Ubuntu Manpage: erl - Start the Erlang runtime system

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NAME

       erl - Start the Erlang runtime system

DESCRIPTION

The erl program starts an Erlang runtime system. The exact details (for example, whether erl is a script
or a program and which other programs it calls) are system-dependent.

NOTE: If you are using Erlang/OTP 25 or earlier on Windows and want to start an Erlang system with full
shell support, you should use werl.exe. See the Erlang/OTP 25 documentation for details on how to do
that.

erl <arguments>
Starts an Erlang runtime system.

The arguments can be divided into emulator flags, flags, and plain arguments:

• Any argument starting with character + is interpreted as an emulator flag.

As indicated by the name, emulator flags control the behavior of the emulator.

• Any argument starting with character - (hyphen) is interpreted as a flag, which is to be passed to
the Erlang part of the runtime system, more specifically to the init system process, see init.

The init process itself interprets some of these flags, the init flags. It also stores any remaining
flags, the user flags. The latter can be retrieved by calling init:get_argument/1.

A small number of "-" flags exist, which now actually are emulator flags, see the description below.

• Plain arguments are not interpreted in any way. They are also stored by the init process and can be
retrieved by calling init:get_plain_arguments/0. Plain arguments can occur before the first flag, or
after a -- flag. Also, the -extra flag causes everything that follows to become plain arguments.

Examples:

% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
(arnie@host)1> init:get_argument(sname).
{ok,[["arnie"]]}
(arnie@host)2> init:get_plain_arguments().
["+bertie"]

Here +W w and +R 9 are emulator flags. -s my_init is an init flag, interpreted by init. -sname arnie is a
user flag, stored by init. It is read by Kernel and causes the Erlang runtime system to become
distributed. Finally, everything after -extra (that is, +bertie) is considered as plain arguments.

% erl -myflag 1
1> init:get_argument(myflag).
{ok,[["1"]]}
2> init:get_plain_arguments().
[]

Here the user flag -myflag 1 is passed to and stored by the init process. It is a user-defined flag,
presumably used by some user-defined application.

Flags
In the following list, init flags are marked "(init flag)". Unless otherwise specified, all other flags
are user flags, for which the values can be retrieved by calling init:get_argument/1. Notice that the
list of user flags is not exhaustive, there can be more application-specific flags that instead are
described in the corresponding application documentation.

-- (init flag)
Everything following -- up to the next flag (-flag or +flag) is considered plain arguments and can be
retrieved using init:get_plain_arguments/0.

-Application Par Val
Sets the application configuration parameter Par to the value Val for the application Application; see
app(4) and application.

-args_file FileName
Command-line arguments are read from the file FileName. The arguments read from the file replace flag
'-args_file FileName' on the resulting command line.

The file FileName is to be a plain text file and can contain comments and command-line arguments. A
comment begins with a # character and continues until the next end of line character. Backslash (\) is
used as quoting character. All command-line arguments accepted by erl are allowed, also flag -args_file
FileName. Be careful not to cause circular dependencies between files containing flag -args_file,
though.

The flag -extra is treated in special way. Its scope ends at the end of the file. Arguments following
an -extra flag are moved on the command line into the -extra section, that is, the end of the command
line following after an -extra flag.

-async_shell_start
The initial Erlang shell does not read user input until the system boot procedure has been completed
(Erlang/OTP 5.4 and later). This flag disables the start synchronization feature and lets the shell
start in parallel with the rest of the system.

-boot File
Specifies the name of the boot file, File.boot, which is used to start the system; see init. Unless
File contains an absolute path, the system searches for File.boot in the current and $ROOT/bin
directories.

Defaults to $ROOT/bin/start.boot.

-boot_var Var Dir
If the boot script contains a path variable Var other than $ROOT, this variable is expanded to Dir.
Used when applications are installed in another directory than $ROOT/lib; see systools:make_script/1,2
in SASL.

-code_path_cache
Enables the code path cache of the code server; see code.

-compile Mod1 Mod2 ...
Compiles the specified modules and then terminates (with non-zero exit code if the compilation of some
file did not succeed). Implies -noinput.

Not recommended; use erlc instead.

-config Config [Config ...]
Specifies the name of one or more configuration files, Config.config, which is used to configure
applications; see app(4) and application. See the documentation for the configuration file format for a
description of the configuration format and the order in which configuration parameters are read.

-configfd FD [FD ...]
Specifies the name of one or more file descriptors (called configuration file descriptors from here on)
with configuration data for applications; see app(4) and application. See the documentation for the
configuration file format for a description of the configuration format and the order in which
configuration parameters are read.

A configuration file descriptor will be read until its end and will then be closed.

The content of a configuration file descriptor is stored so that it can be reused when init:restart/0
or init:restart/1 is called.

The parameter -configfd 0 implies -noinput.

NOTE: It is not recommended to use file descriptors 1 (standard output), and 2 (standard error)
together with -configfd as these file descriptors are typically used to print information to the
console the program is running in.

Examples (Unix shell):

$ erl \
-noshell \
-configfd 3 \
-eval \
<(echo '[{kernel, [{logger_level, warning}]}].')
{ok,warning}

$ echo '[{kernel, [{logger_level, warning}]}].' > test1.config
$ echo '[{kernel, [{logger_level, error}]}].' > test2.config
$ erl \
-noshell \
-configfd 3 \
-configfd 4 \
-eval \
3< test1.config 4< test2.config
{ok,error}

-connect_all false
This flag is deprecated and has been replaced by the kernel application parameter connect_all.

-cookie Cookie
Obsolete flag without any effect and common misspelling for -setcookie. Use -setcookie instead.

-detached
Starts the Erlang runtime system detached from the system console. Useful for running daemons and
backgrounds processes. Implies -noinput.

-disable-feature feature
Disables the feature feature in the runtime system. The special feature all can be used to disable all
non permanent features.

-dist_listen true|false
Specifies whether this node should be listening for incoming distribution connections or not. By
default a node will listen for incoming connections. Setting this option to false implies -hidden.

-emu_args
Useful for debugging. Prints the arguments sent to the emulator.

