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NAME
erts_alloc - An Erlang Run-Time System internal memory allocator library.
DESCRIPTION
erts_alloc is an Erlang Run-Time System internal memory allocator library. erts_alloc
provides the Erlang Run-Time System with a number of memory allocators.
ALLOCATORS
Currently the following allocators are present:
temp_alloc:
Allocator used for temporary allocations.
eheap_alloc:
Allocator used for Erlang heap data, such as Erlang process heaps.
binary_alloc:
Allocator used for Erlang binary data.
ets_alloc:
Allocator used for ETS data.
driver_alloc:
Allocator used for driver data.
sl_alloc:
Allocator used for memory blocks that are expected to be short-lived.
ll_alloc:
Allocator used for memory blocks that are expected to be long-lived, for example
Erlang code.
fix_alloc:
A fast allocator used for some frequently used fixed size data types.
std_alloc:
Allocator used for most memory blocks not allocated via any of the other allocators
described above.
sys_alloc:
This is normally the default malloc implementation used on the specific OS.
mseg_alloc:
A memory segment allocator. mseg_alloc is used by other allocators for allocating
memory segments and is currently only available on systems that have the mmap 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.
sys_alloc is always enabled and cannot be disabled. mseg_alloc is always enabled if it is
available and an allocator that uses it is enabled. All other allocators can be enabled or
disabled. By default all allocators are enabled. When an allocator is disabled, sys_alloc
is used instead of the disabled allocator.
The main idea with the erts_alloc 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.
THE ALLOC_UTIL FRAMEWORK
Internally a framework called alloc_util is used for implementing allocators. sys_alloc,
and mseg_alloc do not use this framework; hence, the following does not apply to them.
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 mseg_alloc), or in
the heap segment (allocated via sys_alloc). 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 (sbct) parameter are placed in
singleblock carriers. Blocks that are smaller than the value of the sbct 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 mmbcs parameter.
Sizes of multiblock carriers allocated via mseg_alloc are decided based on the values of
the largest multiblock carrier size (lmbcs), the smallest multiblock carrier size (smbcs),
and the multiblock carrier growth stages (mbcgs) parameters. If nc is the current number
of multiblock carriers (the main multiblock carrier excluded) managed by an allocator, the
size of the next mseg_alloc multiblock carrier allocated by this allocator will roughly be
smbcs+nc*(lmbcs-smbcs)/mbcgs when nc <= mbcgs, and lmbcs when nc > mbcgs. If the value of
the sbct parameter should be larger than the value of the lmbcs parameter, the allocator
may have to create multiblock carriers that are larger than the value of the lmbcs
parameter, though. Singleblock carriers allocated via mseg_alloc are sized to whole pages.
Sizes of carriers allocated via sys_alloc are decided based on the value of the sys_alloc
carrier size (ycs) parameter. The size of a carrier is the least number of multiples of
the value of the ycs parameter that satisfies the request.
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.
The memory allocation strategy used for multiblock carriers by an allocator is
configurable via the as parameter. Currently the following strategies are available:
Best fit:
Strategy: Find the smallest block that satisfies the requested block size.
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.
Address order best fit:
Strategy: Find the smallest block that satisfies the requested block size. If multiple
blocks are found, choose the one with the lowest address.
Implementation: A balanced binary search tree is used. The time complexity is
proportional to log N, where N is the number of free blocks.
Address order first fit:
Strategy: Find the block with the lowest address that satisfies the requested block
size.
Implementation: A balanced binary search tree is used. The time complexity is
proportional to log N, where N is the number of free blocks.
Address order first fit carrier best fit:
Strategy: Find the carrier with the lowest address that can satisfy the requested
block size, then find a block within that carrier using the "best fit" strategy.
Implementation: Balanced binary search trees are used. The time complexity is
proportional to log N, where N is the number of free blocks.
Address order first fit carrier address order best fit:
Strategy: Find the carrier 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.
Implementation: Balanced binary search trees are used. The time complexity is
proportional to log N, where N is the number of free blocks.
Good fit:
Strategy: Try to find the best fit, but settle for the best fit found during a limited
search.
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 mbsd parameter.
A fit:
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.
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.
As of erts version 5.6.1 the emulator will refuse to use this strategy on other
allocators than temp_alloc. This since it will only cause problems for other
allocators.
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.
SYSTEM FLAGS EFFECTING ERTS_ALLOC
Warning:
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.
Memory allocator system flags have the following syntax: +M<S><P> <V> where <S> is a
letter identifying a subsystem, <P> is a parameter, and <V> is the value to use. The flags
can be passed to the Erlang emulator (erl) as command line arguments.
