erts_alloc - An Erlang runtime system internal memory allocator library.
Please see following description for synopsis
erts_alloc(3) C Library Functions erts_alloc(3)
NAME
erts_alloc - An Erlang runtime system internal memory allocator
library.
DESCRIPTION
erts_alloc is an Erlang runtime system internal memory allocator
library. erts_alloc provides the Erlang runtime system with a number of
memory allocators.
ALLOCATORS
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.
literal_alloc:
Allocator used for constant terms in Erlang code.
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 through any of
the other allocators described above.
sys_alloc:
This is normally the default malloc implementation used on the spe-
cific OS.
mseg_alloc:
A memory segment allocator. It is used by other allocators for
allocating memory segments and is 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 to reduce the number of sys-
tem calls made.
sys_alloc, literal_alloc and temp_alloc are 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, 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 fre-
quently allocated, a performance gain can be achieved.
THE ALLOC_UTIL FRAMEWORK
Internally a framework called alloc_util is used for implementing allo-
cators. sys_alloc and mseg_alloc do not use this framework, so the fol-
lowing 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 seg-
ment (allocated through mseg_alloc), or in the heap segment (allocated
through 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 parameter sbct are placed
in multiblock carriers.
Normally an allocator creates a "main multiblock carrier". Main multi-
block carriers are never deallocated. The size of the main multiblock
carrier is determined by the value of parameter mmbcs.
Sizes of multiblock carriers allocated through mseg_alloc are decided
based on the following parameters:
* The values of the largest multiblock carrier size (lmbcs)
* The smallest multiblock carrier size (smbcs)
* The multiblock carrier growth stages (mbcgs)
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 is roughly
smbcs+nc*(lmbcs-smbcs)/mbcgs when nc <= mbcgs, and lmbcs when nc >
mbcgs. If the value of parameter sbct is larger than the value of
parameter lmbcs, the allocator may have to create multiblock carriers
that are larger than the value of parameter lmbcs, though. Singleblock
carriers allocated through mseg_alloc are sized to whole pages.
Sizes of carriers allocated through sys_alloc are decided based on the
value of the sys_alloc carrier size (ycs) parameter. The size of a car-
rier is the least number of multiples of the value of parameter ycs
satisfying 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 allo-
cator can be configured using parameter as. The following strategies
are available:
Best fit:
Strategy: Find the smallest block satisfying 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 satisfying 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 satisfying 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 "address 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.
Age order first fit carrier address order first fit:
Strategy: Find the oldest carrier that can satisfy the requested
block size, then find a block within that carrier using the
"address order first fit" strategy.
Implementation: A balanced binary search tree is used. The time
complexity is proportional to log N, where N is the number of free
blocks.
Age order first fit carrier best fit:
Strategy: Find the oldest carrier 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.
Age order first fit carrier address order best fit:
Strategy: Find the oldest carrier that can satisfy the requested
block size, then find a block within that carrier using the
"address 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) 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 mbsd.
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 sat-
isfies the request, it is used, otherwise a new carrier is created.
The implementation has a time complexity that is constant.
As from ERTS 5.6.1 the emulator refuses to use this strategy on
other allocators than temp_alloc. This because it only causes prob-
lems for other allocators.
Apart from the ordinary allocators described above, some pre-allocators
are used for some specific data types. These pre-allocators pre-allo-
cate 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.
SYSTEM FLAGS EFFECTING ERTS_ALLOC
Warning:
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.
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(1)) as command-line arguments.
System flags effecting specific allocators have an uppercase letter as
<S>. The following letters are used for the allocators:
* B: binary_alloc
* D: std_alloc
* E: ets_alloc
* F: fix_alloc
* H: eheap_alloc
* I: literal_alloc
* L: ll_alloc
* M: mseg_alloc
* R: driver_alloc
* S: sl_alloc
* T: temp_alloc
* Y: sys_alloc
Flags for Configuration of mseg_alloc
+MMamcbf <size>:
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 4096.
+MMrmcbf <ratio>:
Relative maximum cache bad fit (in percent). A segment in the mem-
ory 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 20.
+MMsco true|false:
Sets super carrier only flag. Defaults to true. When a super car-
rier is used and this flag is true, mseg_alloc only creates carri-
ers in the super carrier. Notice that the alloc_util framework can
create sys_alloc carriers, so if you want all carriers to be cre-
ated 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 tries to create carriers outside of the super
carrier when the super carrier is full.
Note:
Setting this flag to false is not supported on all systems. The flag
is then ignored.
+MMscrfsd <amount>:
Sets super carrier reserved free segment descriptors. Defaults to
65536. This parameter determines the amount of memory to reserve
for free segment descriptors used by the super carrier. If the sys-
tem 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 erts_mmap tuple
part of the result from calling erlang:system_info({allocator,
mseg_alloc}).
+MMscrpm true|false:
Sets super carrier reserve physical memory flag. Defaults to true.
When this flag is true, 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 false, 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.