-emu_flavor emu|jit|smp
Start an emulator of a different flavor. Normally only one flavor is available, more can be added by
building specific flavors. The currently available flavors are: emu and jit. The smp flavor is an alias
for the current default flavor. You can combine this flag with --emu_type. You can get the current
flavor at run-time using erlang:system_info(emu_flavor). (The emulator with this flavor must be built.
You can build a specific flavor by doing make FLAVOR=$FLAVOR in the Erlang/OTP source repository.)

-emu_type Type
Start an emulator of a different type. For example, to start the lock-counter emulator, use -emu_type
lcnt. You can get the current type at run-time using erlang:system_info(build_type). (The emulator of
this type must already be built. Use the configure option --enable-lock-counter to build the lock-
counter emulator.)

-enable-feature feature
Enables the feature feature in the runtime system. The special feature all can be used to enable all
features.

-env Variable Value
Sets the host OS environment variable Variable to the value Value for the Erlang runtime system.
Example:

% erl -env DISPLAY gin:0

In this example, an Erlang runtime system is started with environment variable DISPLAY set to gin:0.

-epmd_module Module
This flag is deprecated and has been replaced by the kernel application parameter epmd_module.

-erl_epmd_port Port
This flag is deprecated and has been replaced by the kernel application parameter
erl_epmd_node_listen_port.

-eval Expr (init flag)
Makes init evaluate the expression Expr; see init.

-extra (init flag)
Everything following -extra is considered plain arguments and can be retrieved using
init:get_plain_arguments/0.

-heart
Starts heartbeat monitoring of the Erlang runtime system; see heart.

-hidden
Starts the Erlang runtime system as a hidden node, if it is run as a distributed node. Hidden nodes
always establish hidden connections to all other nodes except for nodes in the same global group.
Hidden connections are not published on any of the connected nodes, that is, none of the connected
nodes are part of the result from nodes/0 on the other node. See also hidden global groups;
global_group.

-hosts Hosts
Specifies the IP addresses for the hosts on which Erlang boot servers are running, see erl_boot_server.
This flag is mandatory if flag -loader inet is present.

The IP addresses must be specified in the standard form (four decimal numbers separated by periods, for
example, "150.236.20.74"). Hosts names are not acceptable, but a broadcast address (preferably limited
to the local network) is.

-id Id
Specifies the identity of the Erlang runtime system. If it is run as a distributed node, Id must be
identical to the name supplied together with flag -sname or -name.

-init_debug
Makes init write some debug information while interpreting the boot script.

-instr (emulator flag)
Selects an instrumented Erlang runtime system (virtual machine) to run, instead of the ordinary one.
When running an instrumented runtime system, some resource usage data can be obtained and analyzed
using the instrument module. Functionally, it behaves exactly like an ordinary Erlang runtime system.

-loader Loader
Specifies the method used by erl_prim_loader to load Erlang modules into the system; see
erl_prim_loader. Two Loader methods are supported:

• efile, which means use the local file system, this is the default.

• inet, which means use a boot server on another machine. The flags -id, -hosts and -setcookie must
also be specified.

If Loader is something else, the user-supplied Loader port program is started.

-make
Makes the Erlang runtime system invoke make:all() in the current working directory and then terminate;
see make. Implies -noinput.

-man Module
Displays the manual page for the Erlang module Module. Only supported on Unix.

-mode interactive | embedded
Modules are auto loaded when they are first referenced if the runtime system runs in interactive mode,
which is the default. In embedded mode modules are not auto loaded. The latter is recommended when the
boot script preloads all modules, as conventionally happens in OTP releases. See code.

-name Name
Makes the Erlang runtime system into a distributed node. This flag invokes all network servers
necessary for a node to become distributed; see net_kernel. It also ensures that epmd runs on the
current host before Erlang is started (see epmd(1) and the -start_epmd option) and that a magic cookie
has been set (see -setcookie).

The node name will be Name@Host, where Host is the fully qualified host name of the current host. For
short names, use flag -sname instead.

If Name is set to undefined the node will be started in a special mode optimized to be the temporary
client of another node. The node will then request a dynamic node name from the first node it connects
to. Read more in Dynamic Node Name.

WARNING: Starting a distributed node without also specifying -proto_dist inet_tls will expose the node
to attacks that may give the attacker complete access to the node and in extension the cluster. When
using un-secure distributed nodes, make sure that the network is configured to keep potential attackers
out.

-no_epmd
Specifies that the distributed node does not need epmd at all.

This option ensures that the Erlang runtime system does not start epmd and does not start the erl_epmd
process for distribution either.

This option only works if Erlang is started as a distributed node with the -proto_dist option using an
alternative protocol for Erlang distribution which does not rely on epmd for node registration and
discovery. For more information, see How to implement an Alternative Carrier for the Erlang
Distribution.

-noinput
Ensures that the Erlang runtime system never tries to read any input. Implies -noshell.

-noshell
Starts an Erlang runtime system with no shell. This flag makes it possible to have the Erlang runtime
system as a component in a series of Unix pipes.

-nostick
Disables the sticky directory facility of the Erlang code server; see code.

-oldshell
Invokes the old Erlang shell from Erlang/OTP 3.3. The old shell can still be used.

-pa Dir1 Dir2 ...
Adds the specified directories to the beginning of the code path, similar to code:add_pathsa/1. Note
that the order of the given directories will be reversed in the resulting path.

As an alternative to -pa, if several directories are to be prepended to the code path and the
directories have a common parent directory, that parent directory can be specified in environment
variable ERL_LIBS; see code.

-pz Dir1 Dir2 ...
Adds the specified directories to the end of the code path, similar to code:add_pathsz/1; see code.

-path Dir1 Dir2 ...
Replaces the path specified in the boot script; see script(4).

-proto_dist Proto
Specifies a protocol for Erlang distribution:

inet_tcp
TCP over IPv4 (the default)

inet_tls
Distribution over TLS/SSL, See the Using SSL for Erlang Distribution User's Guide for details on how
to setup a secure distributed node.

inet6_tcp
TCP over IPv6

For example, to start up IPv6 distributed nodes:

% erl -name test@ipv6node.example.com -proto_dist inet6_tcp

-remsh Node
Starts Erlang with a remote shell connected to Node.

