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<title>erts_alloc</title>
<prepared>Rickard Green</prepared>
<docno>1</docno>
<date>2003-06-11</date>
<rev>1</rev>
<file>erts_alloc.xml</file>
</header>
<lib>erts_alloc</lib>
<libsummary>An Erlang runtime system internal memory allocator library.
</libsummary>
<description>
<p><c>erts_alloc</c> is an Erlang runtime system internal memory
allocator library. <c>erts_alloc</c> provides the Erlang
runtime system with a number of memory allocators.</p>
</description>
<section>
<title>Allocators</title>
<marker id="allocators"></marker>
<p>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 <c>ets</c> 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 the <seealso marker="hipe:HiPE_app"><c>HiPE</c></seealso>
application for native executable code.</item>
<tag><c>std_alloc</c></tag>
<item>Allocator used for most memory blocks not allocated through 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. It is used by other
allocators for allocating memory segments and is 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 to reduce
the number of system calls made.</item>
</taglist>
<p><c>sys_alloc</c>, <c>literal_alloc</c> and <c>temp_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, to achieve 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, so 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 through <c>mseg_alloc</c>), or in
the heap segment (allocated through <c>sys_alloc</c>).</p>
<list type="bulleted">
<item>
<p>Multiblock carriers are used for storage of several blocks.</p>
</item>
<item>
<p>Singleblock carriers are used for storage of one block.</p>
</item>
<item>
<p>Blocks that are larger than the value of the singleblock carrier
threshold (<seealso marker="#M_sbct"><c>sbct</c></seealso>) parameter
are placed in singleblock carriers.</p>
</item>
<item>
<p>Blocks that are smaller than the value of parameter <c>sbct</c>
are placed in multiblock carriers.</p></item>
</list>
<p>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
parameter <seealso marker="#M_mmbcs"><c>mmbcs</c></seealso>.</p>
<p><marker id="mseg_mbc_sizes"></marker>Sizes of multiblock carriers
allocated through <c>mseg_alloc</c> are decided based on the
following parameters:</p>
<list type="bulleted">
<item>The values of the largest multiblock carrier size
(<seealso marker="#M_lmbcs"><c>lmbcs</c></seealso>)</item>
<item>The smallest multiblock carrier size
(<seealso marker="#M_smbcs"><c>smbcs</c></seealso>)</item>
<item>The multiblock carrier growth stages
(<seealso marker="#M_mbcgs"><c>mbcgs</c></seealso>)</item>
</list>
<p>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 is roughly
<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
parameter <c>sbct</c> is larger than the value of parameter
<c>lmbcs</c>, the allocator may have to create
multiblock carriers that are larger than the value of
parameter <c>lmbcs</c>, though.
Singleblock carriers allocated through <c>mseg_alloc</c> are sized
to whole pages.</p>
<p>Sizes of carriers allocated through <c>sys_alloc</c> are
decided based on the value of the <c>sys_alloc</c> carrier size
(<seealso marker="#Muycs"><c>ycs</c></seealso>) parameter. The size of
a carrier is the least number of multiples of the value of
parameter <c>ycs</c> satisfying 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 can be
configured using parameter <seealso marker="#M_as"><c>as</c></seealso>.
The following strategies are available:</p>
<taglist>
<tag>Best fit</tag>
<item>
<p>Strategy: Find the smallest block satisfying 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 satisfying 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 satisfying 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 "address 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)
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 can be configured using parameter
<seealso marker="#M_mbsd"><c>mbsd</c></seealso>.</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 from ERTS 5.6.1 the emulator refuses to
use this strategy on other allocators than <c>temp_alloc</c>.
This because it only causes problems for other allocators.</p>
</item>
</taglist>
<p>Apart from the ordinary allocators described above, some
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 runtime system starts. As long as pre-allocated memory
is available, it is used. When no pre-allocated memory is
available, memory is allocated in ordinary allocators. These
pre-allocators are typically much faster than the ordinary allocators,
but can only satisfy a limited number 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 sure what you are
doing. Unsuitable settings can 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"><c>erl(1)</c></seealso>) as command-line
arguments.</p>
<p>System flags effecting specific allocators have an uppercase
letter as <c><![CDATA[<S>]]></c>. The following letters are used for
the 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>
<section>
<title>Flags for Configuration of mseg_alloc</title>
<taglist>
<tag><marker id="MMamcbf"/><c><![CDATA[+MMamcbf <size>]]></c></tag>
<item>
<p>Absolute maximum 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. Defaults to <c>4096</c>.</p>
</item>
<tag><marker id="MMrmcbf"/><c><![CDATA[+MMrmcbf <ratio>]]></c></tag>
<item>
<p>Relative maximum 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 maximum cache bad fit
percent of the requested size. Defaults to <c>20</c>.</p>
</item>
<tag><marker id="MMsco"/><c><![CDATA[+MMsco true|false]]></c></tag>
<item>
<p>Sets <seealso marker="#MMscs">super carrier</seealso> only flag.