Note:
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.
Setting this flag to false is possibly not supported on all systems.
The flag is then ignored.
+MMscs <size in MB>:
Sets super carrier size (in MB). Defaults to 0, that is, the super
carrier is by default disabled. The super carrier is a large con-
tinuous area in the virtual address space. mseg_alloc always tries
to create new carriers in the super carrier if it exists. Notice
that the alloc_util framework can create sys_alloc carriers. For
more information, see +MMsco.
+MMmcs <amount>:
Maximum cached segments. The maximum number of memory segments
stored in the memory segment cache. Valid range is [0, 30].
Defaults to 10.
Flags for Configuration of sys_alloc
+MYe true:
Enables sys_alloc.
Note:
sys_alloc cannot be disabled.
+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 is 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 releases it (by calling sbrk). Trim threshold is specified
in kilobytes. Defaults to 128.
Note:
This flag has effect only when the emulator is 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 is allocated by malloc when sbrk is called to get more memory
from the operating system. Defaults to 0.
Note:
This flag has effect only when the emulator is linked with the GNU C
library, and uses its malloc implementation.
Flags for Configuration of Allocators Based on alloc_util
If u is used as subsystem identifier (that is, <S> = u), all allocators
based on alloc_util are effected. If B, D, E, F, H, I, L, R, S, T, X is
used as subsystem identifier, only the specific allocator identifier is
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 > 0 is used, allocator instances are
allowed to abandon multiblock carriers. If de (default enabled) is
passed instead of a <utilization>, a recommended non-zero utiliza-
tion value is used. The value chosen depends on the allocator type
and can be changed between ERTS versions. Defaults to de, but this
can be changed in the future.
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 another allocator instance. If
no abandoned carrier can be fetched, it creates a new empty car-
rier. When an abandoned carrier has been fetched, it will function
as an ordinary carrier. This feature has special requirements on
the allocation strategy used. Only the strategies aoff, aoffcbf,
aoffcaobf, ageffcaoffm, ageffcbf and ageffcaobf support abandoned
carriers.
This feature also requires multiple thread specific instances to be
enabled. When enabling this feature, multiple thread-specific
instances are enabled if not already enabled, and the aoffcbf
strategy is enabled if the current strategy does not support aban-
doned carriers. This feature can be enabled on all allocators based
on the alloc_util framework, except temp_alloc (which would be
pointless).
+M<S>acfml <bytes>:
Abandon carrier free block min limit. A valid <bytes> is a positive
integer representing a block size limit. The largest free block in
a carrier must be at least bytes large, for the carrier to be aban-
doned. The default is zero but can be changed in the future.
See also acul.
+M<S>acnl <amount>:
Abandon carrier number limit. A valid <amount> is a positive inte-
ger representing max number of abandoned carriers per allocator
instance. Defaults to 1000 which will practically disable the
limit, but this can be changed in the future.
See also acul.
+M<S>as bf|aobf|aoff|aoffcbf|aoffcaobf|ageffcaoff|ageffcbf|ageff-
caobf|gf|af:
Allocation strategy. The following strategies are valid:
* 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)
* ageffcaoff (age order first fit carrier address order first fit)
* ageffcbf (age order first fit carrier best fit)
* ageffcaobf (age order first fit carrier address order best fit)
* gf (good fit)
* af (a fit)
See the description of allocation strategies in section The
alloc_util Framework.
+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 is left unchanged if the amount of unused memory is less
than this threshold, otherwise the carrier is shrunk. See also
rsbcst.
+M<S>atags true|false:
Adds a small tag to each allocated block that contains basic infor-
mation about what it is and who allocated it. Use the instrument
module to inspect this information.
The runtime overhead is one word per allocation when enabled. This
may change at any time in the future.
The default is true for binary_alloc and driver_alloc, and false
for the other allocator types.
+M<S>cp B|D|E|F|H||L|R|S|@|::
Set carrier pool to use for the allocator. Memory carriers will
only migrate between allocator instances that use the same carrier
pool. The following carrier pool names exist:
B:
Carrier pool associated with binary_alloc.
D:
Carrier pool associated with std_alloc.
E:
Carrier pool associated with ets_alloc.
F:
Carrier pool associated with fix_alloc.
H:
Carrier pool associated with eheap_alloc.
L:
Carrier pool associated with ll_alloc.
R:
Carrier pool associated with driver_alloc.
S:
Carrier pool associated with sl_alloc.
@:
Carrier pool associated with the system as a whole.
Besides passing carrier pool name as value to the parameter, you
can also pass :. By passing : instead of carrier pool name, the
allocator will use the carrier pool associated with itself. By
passing the command line argument "+Mucg :", all allocators that
have an associated carrier pool will use the carrier pool associ-
ated with themselves.
The association between carrier pool and allocator is very loose.
The associations are more or less only there to get names for the
amount of carrier pools needed and names of carrier pools that can
be easily identified by the : value.
