S>]]></c> is a letter identifying a subsystem,
      <c><![CDATA[<P>]]></c> is a parameter, and <c><![CDATA[<V>]]></c> is the
      value to use. The flags can be passed to the Erlang emulator
      (<seealso marker="erl">erl</seealso>) as command line
      arguments.</p>
    <p>System flags effecting specific allocators have an upper-case
      letter as <c><![CDATA[<S>]]></c>. The following letters are used for
      the currently present allocators:</p>
    <list type="bulleted">
      <item><c>B: binary_alloc</c></item>
      <item><c>D: std_alloc</c></item>
      <item><c>E: ets_alloc</c></item>
      <item><c>F: fix_alloc</c></item>
      <item><c>H: eheap_alloc</c></item>
      <item><c>I: literal_alloc</c></item>
      <item><c>L: ll_alloc</c></item>
      <item><c>M: mseg_alloc</c></item>
      <item><c>R: driver_alloc</c></item>
      <item><c>S: sl_alloc</c></item>
      <item><c>T: temp_alloc</c></item>
      <item><c>X: exec_alloc</c></item>
      <item><c>Y: sys_alloc</c></item>
    </list>
    <p>The following flags are available for configuration of
      <c>mseg_alloc</c>:</p>
    <taglist>
      <tag><marker id="MMamcbf"/><c><![CDATA[+MMamcbf <size>]]></c></tag>
      <item>
       Absolute max cache bad fit (in kilobytes). A segment in the
       memory segment cache is not reused if its size exceeds the
       requested size with more than the value of this
       parameter. Default value is 4096. </item>
      <tag><marker id="MMrmcbf"/><c><![CDATA[+MMrmcbf <ratio>]]></c></tag>
      <item>
       Relative max cache bad fit (in percent). A segment in the
       memory segment cache is not reused if its size exceeds the
       requested size with more than relative max cache bad fit
       percent of the requested size. Default value is 20.</item>
      <tag><marker id="MMsco"/><c><![CDATA[+MMsco true|false]]></c></tag>
      <item>
	Set <seealso marker="#MMscs">super carrier</seealso> only flag. This
	flag defaults to <c>true</c>. When a super carrier is used and this
	flag is <c>true</c>, <c>mseg_alloc</c> will only create carriers
	in the super carrier. Note that the <c>alloc_util</c> framework may
	create <c>sys_alloc</c> carriers, so if you want all carriers to
	be created in the super carrier, you therefore want to disable use
	of <c>sys_alloc</c> carriers by also passing
	<seealso marker="#Musac"><c>+Musac false</c></seealso>. When the flag
	is <c>false</c>, <c>mseg_alloc</c> will try to create carriers outside
	of the super carrier when the super carrier is full.
	<br/><br/>
	<em>NOTE</em>: Setting this flag to <c>false</c> may not be supported
	on all systems. This flag will in that case be ignored.
	<br/><br/>
	<em>NOTE</em>: The super carrier cannot be enabled nor
	disabled on halfword heap systems. This flag will be
	ignored on halfword heap systems.
      </item>
      <tag><marker id="MMscrfsd"/><c><![CDATA[+MMscrfsd <amount>]]></c></tag>
      <item>
	Set <seealso marker="#MMscs">super carrier</seealso> reserved
	free segment descriptors. This parameter defaults to <c>65536</c>.
	This parameter determines the amount of memory to reserve for
	free segment descriptors used by the super carrier. If the system
	runs out of reserved memory for free segment descriptors, other
	memory will be used. This may however cause fragmentation issues,
	so you want to ensure that this	never happens. The maximum amount
	of free segment descriptors used can be retrieved from the
	<c>erts_mmap</c> tuple part of the result from calling
	<seealso marker="erts:erlang#system_info_allocator_tuple">erlang:system_info({allocator, mseg_alloc})</seealso>.