If no -name or -sname is given the node will be started using -sname undefined. If Node does not
contain a hostname, one is automatically taken from -name or -sname

NOTE: Before OTP-23 the user needed to supply a valid -sname or -name for -remsh to work. This is still
the case if the target node is not running OTP-23 or later.

NOTE: The connecting node needs to have a proper shell with terminal emulation. This means that UNIX
users must use an Erlang compiled with terminal capabilities and before Erlang/OTP 25 Windows users
must use werl.

-rsh Program
Specifies an alternative to ssh for starting a slave node on a remote host; see slave.

-S Mod [Func [Arg1, Arg2, ...]] (init flag)
Makes init call the specified function. Func defaults to start. The function is assumed to be of arity
1, taking the list [Arg1,Arg2,...] as argument, or an empty list if no arguments are passed. All
further arguments occurring after this option are passed to the specified function as strings. Implies
-noshell. See init.

-run Mod [Func [Arg1, Arg2, ...]] (init flag)
Makes init call the specified function. Func defaults to start. If no arguments are provided, the
function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as strings. See init.

-s Mod [Func [Arg1, Arg2, ...]] (init flag)
Makes init call the specified function. Func defaults to start. If no arguments are provided, the
function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as atoms. See init.

-setcookie Cookie
Sets the magic cookie of the node to Cookie; see erlang:set_cookie/1. See see section Distributed
Erlang in the Erlang Reference Manual for more details.

-setcookie Node Cookie
Sets the magic cookie for Node to Cookie; see erlang:set_cookie/2.

-shutdown_time Time
Specifies how long time (in milliseconds) the init process is allowed to spend shutting down Erlang
applications in the system. If Time milliseconds have elapsed, all processes still existing are killed.
Defaults to infinity.

-sname Name
Makes the Erlang runtime system into a distributed node, similar to -name, but the host name portion of
the node name Name@Host will be the short name, not fully qualified.

This is sometimes the only way to run distributed Erlang if the Domain Name System (DNS) is not
running. No communication can exist between nodes running with flag -sname and those running with flag
-name, as node names must be unique in distributed Erlang systems.

-start_epmd true | false
Specifies whether Erlang should start epmd on startup. By default this is true, but if you prefer to
start epmd manually, set this to false.

This only applies if Erlang is started as a distributed node, i.e. if -name or -sname is specified.
Otherwise, epmd is not started even if -start_epmd true is given.

Note that a distributed node will fail to start if epmd is not running.

-version (emulator flag)
Makes the emulator print its version number. The same as erl +V.

Emulator Flags
erl invokes the code for the Erlang emulator (virtual machine), which supports the following flags. The
flags are read from left to right and later flags override the behavior of earlier flags.

+a size
Suggested stack size, in kilowords, for threads in the async thread pool. Valid range is 16-8192
kilowords. The default suggested stack size is 16 kilowords, that is, 64 kilobyte on 32-bit
architectures. This small default size has been chosen because the number of async threads can be
large. The default size is enough for drivers delivered with Erlang/OTP, but might not be large enough
for other dynamically linked-in drivers that use the driver_async() functionality. Notice that the
value passed is only a suggestion, and it can even be ignored on some platforms.

+A size
Sets the number of threads in async thread pool. Valid range is 1-1024. The async thread pool is used
by linked-in drivers to handle work that may take a very long time. Since OTP 21, the default
Erlang/OTP distribution includes few linked-in drivers that use the async thread pool. Most of them
have been migrated to dirty IO schedulers. Defaults to 1.

+B [c | d | i]
Option c makes Ctrl-C interrupt the current shell instead of invoking the emulator break handler.
Option d (same as specifying +B without an extra option) disables the break handler. Option i makes the
emulator ignore any break signal.

If option c is used with oldshell on Unix, Ctrl-C will restart the shell process rather than interrupt
it.

+c true | false
Enables or disables time correction:

true
Enables time correction. This is the default if time correction is supported on the specific
platform.

false
Disables time correction.

For backward compatibility, the boolean value can be omitted. This is interpreted as +c false.

+C no_time_warp | single_time_warp | multi_time_warp
Sets time warp mode:

no_time_warp
No time warp mode (the default)

single_time_warp
Single time warp mode

multi_time_warp
Multi-time warp mode

+d
If the emulator detects an internal error (or runs out of memory), it, by default, generates both a
crash dump and a core dump. The core dump is, however, not very useful as the content of process heaps
is destroyed by the crash dump generation.

Option +d instructs the emulator to produce only a core dump and no crash dump if an internal error is
detected.

Calling erlang:halt/1 with a string argument still produces a crash dump. On Unix systems, sending an
emulator process a SIGUSR1 signal also forces a crash dump.

+dcg DecentralizedCounterGroupsLimit
Limits the number of decentralized counter groups used by decentralized counters optimized for update
operations in the Erlang runtime system. By default, the limit is 256.

When the number of schedulers is less than or equal to the limit, each scheduler has its own group.
When the number of schedulers is larger than the groups limit, schedulers share groups. Shared groups
degrade the performance for updating counters while many reader groups degrade the performance for
reading counters. So, the limit is a tradeoff between performance for update operations and performance
for read operations. Each group consumes 64 bytes in each counter.

Note that a runtime system using decentralized counter groups benefits from binding schedulers to
logical processors, as the groups are distributed better between schedulers with this option.

This option only affects decentralized counters used for the counters that are keeping track of the
memory consumption and the number of terms in ETS tables of type ordered_set with the write_concurrency
option activated.

+e Number
Sets the maximum number of ETS tables. This limit is partially obsolete.

+ec
Forces option compressed on all ETS tables. Only intended for test and evaluation.

+fnl
The virtual machine works with filenames as if they are encoded using the ISO Latin-1 encoding,
disallowing Unicode characters with code points > 255.