Defaults to <c>true</c>. When a super carrier is used and this
flag is <c>true</c>, <c>mseg_alloc</c> only creates carriers in
the super carrier. Notice that the <c>alloc_util</c> framework can
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> tries to create carriers
outside of the super carrier when the super carrier is full.</p>
<note>
<p>Setting this flag to <c>false</c> is not supported
on all systems. The flag is then ignored.</p>
</note>
</item>
<tag><marker id="MMscrfsd"/><c><![CDATA[+MMscrfsd <amount>]]></c></tag>
<item>
<p>Sets <seealso marker="#MMscs">super carrier</seealso> reserved
free segment descriptors. 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 is used. This can 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">
<c>erlang:system_info({allocator, mseg_alloc})</c></seealso>.</p>
</item>
<tag><marker id="MMscrpm"/><c><![CDATA[+MMscrpm true|false]]></c></tag>
<item>
<p>Sets <seealso marker="#MMscs">super carrier</seealso> reserve
physical memory flag. Defaults to <c>true</c>. When this flag is
<c>true</c>, physical memory is reserved for the whole super
carrier at once when it is created. The reservation is after that
left unchanged. When this flag is set to <c>false</c>, only virtual
address space is reserved for the super carrier upon creation.
The system attempts to reserve physical memory upon carrier
creations in the super carrier, and attempt to unreserve physical
memory upon carrier destructions in the super carrier.</p>
<note>
<p>What reservation of physical memory means, highly
depends on the operating system, and how it is configured. For
example, different memory overcommit settings on Linux drastically
change the behavior.</p>
<p>Setting this flag to <c>false</c> is possibly not supported on
all systems. The flag is then ignored.</p>
</note>
</item>
<tag><marker id="MMscs"/><c><![CDATA[+MMscs <size in MB>]]></c></tag>
<item>
<p>Sets super carrier size (in MB). Defaults to <c>0</c>, that is,
the super carrier is by default disabled. The super
carrier is a large continuous area in the virtual address space.
<c>mseg_alloc</c> always tries to create new carriers in the super
carrier if it exists. Notice that the <c>alloc_util</c> framework
can create <c>sys_alloc</c> carriers. For more information, see
<seealso marker="#MMsco"><c>+MMsco</c></seealso>.</p>
</item>
<tag><marker id="MMmcs"/><c><![CDATA[+MMmcs <amount>]]></c></tag>
<item>
<p>Maximum cached segments. The maximum number of memory segments
stored in the memory segment cache. Valid range is <c>[0, 30]</c>.
Defaults to <c>10</c>.</p>
</item>
</taglist>
</section>
<section>
<title>Flags for Configuration of sys_alloc</title>
<taglist>
<tag><marker id="MYe"/><c>+MYe true</c></tag>
<item>
<p>Enables <c>sys_alloc</c>.</p>
<note>
<p><c>sys_alloc</c> cannot be disabled.</p>
</note>
</item>
<tag><marker id="MYm"/><c>+MYm libc</c></tag>
<item>
<p><c>malloc</c> library to use. Only
<c>libc</c> is available. <c>libc</c> enables the standard
<c>libc</c> <c>malloc</c> implementation. By default <c>libc</c>
is used.</p>
</item>
<tag><marker id="MYtt"/><c><![CDATA[+MYtt <size>]]></c></tag>
<item>
<p>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 is 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> releases it (by calling <c>sbrk</c>).
Trim threshold is specified in kilobytes.