      </item>
      <tag><marker id="MMscrpm"/><c><![CDATA[+MMscrpm true|false]]></c></tag>
      <item>
	Set <seealso marker="#MMscs">super carrier</seealso> reserve physical
	memory flag. This flag defaults	to <c>true</c>. When this flag is
	<c>true</c>, physical memory will be reserved for the whole super
	carrier at once when it is created. The reservation will after that
	be left unchanged. When this flag is set to <c>false</c> only virtual
	address space will be reserved for the super carrier upon creation.
	The system will attempt to reserve physical memory upon carrier
	creations in the super carrier, and attempt to unreserve physical
	memory upon carrier destructions in the super carrier.
	<br/><br/>
	<em>NOTE</em>: What reservation of physical memory actually means
	highly depends on the operating system, and how it is configured. For
	example, different memory overcommit settings on Linux drastically
	change the behaviour. Also note, setting this flag to <c>false</c>
	may not be supported on all systems. This flag will in that case
	be ignored.
	<br/><br/>
	<em>NOTE</em>: The super carrier cannot be enabled nor
	disabled on halfword heap systems. This flag will be
	ignored on halfword heap systems.
      </item>
      <tag><marker id="MMscs"/><c><![CDATA[+MMscs <size in MB>]]></c></tag>
      <item>
	Set super carrier size (in MB). The super carrier size defaults to
	zero; i.e, the super carrier is by default disabled. The super
	carrier is a large continuous area in the virtual address space.
	<c>mseg_alloc</c> will always try to create new carriers in the super
	carrier if it exists. Note that the <c>alloc_util</c> framework may
	create <c>sys_alloc</c> carriers. For more information on this, see the
	documentation of the <seealso marker="#MMsco"><c>+MMsco</c></seealso>
	flag.
	<br/><br/>
	<em>NOTE</em>: The super carrier cannot be enabled nor
	disabled on halfword heap systems. This flag will be
	ignored on halfword heap systems.
      </item>
      <tag><marker id="MMmcs"/><c><![CDATA[+MMmcs <amount>]]></c></tag>
      <item>
       Max cached segments. The maximum number of memory segments
       stored in the memory segment cache. Valid range is
       0-30. Default value is 10.</item>
    </taglist>
    <p>The following flags are available for configuration of
      <c>sys_alloc</c>:</p>
    <taglist>
      <tag><marker id="MYe"/><c>+MYe true</c></tag>
      <item>
       Enable <c>sys_alloc</c>. Note: <c>sys_alloc</c> cannot be disabled.</item>
      <tag><marker id="MYm"/><c>+MYm libc</c></tag>
      <item>
      <c>malloc</c> library to use. Currently only
      <c>libc</c> is available. <c>libc</c> enables the standard
      <c>libc</c> malloc implementation. By default <c>libc</c> is used.</item>
      <tag><marker id="MYtt"/><c><![CDATA[+MYtt <size>]]></c></tag>
      <item>
       Trim threshold size (in kilobytes). This is the maximum amount
       of free memory at the top of the heap (allocated by
      <c>sbrk</c>) that will be kept by <c>malloc</c> (not
       released to the operating system). When the amount of free
       memory at the top of the heap exceeds the trim threshold,
      <c>malloc</c> will release it (by calling
      <c>sbrk</c>). Trim threshold is given in kilobytes. Default
       trim threshold is 128. <em>Note:</em> This flag will
       only have any effect when the emulator has been linked with
       the GNU C library, and uses its <c>malloc</c> implementation.</item>
      <tag><marker id="MYtp"/><c><![CDATA[+MYtp <size>]]></c></tag>
      <item>
       Top pad size (in kilobytes). This is the amount of extra
       memory that will be allocated by <c>malloc</c> when