For more information about Unicode filenames, see section Unicode Filenames in the STDLIB User's Guide.
Notice that this value also applies to command-line parameters and environment variables (see section
Unicode in Environment and Parameters in the STDLIB User's Guide).

+fnu[{w|i|e}]
The virtual machine works with filenames as if they are encoded using UTF-8 (or some other system-
specific Unicode encoding). This is the default on operating systems that enforce Unicode encoding,
that is, Windows MacOS X and Android.

The +fnu switch can be followed by w, i, or e to control how wrongly encoded filenames are to be
reported:

• w means that a warning is sent to the error_logger whenever a wrongly encoded filename is "skipped"
in directory listings. This is the default.

• i means that those wrongly encoded filenames are silently ignored.

• e means that the API function returns an error whenever a wrongly encoded filename (or directory
name) is encountered.

Notice that file:read_link/1 always returns an error if the link points to an invalid filename.

+fna[{w|i|e}]
Selection between +fnl and +fnu is done based on the current locale settings in the OS. This means that
if you have set your terminal for UTF-8 encoding, the filesystem is expected to use the same encoding
for filenames. This is the default on all operating systems, except Android, MacOS X and Windows.

The +fna switch can be followed by w, i, or e. This has effect if the locale settings cause the
behavior of +fnu to be selected; see the description of +fnu above. If the locale settings cause the
behavior of +fnl to be selected, then w, i, or e have no effect.

+hms Size
Sets the default heap size of processes to the size Size words.

+hmbs Size
Sets the default binary virtual heap size of processes to the size Size words.

+hmax Size
Sets the default maximum heap size of processes to the size Size words. Defaults to 0, which means that
no maximum heap size is used. For more information, see process_flag(max_heap_size, MaxHeapSize).

+hmaxel true|false
Sets whether to send an error logger message or not for processes reaching the maximum heap size.
Defaults to true. For more information, see process_flag(max_heap_size, MaxHeapSize).

+hmaxib true|false
Sets whether to include the size of shared off-heap binaries in the sum compared against the maximum
heap size. Defaults to false. For more information, see process_flag(max_heap_size, MaxHeapSize).

+hmaxk true|false
Sets whether to kill processes reaching the maximum heap size or not. Default to true. For more
information, see process_flag(max_heap_size, MaxHeapSize).

+hpds Size
Sets the initial process dictionary size of processes to the size Size.

+hmqd off_heap|on_heap
Sets the default value of the message_queue_data process flag. Defaults to on_heap. If +hmqd is not
passed, on_heap will be the default. For more information, see process_flag(message_queue_data, MQD).

+IOp PollSets
Sets the number of IO pollsets to use when polling for I/O. This option is only used on platforms that
support concurrent updates of a pollset, otherwise the same number of pollsets are used as IO poll
threads. The default is 1.

+IOt PollThreads
Sets the number of IO poll threads to use when polling for I/O. The maximum number of poll threads
allowed is 1024. The default is 1.

A good way to check if more IO poll threads are needed is to use microstate accounting and see what the
load of the IO poll thread is. If it is high it could be a good idea to add more threads.

+IOPp PollSetsPercentage
Similar to +IOp but uses percentages to set the number of IO pollsets to create, based on the number of
poll threads configured. If both +IOPp and +IOp are used, +IOPp is ignored.

+IOPt PollThreadsPercentage
Similar to +IOt but uses percentages to set the number of IO poll threads to create, based on the
number of schedulers configured. If both +IOPt and +IOt are used, +IOPt is ignored.

+IOs true|false
Enable or disable scheduler thread poll optimization. Default is true.

If enabled, file descriptors that are frequently read may be moved to a special pollset used by
scheduler threads. The objective is to reduce the number of system calls and thereby CPU load, but it
can in some cases increase scheduling latency for individual file descriptor input events.

Enables or disables support for coverage when running with the JIT. Defaults to false.

function
All modules that are loaded will be instrumented to keep track of which functions are executed.
Information about which functions that have been executed can be retrieved by calling
code:get_coverage(function, Module).

function_counters
All modules that are loaded will be instrumented to count how many times each function is executed.
Information about how many times each function has been executed can be retrieved by calling
code:get_coverage(function, Module).

line
When modules that have been compiled with the line_coverage option are loaded, they will be
instrumented to keep track of which lines have been executed. Information about which lines have been
executed can be retrieved by calling code:get_coverage(line, Module), and information about which
functions that have been executed can be retrieved by calling code:get_coverage(function, Module).

line_counters
When modules that have been compiled with the line_coverage option are loaded, they will be
instrumented to count the number of times each line is executed. Information about how many times
each line has been executed can be retrieved by calling code:get_coverage(line, Module), and
information about which functions that have been executed can be retrieved by calling
code:get_coverage(function, Module) (note that in this mode, counters for the number of times each
function has been executed cannot be retrieved).

true
Same as line_counters.

false
Disables coverage.

This option can be combined multiple times to enable several options:

dump
Gives perf detailed line information, so that the perf annotate feature works.

map
Gives perf a map over all module code, letting it translate machine code addresses to Erlang source
code locations. This also enables frame pointers for Erlang code so that perf can walk the call
stacks of Erlang processes, which costs one extra word per stack frame.

fp
Enables frame pointers independently of the map option.

no_fp
Disables the frame pointers added by the map option.

true
Enables map and dump.

false
Disables all other options.

For more details about how to run perf see the perf support section in the BeamAsm internal
documentation.

+JMsingle true|false
Since: OTP-26.0

Enables or disables the use of single-mapped RWX memory for JIT code. The default is to map JIT:ed
machine code into two regions sharing the same physical pages, where one region is executable but not
writable, and the other writable but not executable. As some tools, such as QEMU user mode emulation,
cannot deal with the dual mapping, this flags allows it to be disabled. This flag is automatically
enabled by the +JPperf flag.

+L
Prevents loading information about source filenames and line numbers. This saves some memory, but
exceptions do not contain information about the filenames and line numbers.

+MFlag Value
Memory allocator-specific flags. For more information, see erts_alloc(3).