This flag is only valid for allocators that have an associated car-
rier pool. Besides that, there are no restrictions on carrier pools
to use for an allocator.
Currently each allocator with an associated carrier pool defaults
to using its own associated carrier pool.
+M<S>e true|false:
Enables 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 section The alloc_util Framework. On 32-bit Unix style
OS this limit cannot be set > 64 MB.
+M<S>mbcgs <ratio>:
(mseg_alloc) multiblock carrier growth stages. See the description
on how sizes for mseg_alloc multiblock carriers are decided in sec-
tion The alloc_util Framework.
+M<S>mbsd <depth>:
Maximum block search depth. This flag has effect only if the good
fit strategy is 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
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.
+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
through sys_alloc and is never deallocated.
+M<S>mmmbc <amount>:
Maximum mseg_alloc multiblock carriers. Maximum number of multi-
block carriers allocated through mseg_alloc by allocator <S>. When
this limit is reached, new multiblock carriers are allocated
through sys_alloc.
+M<S>mmsbc <amount>:
Maximum mseg_alloc singleblock carriers. Maximum number of single-
block carriers allocated through mseg_alloc by allocator <S>. When
this limit is reached, new singleblock carriers are allocated
through sys_alloc.
+M<S>ramv <bool>:
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.
+M<S>rmbcmt <ratio>:
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 freed memory compared to the previ-
ous size is more than this threshold, otherwise the block is shrunk
at the 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 parameter sbct, 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.
+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 is left unchanged if the ratio of unused memory is less
than this threshold, otherwise the carrier is shrunk. See also
asbcst.
+M<S>sbct <size>:
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.
+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 section The alloc_util Framework.
+M<S>t true|false:
Multiple, thread-specific instances of the allocator. Default
behavior is NoSchedulers+1 instances. Each scheduler uses a lock-
free instance of its own and other threads use a common instance.
Before ERTS 5.9 it was possible to configure a smaller number of
thread-specific instances than schedulers. This is, however, not
possible anymore.
Flags for Configuration of alloc_util
All allocators based on alloc_util are effected.
+Muycs <size>:
sys_alloc carrier size. Carriers allocated through sys_alloc are
allocated in sizes that 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>:
Maximum mseg_alloc carriers. Maximum number of carriers placed in
separate memory segments. When this limit is reached, new carriers
are placed in memory retrieved from sys_alloc.
+Musac <bool>:
Allow sys_alloc carriers. Defaults to true. If set to false,
sys_alloc carriers are never created by allocators using the
alloc_util framework.
Special Flag for literal_alloc
+MIscs <size in MB>:
literal_alloc super carrier size (in MB). The amount of virtual
address space reserved for literal terms in Erlang code on 64-bit
architectures. Defaults to 1024 (that is, 1 GB), which is usually
sufficient. The flag is ignored on 32-bit architectures.
Instrumentation Flags
+M<S>atags:
Adds a small tag to each allocated block that contains basic infor-
mation about what it is and who allocated it. See +M<S>atags for a
more complete description.
+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:
Options:
min:
Disables all allocators that can be disabled.
max:
Enables all allocators (default).
r9c|r10b|r11b:
Configures all allocators as they were configured in respective
Erlang/OTP release. These will eventually be removed.
config:
Disables features that cannot be enabled while creating an allo-
cator configuration with erts_alloc_config(3).
Note:
This option is to be used only while running erts_alloc_config(3),
not when using the created configuration.
+Mlpm all|no:
Lock physical memory. Defaults to no, that is, no physical memory
is locked. If set to all, all memory mappings made by the runtime
system are locked into physical memory. If set to all, 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 mem-
ory is reached.
+Mdai max|<amount>:
Set amount of dirty allocator instances used. Defaults to 0. That
is, by default no instances will be used. The maximum amount of
instances equals the amount of dirty CPU schedulers on the system.
By default, each normal scheduler thread has its own allocator
instance for each allocator. All other threads in the system,
including dirty schedulers, share one instance for each allocator.
By enabling dirty allocator instances, dirty schedulers will get
and use their own set of allocator instances. Note that these
instances are not exclusive to each dirty scheduler, but instead
shared among dirty schedulers. The more instances used the less
risk of lock contention on these allocator instances. Memory con-
sumption do however increase with increased amount of dirty alloca-
tor instances.
NOTES
Only some default values have been presented here. For information
about the currently used settings and the current status of the alloca-
tors, see erlang:system_info(allocator) and erlang:system_info({alloca-
tor, Alloc}).
Note:
Most of these flags are highly implementation-dependent and can be
changed or removed without prior notice.
erts_alloc is not obliged to strictly use the settings that have been
passed to it (it can even ignore them).
The erts_alloc_config(3) tool can be used to aid creation of an
erts_alloc configuration that is suitable for a limited number of run-
time scenarios.
SEE ALSO
erl(1), erlang(3), erts_alloc_config(3), instrument(3)
Ericsson AB erts 12.2 erts_alloc(3)