Ubuntu Manpage: erl - The Erlang emulator.

Provided by: erlang-base-hipe_22.2.7+dfsg-1ubuntu0.2_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:
Specifies the name of a configuration file, Config.config, which is used to configure applications;
see app(5) and application(3erl).

-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_type Type:
Start an emulator of a different type. For example, to start the lock-counter emualator, use
-emu_type lcnt. (The emulator 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.

-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.

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.

-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. Requires either -name or -sname to be given. If
Node does not contain a hostname, one is automatically taken from -name or -sname

-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.

-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 0-1024. 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.

+rg 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 and MacOS X.

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 default on all operating systems, except 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 for process flag message_queue_data. 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.

+l:
Enables autoload tracing, displaying information while loading code.

+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. 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.

+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 0-1024. By default, the
number of dirty I/O scheduler threads created is 10, same as the default number of threads in the
async thread pool.

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.

+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 be 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.

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