+pad true|false
Since: OTP 25.3

The boolean value used with the +pad parameter determines the default value of the async_dist process
flag of newly spawned processes. By default, if no +pad command line option is passed, the async_dist
flag will be set to false.

The value used in runtime can be inspected by calling erlang:system_info(async_dist).

+pc Range
Sets the range of characters that the system considers printable in heuristic detection of strings.
This typically affects the shell, debugger, and io:format functions (when ~tp is used in the format
string).

Two values are supported for Range:

latin1
The default. Only characters in the ISO Latin-1 range can be considered printable. This means that a
character with a code point > 255 is never considered printable and that lists containing such
characters are displayed as lists of integers rather than text strings by tools.

unicode
All printable Unicode characters are considered when determining if a list of integers is to be
displayed in string syntax. This can give unexpected results if, for example, your font does not
cover all Unicode characters.

See also io:printable_range/0 in STDLIB.

+P Number
Sets the maximum number of simultaneously existing processes for this system if a Number is passed as
value. Valid range for Number is [1024-134217727].

NOTE: The actual maximum chosen may be much larger than the Number passed. Currently the runtime system
often, but not always, chooses a value that is a power of 2. This might, however, be changed in the
future. The actual value chosen can be checked by calling erlang:system_info(process_limit).

The default value is 1048576

+Q Number
Sets the maximum number of simultaneously existing ports for this system if a Number is passed as
value. Valid range for Number is [1024-134217727].

NOTE: The actual maximum chosen may be much larger than the actual Number passed. Currently the runtime
system often, but not always, chooses a value that is a power of 2. This might, however, be changed in
the future. The actual value chosen can be checked by calling erlang:system_info(port_limit).

The default value used is normally 65536. However, if the runtime system is able to determine maximum
amount of file descriptors that it is allowed to open and this value is larger than 65536, the chosen
value will increased to a value larger or equal to the maximum amount of file descriptors that can be
opened.

On Windows the default value is set to 8196 because the normal OS limitations are set higher than most
machines can handle.

+R ReleaseNumber
Sets the compatibility mode.

The distribution mechanism is not backward compatible by default. This flag sets the emulator in
compatibility mode with an earlier Erlang/OTP release ReleaseNumber. The release number must be in the
range <current release>-2 through <current release>. This limits the emulator, making it possible for
it to communicate with Erlang nodes (as well as C and Java nodes) running that earlier release.

NOTE: Ensure that all nodes (Erlang-, C-, and Java nodes) of a distributed Erlang system is of the same
Erlang/OTP release, or from two different Erlang/OTP releases X and Y, where all Y nodes have
compatibility mode X.

+r
Forces ETS memory blocks to be moved on reallocation.

+rg ReaderGroupsLimit
Limits the number of reader groups used by read/write locks optimized for read operations in the Erlang
runtime system. By default the reader groups limit is 64.

When the number of schedulers is less than or equal to the reader groups limit, each scheduler has its
own reader group. When the number of schedulers is larger than the reader groups limit, schedulers
share reader groups. Shared reader groups degrade read lock and read unlock performance while many
reader groups degrade write lock performance. So, the limit is a tradeoff between performance for read
operations and performance for write operations. Each reader group consumes 64 byte in each read/write
lock.

Notice that a runtime system using shared reader groups benefits from binding schedulers to logical
processors, as the reader groups are distributed better between schedulers.

+S Schedulers:SchedulerOnline
Sets the number of scheduler threads to create and scheduler threads to set online. The maximum for
both values is 1024. If the Erlang runtime system is able to determine the number of logical processors
configured and logical processors available, Schedulers defaults to logical processors configured, and
SchedulersOnline defaults to logical processors available; otherwise the default values are 1. If the
emulator detects that it is subject to a CPU quota, the default value for SchedulersOnline will be
limited accordingly.

Schedulers can be omitted if :SchedulerOnline is not and conversely. The number of schedulers online
can be changed at runtime through erlang:system_flag(schedulers_online, SchedulersOnline).

If Schedulers or SchedulersOnline is specified as a negative number, the value is subtracted from the
default number of logical processors configured or logical processors available, respectively.

Specifying value 0 for Schedulers or SchedulersOnline resets the number of scheduler threads or
scheduler threads online, respectively, to its default value.

+SP SchedulersPercentage:SchedulersOnlinePercentage
Similar to +S but uses percentages to set the number of scheduler threads to create, based on logical
processors configured, and scheduler threads to set online, based on logical processors available.
Specified values must be > 0. For example, +SP 50:25 sets the number of scheduler threads to 50% of the
logical processors configured, and the number of scheduler threads online to 25% of the logical
processors available. SchedulersPercentage can be omitted if :SchedulersOnlinePercentage is not and
conversely. The number of schedulers online can be changed at runtime through
erlang:system_flag(schedulers_online, SchedulersOnline).

This option interacts with +S settings. For example, on a system with 8 logical cores configured and 8
logical cores available, the combination of the options +S 4:4 +SP 50:25 (in either order) results in 2
scheduler threads (50% of 4) and 1 scheduler thread online (25% of 4).

+SDcpu DirtyCPUSchedulers:DirtyCPUSchedulersOnline
Sets the number of dirty CPU scheduler threads to create and dirty CPU scheduler threads to set online.
The maximum for both values is 1024, and each value is further limited by the settings for normal
schedulers:

• The number of dirty CPU scheduler threads created cannot exceed the number of normal scheduler
threads created.

• The number of dirty CPU scheduler threads online cannot exceed the number of normal scheduler
threads online.

For details, see +S and +SP. By default, the number of dirty CPU scheduler threads created equals the
number of normal scheduler threads created, and the number of dirty CPU scheduler threads online equals
the number of normal scheduler threads online. DirtyCPUSchedulers can be omitted if
:DirtyCPUSchedulersOnline is not and conversely. The number of dirty CPU schedulers online can be
changed at runtime through erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).

The amount of dirty CPU schedulers is limited by the amount of normal schedulers in order to limit the
effect on processes executing on ordinary schedulers. If the amount of dirty CPU schedulers was allowed
to be unlimited, dirty CPU bound jobs would potentially starve normal jobs.