System flags effecting specific allocators have an upper-case letter as <S>. The following
letters are used for the currently present allocators:
* B: binary_alloc
* D: std_alloc
* E: ets_alloc
* F: fix_alloc
* H: eheap_alloc
* L: ll_alloc
* M: mseg_alloc
* R: driver_alloc
* S: sl_alloc
* T: temp_alloc
* Y: sys_alloc
The following flags are available for configuration of mseg_alloc:
+MMamcbf <size>:
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.
+MMrmcbf <ratio>:
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.
+MMsco true|false:
Set super carrier only flag. This flag defaults to true. When a super carrier is used
and this flag is true, mseg_alloc will only create carriers in the super carrier. Note
that the alloc_util framework may create sys_alloc carriers, so if you want all
carriers to be created in the super carrier, you therefore want to disable use of
sys_alloc carriers by also passing +Musac false. When the flag is false, mseg_alloc
will try to create carriers outside of the super carrier when the super carrier is
full.
NOTE: Setting this flag to false may not be supported on all systems. This flag will
in that case be ignored.
NOTE: The super carrier cannot be enabled nor disabled on halfword heap systems. This
flag will be ignored on halfword heap systems.
+MMscrfsd <amount>:
Set super carrier reserved free segment descriptors. This parameter defaults to
65536. 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 erts_mmap tuple part
of the result from calling erlang:system_info({allocator, mseg_alloc}).
+MMscrpm true|false:
Set super carrier reserve physical memory flag. This flag defaults to true. When this
flag is true, 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 false 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.
NOTE: 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
false may not be supported on all systems. This flag will in that case be ignored.
NOTE: The super carrier cannot be enabled nor disabled on halfword heap systems. This
flag will be ignored on halfword heap systems.
+MMscs <size in MB>:
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. mseg_alloc will always try to create new carriers in the
super carrier if it exists. Note that the alloc_util framework may create sys_alloc
carriers. For more information on this, see the documentation of the +MMsco flag.
NOTE: The super carrier cannot be enabled nor disabled on halfword heap systems. This
flag will be ignored on halfword heap systems.
+MMmcs <amount>:
Max cached segments. The maximum number of memory segments stored in the memory
segment cache. Valid range is 0-30. Default value is 10.
The following flags are available for configuration of sys_alloc:
+MYe true:
Enable sys_alloc. Note: sys_alloc cannot be disabled.
+MYm libc:
malloc library to use. Currently only libc is available. libc enables the standard
libc malloc implementation. By default libc is used.
+MYtt <size>:
Trim threshold size (in kilobytes). This is the maximum amount of free memory at the
top of the heap (allocated by sbrk) that will be kept by malloc (not released to the
operating system). When the amount of free memory at the top of the heap exceeds the
trim threshold, malloc will release it (by calling sbrk). Trim threshold is given in
kilobytes. Default trim threshold is 128. Note: This flag will only have any effect
when the emulator has been linked with the GNU C library, and uses its malloc
implementation.
+MYtp <size>:
Top pad size (in kilobytes). This is the amount of extra memory that will be
allocated by malloc when sbrk is called to get more memory from the operating system.
Default top pad size is 0. Note: This flag will only have any effect when the emulator
has been linked with the GNU C library, and uses its malloc implementation.
The following flags are available for configuration of allocators based on alloc_util. If
u is used as subsystem identifier (i.e., <S> = u) all allocators based on alloc_util will
be effected. If B, D, E, F, H, L, R, S, or T is used as subsystem identifier, only the
specific allocator identified will be effected:
+M<S>acul <utilization>|de:
Abandon carrier utilization limit. A valid <utilization> is an integer in the range
[0, 100] representing utilization in percent. When a utilization value larger than
zero is used, allocator instances are allowed to abandon multiblock carriers. If de
(default enabled) is passed instead of a <utilization>, 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 de, 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 allocation strategy used. Currently only the strategies aoff, aoffcbf and
aoffcaobf support abandoned carriers. This feature also requires multiple thread
specific instances to be enabled. When enabling this feature, multiple thread specific
instances will be enabled if not already enabled, and the aoffcbf strategy will be
enabled if current strategy does not support abandoned carriers. This feature can be
enabled on all allocators based on the alloc_util framework with the exception of
temp_alloc (which would be pointless).
+M<S>as bf|aobf|aoff|aoffcbf|aoffcaobf|gf|af:
Allocation strategy. Valid strategies are bf (best fit), aobf (address order best
fit), aoff (address order first fit), aoffcbf (address order first fit carrier best
fit), aoffcaobf (address order first fit carrier address order best fit), gf (good
fit), and af (a fit). See the description of allocation strategies in "the alloc_util
framework" section.