      <c>sbrk</c> is called to get more memory from the operating
       system. Default top pad size is 0. <em>Note:</em> This flag
       will only have any effect when the emulator has been linked
       with the GNU C library, and uses its <c>malloc</c>
       implementation.</item>
    </taglist>
    <p>The following flags are available for configuration of allocators
       based on <c>alloc_util</c>. If <c>u</c> is used as subsystem
       identifier (i.e., <c><![CDATA[<S> = u]]></c>) all allocators based on
       <c>alloc_util</c> will be effected. If <c>B</c>, <c>D</c>, <c>E</c>,
        <c>F</c>, <c>H</c>, <c>L</c>, <c>R</c>, <c>S</c>, or <c>T</c> is used as
       subsystem identifier, only the specific allocator identified will be
       effected:</p>
    <taglist>
      <tag><marker id="M_acul"/><c><![CDATA[+M<S>acul <utilization>|de]]></c></tag>
      <item>
	Abandon carrier utilization limit. A valid
	<c><![CDATA[<utilization>]]></c> is an integer in the range
	<c>[0, 100]</c> representing utilization in percent. When a
	utilization value larger than zero is used, allocator instances
	are allowed to abandon multiblock carriers. If <c>de</c> (default
	enabled) is passed instead of a <c><![CDATA[<utilization>]]></c>,
	a recomended non zero utilization value will be used. The actual
	value chosen depend on allocator type and may be changed between
	ERTS versions. Currently the default equals <c>de</c>, but this
	may be changed in the future. Carriers will be abandoned when
	memory utilization in the allocator instance falls below the
	utilization value used. Once a carrier has been abandoned, no new
	allocations will be made in it. When an allocator instance gets an
	increased multiblock carrier need, it will first try to fetch an
	abandoned carrier from an allocator instances of the same
	allocator type. If no abandoned carrier could be fetched, it will
	create a new empty carrier. When an abandoned carrier has been
	fetched it will function as an ordinary carrier. This feature has
	special requirements on the
	<seealso marker="#M_as">allocation strategy</seealso> used. Currently
	only the strategies <c>aoff</c>, <c>aoffcbf</c> and <c>aoffcaobf</c> support
	abandoned carriers. This feature also requires
	<seealso marker="#M_t">multiple thread specific instances</seealso>
	to be enabled. When enabling this feature, multiple thread specific
	instances will be enabled if not already enabled, and the
	<c>aoffcbf</c> strategy will be enabled if current strategy does not
	support abandoned carriers. This feature can be enabled on all
	allocators based on the <c>alloc_util</c> framework with the
	exception of <c>temp_alloc</c> (which would be pointless).
      </item>
      <tag><marker id="M_as"/><c><![CDATA[+M<S>as bf|aobf|aoff|aoffcbf|aoffcaobf|gf|af]]></c></tag>
      <item>
       Allocation strategy. Valid strategies are <c>bf</c> (best fit),
      <c>aobf</c> (address order best fit), <c>aoff</c> (address order first fit),
      <c>aoffcbf</c> (address order first fit carrier best fit),
      <c>aoffcaobf</c> (address order first fit carrier address order best fit),
      <c>gf</c> (good fit), and <c>af</c> (a fit). See 
      <seealso marker="#strategy">the description of allocation strategies</seealso> in "the <c>alloc_util</c> framework" section.</item>
      <tag><marker id="M_asbcst"/><c><![CDATA[+M<S>asbcst <size>]]></c></tag>
      <item>
       Absolute singleblock carrier shrink threshold (in
       kilobytes). When a block located in an
      <c>mseg_alloc</c> singleblock carrier is shrunk, the carrier
       will be left unchanged if the amount of unused memory is less
       than this threshold; otherwise, the carrier will be shrunk.