Typical users of the dirty CPU schedulers are large garbage collections, json protocol encode/decoders
written as nifs and matrix manipulation libraries.

You can use msacc in order to see the current load of the dirty CPU schedulers threads and adjust the
number used accordingly.

+SDPcpu DirtyCPUSchedulersPercentage:DirtyCPUSchedulersOnlinePercentage
Similar to +SDcpu but uses percentages to set the number of dirty CPU scheduler threads to create and
the number of dirty CPU scheduler threads to set online. Specified values must be > 0. For example,
+SDPcpu 50:25 sets the number of dirty CPU scheduler threads to 50% of the logical processors
configured and the number of dirty CPU scheduler threads online to 25% of the logical processors
available. DirtyCPUSchedulersPercentage can be omitted if :DirtyCPUSchedulersOnlinePercentage is not
and conversely. The number of dirty CPU schedulers online can be changed at runtime through
erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).

This option interacts with +SDcpu settings. For example, on a system with 8 logical cores configured
and 8 logical cores available, the combination of the options +SDcpu 4:4 +SDPcpu 50:25 (in either
order) results in 2 dirty CPU scheduler threads (50% of 4) and 1 dirty CPU scheduler thread online (25%
of 4).

+SDio DirtyIOSchedulers
Sets the number of dirty I/O scheduler threads to create. Valid range is 1-1024. By default, the number
of dirty I/O scheduler threads created is 10.

The amount of dirty IO schedulers is not limited by the amount of normal schedulers like the amount of
dirty CPU schedulers. This since only I/O bound work is expected to execute on dirty I/O schedulers. If
the user should schedule CPU bound jobs on dirty I/O schedulers, these jobs might starve ordinary jobs
executing on ordinary schedulers.

Typical users of the dirty IO schedulers are reading and writing to files.

You can use msacc in order to see the current load of the dirty IO schedulers threads and adjust the
number used accordingly.

+sFlag Value
Scheduling specific flags.

+sbt BindType
Sets scheduler bind type.

Schedulers can also be bound using flag +stbt. The only difference between these two flags is how the
following errors are handled:

• Binding of schedulers is not supported on the specific platform.

• No available CPU topology. That is, the runtime system was not able to detect the CPU topology
automatically, and no user-defined CPU topology was set.

If any of these errors occur when +sbt has been passed, the runtime system prints an error message,
and refuses to start. If any of these errors occur when +stbt has been passed, the runtime system
silently ignores the error, and start up using unbound schedulers.

Valid BindTypes:

unbound - Schedulers are not bound to logical processors, that is, the operating system decides
where the scheduler threads execute, and when to migrate them. This is the default.

no_spread - Schedulers with close scheduler identifiers are bound as close as possible in hardware.

thread_spread - Thread refers to hardware threads (such as Intel's hyper-threads). Schedulers with
low scheduler identifiers, are bound to the first hardware thread of each core, then schedulers
with higher scheduler identifiers are bound to the second hardware thread of each core,and so on.

processor_spread - Schedulers are spread like thread_spread, but also over physical processor
chips.

spread - Schedulers are spread as much as possible.

nnts

no_node_thread_spread - Like thread_spread, but if multiple Non-Uniform Memory Access (NUMA) nodes
exist, schedulers are spread over one NUMA node at a time, that is, all logical processors of one
NUMA node are bound to schedulers in sequence.

nnps

no_node_processor_spread - Like processor_spread, but if multiple NUMA nodes exist, schedulers are
spread over one NUMA node at a time, that is, all logical processors of one NUMA node are bound to
schedulers in sequence.

tnnps

thread_no_node_processor_spread - A combination of thread_spread, and no_node_processor_spread.
Schedulers are spread over hardware threads across NUMA nodes, but schedulers are only spread over
processors internally in one NUMA node at a time.

default_bind - Binds schedulers the default way. Defaults to thread_no_node_processor_spread (which
can change in the future).

Binding of schedulers is only supported on newer Linux, Solaris, FreeBSD, and Windows systems.

If no CPU topology is available when flag +sbt is processed and BindType is any other type than u,
the runtime system fails to start. CPU topology can be defined using flag +sct. Notice that flag +sct
can have to be passed before flag +sbt on the command line (if no CPU topology has been automatically
detected).

The runtime system does by default not bind schedulers to logical processors.

NOTE: If the Erlang runtime system is the only operating system process that binds threads to logical
processors, this improves the performance of the runtime system. However, if other operating system
processes (for example another Erlang runtime system) also bind threads to logical processors, there
can be a performance penalty instead. This performance penalty can sometimes be severe. If so, you
are advised not to bind the schedulers.

How schedulers are bound matters. For example, in situations when there are fewer running processes
than schedulers online, the runtime system tries to migrate processes to schedulers with low
scheduler identifiers. The more the schedulers are spread over the hardware, the more resources are
available to the runtime system in such situations.

NOTE: If a scheduler fails to bind, this is often silently ignored, as it is not always possible to
verify valid logical processor identifiers. If an error is reported, it is reported to the
error_logger. If you want to verify that the schedulers have bound as requested, call
erlang:system_info(scheduler_bindings).

NOTE: This flag can be removed or changed at any time without prior notice.

NOTE: This flag can be removed or changed at any time without prior notice.

NOTE: This flag can be removed or changed at any time without prior notice.

+scl true|false
Enables or disables scheduler compaction of load. By default scheduler compaction of load is enabled.
When enabled, load balancing strives for a load distribution, which causes as many scheduler threads
as possible to be fully loaded (that is, not run out of work). This is accomplished by migrating load
(for example, runnable processes) into a smaller set of schedulers when schedulers frequently run out
of work. When disabled, the frequency with which schedulers run out of work is not taken into account
by the load balancing logic.

+scl false is similar to +sub true, but +sub true also balances scheduler utilization between
schedulers.

+sct CpuTopology
Sets a user-defined CPU topology. The user-defined CPU topology overrides any automatically detected
CPU topology. The CPU topology is used when binding schedulers to logical processors. This option
must be before +sbt on the command-line.