Defaults to <c>128</c>.</p>
<note>
<p>This flag has effect only when the emulator is linked with
the GNU C library, and uses its <c>malloc</c> implementation.</p>
</note>
</item>
<tag><marker id="MYtp"/><c><![CDATA[+MYtp <size>]]></c></tag>
<item>
<p>Top pad size (in kilobytes). This is the amount of extra
memory that is allocated by <c>malloc</c> when
<c>sbrk</c> is called to get more memory from the operating
system. Defaults to <c>0</c>.</p>
<note>
<p>This flag has effect only when the emulator is linked with
the GNU C library, and uses its <c>malloc</c> implementation.</p>
</note>
</item>
</taglist>
</section>
<section>
<title>Flags for Configuration of Allocators Based on alloc_util</title>
<p>If <c>u</c> is used as subsystem identifier (that is,
<c><![CDATA[<S> = u]]></c>), all allocators based on
<c>alloc_util</c> are 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 identifier is
effected.</p>
<taglist>
<tag><marker id="M_acul"/><c><![CDATA[+M<S>acul <utilization>|de]]></c>
</tag>
<item>
<p>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 > 0 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 recommended non-zero utilization value is used. The value
chosen depends on the allocator type and can be changed between
ERTS versions. Defaults to <c>de</c>, but this
can be changed in the future.</p>
<p>Carriers are abandoned when
memory utilization in the allocator instance falls below the
utilization value used. Once a carrier is abandoned, no new
allocations are made in it. When an allocator instance gets an
increased multiblock carrier need, it first tries to fetch an
abandoned carrier from an allocator instance of the same
allocator type. If no abandoned carrier can be fetched, it
creates 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. Only
the strategies <c>aoff</c>, <c>aoffcbf</c>, and <c>aoffcaobf</c>
support abandoned carriers.</p>
<p>This feature also requires
<seealso marker="#M_t">multiple thread specific instances</seealso>
to be enabled. When enabling this feature, multiple thread-specific
instances are enabled if not already enabled, and the
<c>aoffcbf</c> strategy is enabled if the current strategy does not
support abandoned carriers. This feature can be enabled on all
allocators based on the <c>alloc_util</c> framework, except
<c>temp_alloc</c> (which would be pointless).</p>
</item>
<tag><marker id="M_as"/>
<c><![CDATA[+M<S>as bf|aobf|aoff|aoffcbf|aoffcaobf|gf|af]]></c></tag>
<item>
<p>Allocation strategy. The following strategies are valid:</p>
<list type="bulleted">
<item><c>bf</c> (best fit)</item>
<item><c>aobf</c> (address order best fit)</item>
<item><c>aoff</c> (address order first fit)</item>
<item><c>aoffcbf</c> (address order first fit carrier best fit)
</item>
<item><c>aoffcaobf</c> (address order first fit carrier address
order best fit)</item>
<item><c>gf</c> (good fit)</item>
<item><c>af</c> (a fit)</item>
</list>
<p>See the description of allocation strategies in section
<seealso marker="#strategy">The alloc_util Framework</seealso>.</p>
</item>
<tag><marker id="M_asbcst"/><c><![CDATA[+M<S>asbcst <size>]]></c></tag>
<item>
<p>Absolute singleblock carrier shrink threshold (in
kilobytes). When a block located in an
<c>mseg_alloc</c> singleblock carrier is shrunk, the carrier
is left unchanged if the amount of unused memory is less
than this threshold, otherwise the carrier is shrunk.
See also <seealso marker="#M_rsbcst"><c>rsbcst</c></seealso>.</p>
</item>
<tag><marker id="M_e"/><c><![CDATA[+M<S>e true|false]]></c></tag>
<item>
<p>Enables allocator <c><![CDATA[<S>]]></c>.</p>
</item>
<tag><marker id="M_lmbcs"/><c><![CDATA[+M<S>lmbcs <size>]]></c></tag>
<item>
<p>Largest (<c>mseg_alloc</c>) multiblock carrier size (in kilobytes).
See the description on how sizes for <c>mseg_alloc</c> multiblock
carriers are decided in section
<seealso marker="#mseg_mbc_sizes">
The alloc_util Framework</seealso>. On
32-bit Unix style OS this limit cannot be set > 128 MB.</p>
</item>
<tag><marker id="M_mbcgs"/><c><![CDATA[+M<S>mbcgs <ratio>]]></c></tag>
<item>
<p>(<c>mseg_alloc</c>) multiblock carrier growth stages.