       See also <seealso marker="#M_rsbcst">rsbcst</seealso>.</item>
      <tag><marker id="M_e"/><c><![CDATA[+M<S>e true|false]]></c></tag>
      <item>
       Enable allocator <c><![CDATA[<S>]]></c>.</item>
      <tag><marker id="M_lmbcs"/><c><![CDATA[+M<S>lmbcs <size>]]></c></tag>
      <item>
       Largest (<c>mseg_alloc</c>) multiblock carrier size (in
       kilobytes).  See <seealso marker="#mseg_mbc_sizes">the description
       on how sizes for mseg_alloc multiblock carriers are decided</seealso>
       in "the <c>alloc_util</c> framework" section. On 32-bit Unix style OS
       this limit can not be set higher than 128 megabyte.</item>
      <tag><marker id="M_mbcgs"/><c><![CDATA[+M<S>mbcgs <ratio>]]></c></tag>
      <item>
       (<c>mseg_alloc</c>) multiblock carrier growth stages. See
      <seealso marker="#mseg_mbc_sizes">the description on how sizes for
       mseg_alloc multiblock carriers are decided</seealso>
       in "the <c>alloc_util</c> framework" section.</item>
      <tag><marker id="M_mbsd"/><c><![CDATA[+M<S>mbsd <depth>]]></c></tag>
      <item>
       Max block search depth. This flag has effect only if the
       good fit strategy has been selected for allocator
      <c><![CDATA[<S>]]></c>. When the good fit strategy is used, free
       blocks are placed in segregated free-lists. Each free list
       contains blocks of sizes in a specific range. The max block
       search depth sets a limit on the maximum number of blocks to
       inspect in a free list during a search for suitable block
       satisfying the request.</item>
      <tag><marker id="M_mmbcs"/><c><![CDATA[+M<S>mmbcs <size>]]></c></tag>
      <item>
       Main multiblock carrier size. Sets the size of the main
       multiblock carrier for allocator <c><![CDATA[<S>]]></c>. The main
       multiblock carrier is allocated via <c><![CDATA[sys_alloc]]></c> and is
       never deallocated.</item>
      <tag><marker id="M_mmmbc"/><c><![CDATA[+M<S>mmmbc <amount>]]></c></tag>
      <item>
       Max <c>mseg_alloc</c> multiblock carriers. Maximum number of
       multiblock carriers allocated via <c>mseg_alloc</c> by
       allocator <c><![CDATA[<S>]]></c>. When this limit has been reached,
       new multiblock carriers will be allocated via
      <c>sys_alloc</c>.</item>
      <tag><marker id="M_mmsbc"/><c><![CDATA[+M<S>mmsbc <amount>]]></c></tag>
      <item>
       Max <c>mseg_alloc</c> singleblock carriers. Maximum number of
       singleblock carriers allocated via <c>mseg_alloc</c> by
       allocator <c><![CDATA[<S>]]></c>. When this limit has been reached,
       new singleblock carriers will be allocated via
      <c>sys_alloc</c>.</item>
      <tag><marker id="M_ramv"/><c><![CDATA[+M<S>ramv <bool>]]></c></tag>
      <item>
       Realloc always moves. When enabled, reallocate operations will
       more or less be translated into an allocate, copy, free sequence.
       This often reduce memory fragmentation, but costs performance.
      </item>
      <tag><marker id="M_rmbcmt"/><c><![CDATA[+M<S>rmbcmt <ratio>]]></c></tag>
      <item>
       Relative multiblock carrier move threshold (in percent). When
       a block located in a multiblock carrier is shrunk,
       the block will be moved if the ratio of the size of the returned
       memory compared to the previous size is more than this threshold;
       otherwise, the block will be shrunk at current location.</item>
      <tag><marker id="M_rsbcmt"/><c><![CDATA[+M<S>rsbcmt <ratio>]]></c></tag>
      <item>
       Relative singleblock carrier move threshold (in percent). When
       a block located in a singleblock carrier is shrunk to
       a size smaller than the value of the
      <seealso marker="#M_sbct">sbct</seealso> parameter,
       the block will be left unchanged in the singleblock carrier if
       the ratio of unused memory is less than this threshold;
       otherwise, it will be moved into a multiblock carrier. </item>
      <tag><marker id="M_rsbcst"/><c><![CDATA[+M<S>rsbcst <ratio>]]></c></tag>
      <item>
       Relative singleblock carrier shrink threshold (in
       percent). When a block located in an <c>mseg_alloc</c>
       singleblock carrier is shrunk, the carrier will be left
       unchanged if the ratio of unused memory is less than this
       threshold; otherwise, the carrier will be shrunk.