<Id> = integer(); when 0 =< <Id> =< 65535
<IdRange> = <Id>-<Id>
<IdOrIdRange> = <Id> | <IdRange>
<IdList> = <IdOrIdRange>,<IdOrIdRange> | <IdOrIdRange>
<LogicalIds> = L<IdList>
<ThreadIds> = T<IdList> | t<IdList>
<CoreIds> = C<IdList> | c<IdList>
<ProcessorIds> = P<IdList> | p<IdList>
<NodeIds> = N<IdList> | n<IdList>
<IdDefs> = <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds> |
<LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>
CpuTopology = <IdDefs>:<IdDefs> | <IdDefs>

Uppercase letters signify real identifiers and lowercase letters signify fake identifiers only used
for description of the topology. Identifiers passed as real identifiers can be used by the runtime
system when trying to access specific hardware; if they are incorrect the behavior is undefined.
Faked logical CPU identifiers are not accepted, as there is no point in defining the CPU topology
without real logical CPU identifiers. Thread, core, processor, and node identifiers can be omitted.
If omitted, the thread ID defaults to t0, the core ID defaults to c0, the processor ID defaults to
p0, and the node ID is left undefined. Either each logical processor must belong to only one NUMA
node, or no logical processors must belong to any NUMA nodes.

Both increasing and decreasing <IdRange>s are allowed.

NUMA node identifiers are system wide. That is, each NUMA node on the system must have a unique
identifier. Processor identifiers are also system wide. Core identifiers are processor wide. Thread
identifiers are core wide.

The order of the identifier types implies the hierarchy of the CPU topology. The valid orders are as
follows:

• <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds>, that is, thread is part of a core that
is part of a processor, which is part of a NUMA node.

• <LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>, that is, thread is part of a core that
is part of a NUMA node, which is part of a processor.

A CPU topology can consist of both processor external, and processor internal NUMA nodes as long as
each logical processor belongs to only one NUMA node. If <ProcessorIds> is omitted, its default
position is before <NodeIds>. That is, the default is processor external NUMA nodes.

If a list of identifiers is used in an <IdDefs>:

• <LogicalIds> must be a list of identifiers.

• At least one other identifier type besides <LogicalIds> must also have a list of identifiers.

• All lists of identifiers must produce the same number of identifiers.

A simple example. A single quad core processor can be described as follows:

% erl +sct L0-3c0-3
1> erlang:system_info(cpu_topology).
[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,2}},
{core,{logical,3}}]}]

A more complicated example with two quad core processors, each processor in its own NUMA node. The
ordering of logical processors is a bit weird. This to give a better example of identifier lists:

% erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1
1> erlang:system_info(cpu_topology).
[{node,[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,3}},
{core,{logical,2}}]}]},
{node,[{processor,[{core,{logical,7}},
{core,{logical,4}},
{core,{logical,6}},
{core,{logical,5}}]}]}]

As long as real identifiers are correct, it is OK to pass a CPU topology that is not a correct
description of the CPU topology. When used with care this can be very useful. This to trick the
emulator to bind its schedulers as you want. For example, if you want to run multiple Erlang runtime
systems on the same machine, you want to reduce the number of schedulers used and manipulate the CPU
topology so that they bind to different logical CPUs. An example, with two Erlang runtime systems on
a quad core machine:

% erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one
% erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two

In this example, each runtime system have two schedulers each online, and all schedulers online will
run on different cores. If we change to one scheduler online on one runtime system, and three
schedulers online on the other, all schedulers online will still run on different cores.

Notice that a faked CPU topology that does not reflect how the real CPU topology looks like is likely
to decrease the performance of the runtime system.

For more information, see erlang:system_info(cpu_topology).

+ssrct
Skips reading CPU topology.

NOTE: Reading CPU topology slows down startup when starting many parallel instances of ERTS on
systems with large amount of cores; using this flag might speed up execution in such scenarios.

+sfwi Interval
Sets scheduler-forced wakeup interval. All run queues are scanned each Interval milliseconds. While
there are sleeping schedulers in the system, one scheduler is woken for each non-empty run queue
found. Interval default to 0, meaning this feature is disabled.

NOTE: This feature has been introduced as a temporary workaround for long-executing native code, and
native code that does not bump reductions properly in OTP. When these bugs have been fixed, this flag
will be removed.

+spp Bool
Sets default scheduler hint for port parallelism. If set to true, the virtual machine schedules port
tasks when it improves parallelism in the system. If set to false, the virtual machine tries to
perform port tasks immediately, improving latency at the expense of parallelism. Default to false.
The default used can be inspected in runtime by calling erlang:system_info(port_parallelism). The
default can be overridden on port creation by passing option parallelism to erlang:open_port/2.

+sss size
Suggested stack size, in kilowords, for scheduler threads. Valid range is 20-8192 kilowords. The
default suggested stack size is 128 kilowords.

+sssdcpu size
Suggested stack size, in kilowords, for dirty CPU scheduler threads. Valid range is 20-8192
kilowords. The default suggested stack size is 40 kilowords.

+sssdio size
Suggested stack size, in kilowords, for dirty IO scheduler threads. Valid range is 20-8192 kilowords.
The default suggested stack size is 40 kilowords.

+stbt BindType
Tries to set the scheduler bind type. The same as flag +sbt except how some errors are handled. For
more information, see +sbt.

+sub true|false
Enables or disables scheduler utilization balancing of load. By default scheduler utilization
balancing is disabled and instead scheduler compaction of load is enabled, which strives for a load
distribution that causes as many scheduler threads as possible to be fully loaded (that is, not run
out of work). When scheduler utilization balancing is enabled, the system instead tries to balance
scheduler utilization between schedulers. That is, strive for equal scheduler utilization on all
schedulers.

+sub true is only supported on systems where the runtime system detects and uses a monotonically
increasing high-resolution clock. On other systems, the runtime system fails to start.

+sub true implies +scl false. The difference between +sub true and +scl false is that +scl false does
not try to balance the scheduler utilization.