See the description on how sizes for <c>mseg_alloc</c> multiblock
carriers are decided in section
<seealso marker="#mseg_mbc_sizes">
The alloc_util Framework</seealso>.</p>
</item>
<tag><marker id="M_mbsd"/><c><![CDATA[+M<S>mbsd <depth>]]></c></tag>
<item>
<p>Maximum block search depth. This flag has effect only if the
good fit strategy is 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 maxiumum 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.</p>
</item>
<tag><marker id="M_mmbcs"/><c><![CDATA[+M<S>mmbcs <size>]]></c></tag>
<item>
<p>Main multiblock carrier size. Sets the size of the main
multiblock carrier for allocator <c><![CDATA[<S>]]></c>. The main
multiblock carrier is allocated through <c><![CDATA[sys_alloc]]></c>
and is never deallocated.</p>
</item>
<tag><marker id="M_mmmbc"/><c><![CDATA[+M<S>mmmbc <amount>]]></c></tag>
<item>
<p>Maximum <c>mseg_alloc</c> multiblock carriers. Maximum number of
multiblock carriers allocated through <c>mseg_alloc</c> by
allocator <c><![CDATA[<S>]]></c>. When this limit is reached,
new multiblock carriers are allocated through
<c>sys_alloc</c>.</p>
</item>
<tag><marker id="M_mmsbc"/><c><![CDATA[+M<S>mmsbc <amount>]]></c></tag>
<item>
<p>Maximum <c>mseg_alloc</c> singleblock carriers. Maximum number of
singleblock carriers allocated through <c>mseg_alloc</c> by
allocator <c><![CDATA[<S>]]></c>. When this limit is reached,
new singleblock carriers are allocated through
<c>sys_alloc</c>.</p>
</item>
<tag><marker id="M_ramv"/><c><![CDATA[+M<S>ramv <bool>]]></c></tag>
<item>
<p>Realloc always moves. When enabled, reallocate operations are
more or less translated into an allocate, copy, free sequence.
This often reduces memory fragmentation, but costs performance.</p>
</item>
<tag><marker id="M_rmbcmt"/><c><![CDATA[+M<S>rmbcmt <ratio>]]></c></tag>
<item>
<p>Relative multiblock carrier move threshold (in percent). When
a block located in a multiblock carrier is shrunk,
the block is moved if the ratio of the size of the returned
memory compared to the previous size is more than this threshold,
otherwise the block is shrunk at the current location.</p>
</item>
<tag><marker id="M_rsbcmt"/><c><![CDATA[+M<S>rsbcmt <ratio>]]></c></tag>
<item>
<p>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 parameter
<seealso marker="#M_sbct"><c>sbct</c></seealso>,
the block is left unchanged in the singleblock carrier if
the ratio of unused memory is less than this threshold,
otherwise it is moved into a multiblock carrier.</p>
</item>
<tag><marker id="M_rsbcst"/><c><![CDATA[+M<S>rsbcst <ratio>]]></c></tag>
<item>
<p>Relative singleblock carrier shrink threshold (in
percent). When a block located in an <c>mseg_alloc</c>
singleblock carrier is shrunk, the carrier is left
unchanged if the ratio of unused memory is less than this
threshold, otherwise the carrier is shrunk.
See also <seealso marker="#M_asbcst"><c>asbcst</c></seealso>.</p>
</item>
<tag><marker id="M_sbct"/><c><![CDATA[+M<S>sbct <size>]]></c></tag>
<item>
<p>Singleblock carrier threshold (in kilobytes). Blocks larger than this
threshold are placed in singleblock carriers. Blocks
smaller than this threshold are placed in multiblock
carriers. On 32-bit Unix style OS this threshold cannot be set
> 8 MB.</p>
</item>
<tag><marker id="M_smbcs"/><c><![CDATA[+M<S>smbcs <size>]]></c></tag>
<item>
<p>Smallest (<c>mseg_alloc</c>) multiblock carrier size (in
kilobytes). See the description on how sizes for <c>mseg_alloc</c>
multiblock carriers are decided in section
<seealso marker="#mseg_mbc_sizes">
The alloc_util Framework</seealso>.</p>
</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 has only effect on the runtime system
with SMP support. Default behavior on the runtime system with
SMP support is <c>NoSchedulers+1</c> instances. Each scheduler
uses a lock-free instance of its own and other threads use
a common instance.</p>
<p>Before ERTS 5.9 it was possible to configure
a smaller number of thread-specific instances than schedulers.
This is, however, not possible anymore.</p>
</item>
</taglist>
</section>
<section>
<title>Flags for Configuration of alloc_util</title>
<p>All allocators based on <c>alloc_util</c> are effected.</p>
<taglist>
<tag><marker id="Muycs"/><c><![CDATA[+Muycs <size>]]></c></tag>
<item>
<p><c>sys_alloc</c> carrier size. Carriers allocated through
<c>sys_alloc</c> are allocated in sizes that 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.</p>
</item>
<tag><marker id="Mummc"/><c><![CDATA[+Mummc <amount>]]></c></tag>
<item>
<p>Maximum <c>mseg_alloc</c> carriers. Maximum number of carriers
placed in separate memory segments. When this limit is
reached, new carriers are placed in memory retrieved from
<c>sys_alloc</c>.</p>
</item>
<tag><marker id="Musac"/><c><![CDATA[+Musac <bool>]]></c></tag>
<item>
<p>Allow <c>sys_alloc</c> carriers. Defaults to <c>true</c>.