       See also <seealso marker="#M_asbcst">asbcst</seealso>.</item>
      <tag><marker id="M_sbct"/><c><![CDATA[+M<S>sbct <size>]]></c></tag>
      <item>
       Singleblock carrier threshold. Blocks larger than this
       threshold will be placed in singleblock carriers. Blocks
       smaller than this threshold will be placed in multiblock
       carriers. On 32-bit Unix style OS this threshold can not be set higher
       than 8 megabytes.</item>
      <tag><marker id="M_smbcs"/><c><![CDATA[+M<S>smbcs <size>]]></c></tag>
      <item>
       Smallest (<c>mseg_alloc</c>) multiblock carrier size (in
       kilobytes). See <seealso marker="#mseg_mbc_sizes">the description
       on how sizes for mseg_alloc multiblock carriers are decided</seealso>
       in "the <c>alloc_util</c> framework" section.</item>
      <tag><marker id="M_t"/><c><![CDATA[+M<S>t true|false]]></c></tag>
      <item>
       <p>Multiple, thread specific instances of the allocator.
       This option will only have any effect on the runtime system
       with SMP support. Default behaviour on the runtime system with
       SMP support is <c>NoSchedulers+1</c> instances. Each scheduler will use
       a lock-free instance of its own and other threads will use
       a common instance.</p>
       <p>It was previously (before ERTS version 5.9) possible to configure
       a smaller amount of thread specific instances than schedulers.
       This is, however, not possible any more.</p>
      </item>
    </taglist>
    <p>Currently the following flags are available for configuration of
      <c>alloc_util</c>, i.e. all allocators based on <c>alloc_util</c>
      will be effected:</p>
    <taglist>
      <tag><marker id="Muycs"/><c><![CDATA[+Muycs <size>]]></c></tag>
      <item>
      <c>sys_alloc</c> carrier size. Carriers allocated via
      <c>sys_alloc</c> will be allocated in sizes which are
       multiples of the <c>sys_alloc</c> carrier size. This is not
       true for main multiblock carriers and carriers allocated
       during a memory shortage, though.</item>
      <tag><marker id="Mummc"/><c><![CDATA[+Mummc <amount>]]></c></tag>
      <item>
       Max <c>mseg_alloc</c> carriers. Maximum number of carriers
       placed in separate memory segments. When this limit has been
       reached, new carriers will be placed in memory retrieved from
      <c>sys_alloc</c>.</item>
      <tag><marker id="Musac"/><c><![CDATA[+Musac <bool>]]></c></tag>
      <item>
	Allow <c>sys_alloc</c> carriers. By default <c>true</c>. If
	set to <c>false</c>, <c>sys_alloc</c> carriers will never be
	created by allocators using the <c>alloc_util</c> framework.</item>
    </taglist>
    <p>The following flag is special for <c>literal_alloc</c>:</p>
    <taglist>
      <tag><marker id="MIscs"/><c><![CDATA[+MIscs <size in MB>]]></c></tag>
      <item>
      <c>literal_alloc</c> super carrier size (in MB). The amount of
      <em>virtual</em> address space reserved for literal terms in
      Erlang code on 64-bit architectures. The default is 1024 (1GB)
      and is usually sufficient. The flag is ignored on 32-bit
      architectures.</item>
    </taglist>
    <p>The following flag is special for <c>exec_alloc</c>:</p>
    <taglist>
      <tag><marker id="MIscs"/><c><![CDATA[+MXscs <size in MB>]]></c></tag>
      <item>
      <c>exec_alloc</c> super carrier size (in MB). The amount of
      <em>virtual</em> address space reserved for native executable code
      used by hipe on specific architectures (x86_64). The default is 512 MB.