+M<S>asbcst <size>:
Absolute singleblock carrier shrink threshold (in kilobytes). When a block located in
an mseg_alloc 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 rsbcst.
+M<S>e true|false:
Enable allocator <S>.
+M<S>lmbcs <size>:
Largest (mseg_alloc) multiblock carrier size (in kilobytes). See the description on
how sizes for mseg_alloc multiblock carriers are decided in "the alloc_util framework"
section. On 32-bit Unix style OS this limit can not be set higher than 128 megabyte.
+M<S>mbcgs <ratio>:
(mseg_alloc) multiblock carrier growth stages. See the description on how sizes for
mseg_alloc multiblock carriers are decided in "the alloc_util framework" section.
+M<S>mbsd <depth>:
Max block search depth. This flag has effect only if the good fit strategy has been
selected for allocator <S>. 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.
+M<S>mmbcs <size>:
Main multiblock carrier size. Sets the size of the main multiblock carrier for
allocator <S>. The main multiblock carrier is allocated via sys_alloc and is never
deallocated.
+M<S>mmmbc <amount>:
Max mseg_alloc multiblock carriers. Maximum number of multiblock carriers allocated
via mseg_alloc by allocator <S>. When this limit has been reached, new multiblock
carriers will be allocated via sys_alloc.
+M<S>mmsbc <amount>:
Max mseg_alloc singleblock carriers. Maximum number of singleblock carriers allocated
via mseg_alloc by allocator <S>. When this limit has been reached, new singleblock
carriers will be allocated via sys_alloc.
+M<S>ramv <bool>:
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.
+M<S>rmbcmt <ratio>:
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.
+M<S>rsbcmt <ratio>:
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 sbct 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.
+M<S>rsbcst <ratio>:
Relative singleblock carrier shrink threshold (in percent). When a block located in
an mseg_alloc 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 asbcst.
+M<S>sbct <size>:
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.
+M<S>smbcs <size>:
Smallest (mseg_alloc) multiblock carrier size (in kilobytes). See the description on
how sizes for mseg_alloc multiblock carriers are decided in "the alloc_util framework"
section.
+M<S>t true|false:
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 NoSchedulers+1 instances. Each scheduler will use a lock-free
instance of its own and other threads will use a common instance.
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.
Currently the following flags are available for configuration of alloc_util, i.e. all
allocators based on alloc_util will be effected:
+Muycs <size>:
sys_alloc carrier size. Carriers allocated via sys_alloc will be allocated in sizes
which are multiples of the sys_alloc carrier size. This is not true for main
multiblock carriers and carriers allocated during a memory shortage, though.
+Mummc <amount>:
Max mseg_alloc 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 sys_alloc.
+Musac <bool>:
Allow sys_alloc carriers. By default true. If set to false, sys_alloc carriers will
never be created by allocators using the alloc_util framework.
Instrumentation flags:
+Mim true|false:
A map over current allocations is kept by the emulator. The allocation map can be
retrieved via the instrument module. +Mim true implies +Mis true. +Mim true is the
same as -instr.
+Mis true|false:
Status over allocated memory is kept by the emulator. The allocation status can be
retrieved via the instrument module.
+Mit X:
Reserved for future use. Do not use this flag.
Note:
When instrumentation of the emulator is enabled, the emulator uses more memory and runs
slower.
Other flags:
+Mea min|max|r9c|r10b|r11b|config:
min:
Disables all allocators that can be disabled.
max:
Enables all allocators (currently default).
r9c|r10b|r11b:
Configures all allocators as they were configured in respective OTP release. These
will eventually be removed.
config:
Disables features that cannot be enabled while creating an allocator configuration
with erts_alloc_config(3erl). Note, this option should only be used while running
erts_alloc_config, not when using the created configuration.
+Mlpm all|no:
Lock physical memory. The default value is no, i.e., no physical memory will be
locked. If set to all, all memory mappings made by the runtime system, will be locked
into physical memory. If set to all, 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.
Only some default values have been presented here. erlang:system_info(allocator), and
erlang:system_info({allocator, Alloc}) can be used in order to obtain currently used
settings and current status of the allocators.
Note:
Most of these flags are highly implementation dependent, and they may be changed or
removed without prior notice.
erts_alloc is not obliged to strictly use the settings that have been passed to it (it may
even ignore them).
erts_alloc_config(3erl) is a tool that can be used to aid creation of an erts_alloc
configuration that is suitable for a limited number of runtime scenarios.
SEE ALSO
erts_alloc_config(3erl), erl(1), instrument(3erl), erlang(3erl)