Ubuntu Manpage: erl - The Erlang emulator.

Provided by: erlang-base_24.3.4.1+dfsg-1_amd64

NAME

       erl - The Erlang emulator.

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.

       Windows users probably want to use the werl program instead, which runs in its own  window
       with  scrollbars and supports command-line editing. The erl program on Windows provides no
       line editing in its shell, and on Windows 95 there is no way to scroll back to  text  that
       has scrolled off the screen. The erl program must be used, however, in pipelines or if you
       want to redirect standard input or output.

   Note:
       As from ERTS 5.9 (Erlang/OTP R15B) the runtime system does by default not bind  schedulers
       to logical processors. For more information, see system flag +sbt.

EXPORTS

       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(3erl).

                  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(5) and application(3erl).

-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(3erl). 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(3erl).

-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(5) and application(3erl). 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(5) and
application(3erl). 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 \ 'io:format("~p~n",[application:get_env(kernel, logger_level)]),erlang:halt()' 3< \ <(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 \ 'io:format("~p~n",[application:get_env(kernel, logger_level)]),erlang:halt()' \ 3< test1.config 4< test2.config
{ok,error}

-connect_all false:
If this flag is present, global does not maintain a fully connected network of
distributed Erlang nodes, and then global name registration cannot be used; see
global(3erl).

-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.

-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.)

-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 (init flag):
Configures the module responsible to communicate to epmd. Defaults to erl_epmd.

-erl_epmd_port Port (init flag):
Configures the port used by erl_epmd to listen for connection and connect to other
nodes. See erl_epmd for more details. Defaults to 0.

-eval Expr (init flag):
Makes init evaluate the expression Expr; see init(3erl).

-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(3erl).

-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(3erl).

-hosts Hosts:
Specifies the IP addresses for the hosts on which Erlang boot servers are running, see
erl_boot_server(3erl). 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(3erl). 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(3erl). 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(3erl).

-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(3erl). It is also
ensured that epmd runs on the current host before Erlang is started; see epmd(1) and
the -start_epmd option.

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(3erl).

-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(3erl).

-pz Dir1 Dir2 ...:
Adds the specified directories to the end of the code path, similar to
code:add_pathsz/1; see code(3erl).

-path Dir1 Dir2 ...:
Replaces the path specified in the boot script; see script(5).

-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.

-rsh Program:
Specifies an alternative to ssh for starting a slave node on a remote host; see
slave(3erl).

-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(3erl).

-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(3erl).

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

-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 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:

+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 there are very few linked-in drivers in the default Erlang/OTP
distribution that uses 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.

Notice that on Windows, this flag is only applicable for werl, not erl (oldshell).
Notice also that Ctrl-Break is used instead of Ctrl-C on Windows.

+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.

Notice 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.

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

+hmax Size:
Sets the default maximum heap size of processes to the size Size. 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).

+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.

+JPperf true|false:
Enables or disables support for the `perf` profiler when running with the JIT on
Linux. Defaults to false.

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

+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(3erl).

+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 262144

+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..<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 block to be moved on realloc.

+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 the +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(3erl) 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(3erl) 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:

u:
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.

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

ts:
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.

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

s:
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.

db:
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:

* <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>

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.

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).

+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.

+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.

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 completly written. This is the default if  option
             -heart is not passed to erl and ERL_CRASH_DUMP_SECONDS is not set.

           See also heart(3erl).

         ERL_CRASH_DUMP_BYTES:
           This variable sets the maximum size of a crash dump file in bytes. The crash dump will
           be truncated if this limit is exceeded. If the variable is not set, no size  limit  is
           enforced  by  default.  If  the variable is set to 0, the runtime system does not even
           attempt to write a crash dump file.

           Introduced in ERTS 8.1.2 (Erlang/OTP 19.2).

         ERL_AFLAGS:
           The content of this variable is added to the beginning of the command line for erl.

           Flag -extra is treated in a special way. Its scope ends at the end of the  environment
           variable  content.  Arguments  following  an -extra flag are moved on the command line
           into section -extra, that is, the end of the command line following an -extra flag.

         ERL_ZFLAGS and ERL_FLAGS:
           The content of these variables are added to the end of the command line for erl.

           Flag -extra is treated in a special way. Its scope ends at the end of the  environment
           variable  content.  Arguments  following  an -extra flag are moved on the command line
           into section -extra, that is, the end of the command line following an -extra flag.

         ERL_LIBS:
           Contains a list of additional library directories that the code  server  searches  for
           applications and adds to the code path; see code(3erl).

         ERL_EPMD_ADDRESS:
           Can  be  set  to a comma-separated list of IP addresses, in which case the epmd daemon
           listens only on the specified address(es)  and  on  the  loopback  address  (which  is
           implicitly added to the list if it has not been specified).

         ERL_EPMD_PORT:
           Can  contain  the  port  number  to use when communicating with epmd. The default port
           works fine in most cases. A  different  port  can  be  specified  to  allow  nodes  of
           independent clusters to co-exist on the same host. All nodes in a cluster must use the
           same epmd port number.

SIGNALS

       On Unix systems, the Erlang runtime will interpret two types of signals.

         SIGUSR1:
           A SIGUSR1 signal forces a crash dump.

         SIGTERM:
           A SIGTERM will produce a stop message to the init process. This  is  equivalent  to  a
           init:stop/0 call.

           Introduced in ERTS 8.3 (Erlang/OTP 19.3)

       The signal SIGUSR2 is reserved for internal usage. No other signals are handled.

CONFIGURATION

       The  standard  Erlang/OTP  system  can  be  reconfigured to change the default behavior on
       startup.

         The .erlang startup file:
           When Erlang/OTP is started, the system searches for a file named .erlang in the user's
           home directory.

           If  an .erlang file is found, it is assumed to contain valid Erlang expressions. These
           expressions are evaluated as if they were input to the shell.

           A typical .erlang file contains a set of search paths, for example:

         io:format("executing user profile in HOME/.erlang\n",[]).
         code:add_path("/home/calvin/test/ebin").
         code:add_path("/home/hobbes/bigappl-1.2/ebin").
         io:format(".erlang rc finished\n",[]).

         user_default and shell_default:
           Functions in the shell that are not prefixed by  a  module  name  are  assumed  to  be
           functional  objects  (funs),  built-in  functions  (BIFs),  or  belong  to  the module
           user_default or shell_default.

           To include private shell commands, define them in a module user_default  and  add  the
           following argument as the first line in the .erlang file:

         code:load_abs("..../user_default").

         erl:
           If  the  contents  of  .erlang  are  changed  and a private version of user_default is
           defined, the Erlang/OTP environment can be customized. More powerful  changes  can  be
           made  by  supplying  command-line  arguments  in  the  startup  script  erl.  For more
           information, see init(3erl).

NAME

DESCRIPTION

EXPORTS

FLAGS

EMULATOR FLAGS

ENVIRONMENT VARIABLES

SIGNALS

CONFIGURATION

SEE ALSO