      </item>
    </taglist>
    <p>Instrumentation flags:</p>
    <taglist>
      <tag><marker id="Mim"/><c>+Mim true|false</c></tag>
      <item>
       A map over current allocations is kept by the emulator. The
       allocation map can be retrieved via the <c>instrument</c>
       module. <c>+Mim true</c> implies <c>+Mis true</c>.
      <c>+Mim true</c> is the same as
      <seealso marker="erl#instr">-instr</seealso>.</item>
      <tag><marker id="Mis"/><c>+Mis true|false</c></tag>
      <item>
       Status over allocated memory is kept by the emulator. The
       allocation status can be retrieved via the <c>instrument</c>
       module.</item>
      <tag><marker id="Mit"/><c>+Mit X</c></tag>
      <item>
       Reserved for future use. Do <em>not</em> use this flag.</item>
    </taglist>
    <note>
      <p>When instrumentation of the emulator is enabled, the emulator
        uses more memory and runs slower.</p>
    </note>
    <p>Other flags:</p>
    <taglist>
      <tag><marker id="Mea"/><c>+Mea min|max|r9c|r10b|r11b|config</c></tag>
      <item>
      <taglist>
        <tag><c>min</c></tag>
        <item>
	  Disables all allocators that can be disabled.
	</item>

        <tag><c>max</c></tag>
        <item>
	  Enables all allocators (currently default).
	</item>

        <tag><c>r9c|r10b|r11b</c></tag>
        <item>
	  Configures all allocators as they were configured in respective
	  OTP release. These will eventually be removed.
	</item>

        <tag><c>config</c></tag>
        <item>
	  Disables features that cannot be enabled while creating an
	  allocator configuration with
	  <seealso marker="runtime_tools:erts_alloc_config">erts_alloc_config(3)</seealso>.
	  Note, this option should only be used while running
	  <c>erts_alloc_config</c>, <em>not</em> when using the created
	  configuration.
        </item>
      </taglist>
      </item>
      <tag><marker id="Mlpm"/><c>+Mlpm all|no</c></tag>
      <item>Lock physical memory. The default value is <c>no</c>, i.e.,
      no physical memory will be locked. If set to <c>all</c>, all
      memory mappings made by the runtime system, will be locked into
      physical memory. If set to <c>all</c>, the runtime system will fail
      to start if this feature is not supported, the user has not got enough
      privileges, or the user is not allowed to lock enough physical memory.
      The runtime system will also fail with an out of memory condition
      if the user limit on the amount of locked memory is reached.
      </item>
    </taglist>
    <p>Only some default values have been presented
      here.
      <seealso marker="erts:erlang#system_info_allocator">erlang:system_info(allocator)</seealso>,
      and
      <seealso marker="erts:erlang#system_info_allocator_tuple">erlang:system_info({allocator, Alloc})</seealso>
      can be used in order to obtain currently used settings and current
      status of the allocators.</p>
    <note>
      <p>Most of these flags are highly implementation dependent, and they
        may be changed or removed without prior notice.</p>
      <p><c>erts_alloc</c> is not obliged to strictly use the settings that
        have been passed to it (it may even ignore them).</p>
    </note>
    <p><seealso marker="runtime_tools:erts_alloc_config">erts_alloc_config(3)</seealso>
      is a tool that can be used to aid creation of an
      <c>erts_alloc</c> configuration that is suitable for a limited
      number of runtime scenarios.</p>
  </section>

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="runtime_tools:erts_alloc_config">erts_alloc_config(3)</seealso>,
      <seealso marker="erl">erl(1)</seealso>,
      <seealso marker="tools:instrument">instrument(3)</seealso>,
      <seealso marker="erts:erlang">erlang(3)</seealso></p>
  </section>
</cref>