If set to <c>false</c>, <c>sys_alloc</c> carriers are never
created by allocators using the <c>alloc_util</c> framework.</p>
</item>
</taglist>
</section>
<section>
<title>Special Flag for literal_alloc</title>
<taglist>
<tag><marker id="MIscs"/><c><![CDATA[+MIscs <size in MB>]]></c></tag>
<item>
<p><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. Defaults to <c>1024</c>
(that is, 1 GB), which is usually sufficient.
The flag is ignored on 32-bit architectures.</p>
</item>
</taglist>
</section>
<section>
<title>Instrumentation Flags</title>
<taglist>
<tag><marker id="Mim"/><c>+Mim true|false</c></tag>
<item>
<p>A map over current allocations is kept by the emulator.
The allocation map can be retrieved through module
<seealso marker="tools:instrument">
<c>instrument(3)</c></seealso>. <c>+Mim true</c>
implies <c>+Mis true</c>. <c>+Mim true</c> is the same as flag
<seealso marker="erl#instr"><c>-instr</c></seealso> in
<c>erl(1)</c>.</p>
</item>
<tag><marker id="Mis"/><c>+Mis true|false</c></tag>
<item>
<p>Status over allocated memory is kept by the emulator.
The allocation status can be retrieved through module
<seealso marker="tools:instrument">
<c>instrument(3)</c></seealso>.</p>
</item>
<tag><marker id="Mit"/><c>+Mit X</c></tag>
<item>
<p>Reserved for future use. Do <em>not</em> use this flag.</p>
</item>
</taglist>
<note>
<p>When instrumentation of the emulator is enabled, the emulator
uses more memory and runs slower.</p>
</note>
</section>
<section>
<title>Other Flags</title>
<taglist>
<tag><marker id="Mea"/><c>+Mea min|max|r9c|r10b|r11b|config</c></tag>
<item>
<p>Options:</p>
<taglist>
<tag><c>min</c></tag>
<item>
<p>Disables all allocators that can be disabled.</p>
</item>
<tag><c>max</c></tag>
<item>
<p>Enables all allocators (default).</p>
</item>
<tag><c>r9c|r10b|r11b</c></tag>
<item>
<p>Configures all allocators as they were configured in respective
Erlang/OTP release. These will eventually be removed.</p>
</item>
<tag><c>config</c></tag>
<item>
<p>Disables features that cannot be enabled while creating an
allocator configuration with
<seealso marker="runtime_tools:erts_alloc_config">
<c>erts_alloc_config(3)</c></seealso>.</p>
<note>
<p>This option is to be used only while running
<c>erts_alloc_config(3)</c>, <em>not</em> when
using the created configuration.</p>
</note>
</item>
</taglist>
</item>
<tag><marker id="Mlpm"/><c>+Mlpm all|no</c></tag>
<item>
<p>Lock physical memory. Defaults to <c>no</c>, that is,
no physical memory is locked. If set to <c>all</c>, all
memory mappings made by the runtime system are locked into
physical memory. If set to <c>all</c>, the runtime system fails 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 also fails with an out of memory
condition if the user limit on the amount of locked memory is
reached.</p>
</item>
</taglist>
</section>
</section>
<section>
<title>Notes</title>
<p>Only some default values have been presented here. For information
about the currently used settings and the current status of the
allocators, see
<seealso marker="erts:erlang#system_info_allocator">
<c>erlang:system_info(allocator)</c></seealso> and
<seealso marker="erts:erlang#system_info_allocator_tuple">
<c>erlang:system_info({allocator, Alloc})</c></seealso>.</p>
<note>
<p>Most of these flags are highly implementation-dependent and
can 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 can even ignore them).</p>
</note>
<p>The <seealso marker="runtime_tools:erts_alloc_config">
<c>erts_alloc_config(3)</c></seealso>
tool 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="erl"><c>erl(1)</c></seealso>,
<seealso marker="erlang"><c>erlang(3)</c></seealso>,
<seealso marker="runtime_tools:erts_alloc_config">
<c>erts_alloc_config(3)</c></seealso>,
<seealso marker="tools:instrument">
<c>instrument(3)</c></seealso></p>
</section>
</cref>