+swct very_eager|eager|medium|lazy|very_lazy
Sets scheduler wake cleanup threshold. Defaults to medium. Controls how eager schedulers are to be
requesting wakeup because of certain cleanup operations. When a lazy setting is used, more
outstanding cleanup operations can be left undone while a scheduler is idling. When an eager setting
is used, schedulers are more frequently woken, potentially increasing CPU-utilization.

NOTE: This flag can be removed or changed at any time without prior notice.

+sws default|legacy
Sets scheduler wakeup strategy. Default strategy changed in ERTS 5.10 (Erlang/OTP R16A). This
strategy was known as proposal in Erlang/OTP R15. The legacy strategy was used as default from R13 up
to and including R15.

NOTE: This flag can be removed or changed at any time without prior notice.

+swt very_low|low|medium|high|very_high
Sets scheduler wakeup threshold. Defaults to medium. The threshold determines when to wake up
sleeping schedulers when more work than can be handled by currently awake schedulers exists. A low
threshold causes earlier wakeups, and a high threshold causes later wakeups. Early wakeups distribute
work over multiple schedulers faster, but work does more easily bounce between schedulers.

NOTE: This flag can be removed or changed at any time without prior notice.

+swtdcpu very_low|low|medium|high|very_high
As +swt but affects dirty CPU schedulers. Defaults to medium.

NOTE: This flag can be removed or changed at any time without prior notice.

+swtdio very_low|low|medium|high|very_high
As +swt but affects dirty IO schedulers. Defaults to medium.

NOTE: This flag can be removed or changed at any time without prior notice.

+t size
Sets the maximum number of atoms the virtual machine can handle. Defaults to 1,048,576.

+T Level
Enables modified timing and sets the modified timing level. Valid range is 0-9. The timing of the
runtime system is changed. A high level usually means a greater change than a low level. Changing the
timing can be very useful for finding timing-related bugs.

Modified timing affects the following:

Process spawning
A process calling spawn, spawn_link, spawn_monitor, or spawn_opt is scheduled out immediately after
completing the call. When higher modified timing levels are used, the caller also sleeps for a while
after it is scheduled out.

Context reductions
The number of reductions a process is allowed to use before it is scheduled out is increased or
reduced.

Input reductions
The number of reductions performed before checking I/O is increased or reduced.

NOTE: Performance suffers when modified timing is enabled. This flag is only intended for testing and
debugging. return_to and return_from trace messages are lost when tracing on the spawn BIFs. This flag
can be removed or changed at any time without prior notice.

+v
Verbose.

+V
Makes the emulator print its version number.

+W w | i | e
Sets the mapping of warning messages for error_logger. Messages sent to the error logger using one of
the warning routines can be mapped to errors (+W e), warnings (+W w), or information reports (+W i).
Defaults to warnings. The current mapping can be retrieved using error_logger:warning_map/0. For more
information, see error_logger:warning_map/0 in Kernel.

+zFlag Value
Miscellaneous flags:

+zdbbl size
Sets the distribution buffer busy limit ( dist_buf_busy_limit) in kilobytes. Valid range is
1-2097151. Defaults to 1024.

A larger buffer limit allows processes to buffer more outgoing messages over the distribution. When
the buffer limit has been reached, sending processes will be suspended until the buffer size has
shrunk. The buffer limit is per distribution channel. A higher limit gives lower latency and higher
throughput at the expense of higher memory use.

This limit only affects processes that have disabled fully asynchronous distributed signaling.

+zdntgc time
Sets the delayed node table garbage collection time ( delayed_node_table_gc) in seconds. Valid values
are either infinity or an integer in the range 0-100000000. Defaults to 60.

Node table entries that are not referred linger in the table for at least the amount of time that
this parameter determines. The lingering prevents repeated deletions and insertions in the tables
from occurring.

+zosrl limit
Sets a limit on the amount of outstanding requests made by a system process orchestrating system wide
changes. Valid range of this limit is [1, 134217727]. See
erlang:system_flag(outstanding_system_requests_limit, Limit) for more information.

+zhft limit
Sets a limit on how long the runtime system is allowed to perform flush operations while halting.
Valid <timeout> values are integers in the range 0..2147483647 or the word infinity. <timeout> is in
milliseconds and is by default infinity.

If flushing during a halt operation has been ongoing for <timeout> milliseconds, the flushing will be
interrupted and the runtime system will be immediately terminated with exit code 255. If halting
without flushing, the <timeout> will have no effect on the system.

The value set by this flag can be read by Erlang code by calling
erlang:system_info(halt_flush_timeout).

See also the flush_timeout option of the erlang:halt/2 BIF. Note that the shortest timeout of this
command line argument and the flush_timeout option will be the actual timeout value in effect.

Since: OTP 27.0

Environment Variables
ERL_CRASH_DUMP
If the emulator needs to write a crash dump, the value of this variable is the filename of the crash
dump file. If the variable is not set, the name of the crash dump file is erl_crash.dump in the current
directory.

ERL_CRASH_DUMP_NICE

Unix systems: If the emulator needs to write a crash dump, it uses the value of this variable to set
the nice value for the process, thus lowering its priority. Valid range is 1-39 (higher values are
replaced with 39). The highest value, 39, gives the process the lowest priority.

ERL_CRASH_DUMP_SECONDS

Unix systems: This variable gives the number of seconds that the emulator is allowed to spend writing a
crash dump. When the given number of seconds have elapsed, the emulator is terminated.
ERL_CRASH_DUMP_SECONDS=0
If the variable is set to 0 seconds, the runtime system does not even attempt to write the crash dump
file. It only terminates. This is the default if option -heart is passed to erl and
ERL_CRASH_DUMP_SECONDS is not set.

ERL_CRASH_DUMP_SECONDS=S
If the variable is set to a positive value S, wait for S seconds to complete the crash dump file and
then terminates the runtime system with a SIGALRM signal.

ERL_CRASH_DUMP_SECONDS=-1
A negative value causes the termination of the runtime system to wait indefinitely until the crash
dump file has been completely written. This is the default if option -heart is not passed to erl and
ERL_CRASH_DUMP_SECONDS is not set.